Here are the First Pictures From the Parker Solar Probe. Wait… That’s Not the Sun

On August 12th, 2018, NASA launched the first spacecraft that will ever “touch” the face of the Sun. This was none other than the Parker Solar Probe, a mission that will revolutionize our understanding of the Sun, solar wind, and “space weather” events like solar flares. Whereas previous missions have observed the Sun, the Parker Solar Probe will provide the closest observations in history by entering the Sun’s atmosphere (aka. the corona).

And now, just over a month into the its mission, the Parker Solar Probe has captured and returned its first-light data. This data, which consisted of images of the Milky Way and Jupiter, was collected by the probe’s four instrument suites. While the images were not aimed at the Sun, the probe’s primary focus of study, they successfully demonstrated that the Parker probe’s instruments are in good working order.

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The Sun is Actually One of the Most Difficult Places to Reach in the Solar System. Here’s how the Parker Solar Probe Will Do It

When it comes to exploring our Solar System, there are few missions more ambitious than those that seek to study the Sun. While NASA and other space agencies have been observing the Sun for decades, the majority of these missions were conducted in orbit around Earth. To date, the closest any probes have gotten to the Sun were the Helios 1 and 2 probes, which studied the Sun during the 1970s from inside Mercury’s orbit at perihelion.

NASA intends to change all that with the Parker Solar Probe, the space probe that recently launched from Cape Canaveral, which will revolutionize our understanding of the Sun by entering it’s atmosphere (aka. the corona). Over the next seven years, the probe will use Venus’ gravity to conduct a series of slingshots that will gradually bring it closer the Sun than any mission in the history of spaceflight!

The spacecraft lifted off at 3:31 a.m. EDT on Sunday August 12th, from Space Launch Complex-37 at Cape Canaveral Air Force Station atop a United Launch Alliance Delta IV Heavy rocket. At 5:33 a.m., the mission operations manager reported that the spacecraft was healthy and operating normally. Over the course of the next week, it will begin deploying its instruments in preparation for its science mission.

Once inside the Sun’s corona, the Parker Solar Probe will employ an advanced suite of instruments to revolutionize our understanding of the Sun’s atmosphere and the origin and evolution of solar wind. These and other findings will allow researchers and astronomers to improve their ability to forecast space weather events (such as solar flares), which can cause harm to astronauts and orbiting missions, disrupt radio communications and damage power grids.

As Thomas Zurbuchen, the associate administrator of NASA’s Science Mission Directorate, said in a recent NASA press release:

“This mission truly marks humanity’s first visit to a star that will have implications not just here on Earth, but how we better understand our universe. We’ve accomplished something that decades ago, lived solely in the realm of science fiction.”

The Parker Probes mission certainly comes with its share of challenges. In addition to the incredible heat it will have to endure, there is also the challenge of simply getting there. This is due to Earth’s orbital velocity, which travels around the Sun at a speed of 30 km/s (18.64 mps) – or about 108,000 km/h (67,000 mph). Cancelling out this velocity and traveling towards the Sun would take 55 times as much energy as it would for a craft to travel to Mars.

To address this challenge, the Parker Probe has been launched by a very powerful rocket – the ULA Delta IV, which is capable of generating 9,700 kN of thrust. In addition, it will be relying on a series of gravity assists (aka. gravitational slingshots) with Venus. These will consist of the probe conducting flybys of the Sun, then circling around Venus to get a boost in speed from the force of the planet’s gravity, and then slingshoting around the Sun again.

The launch of the Parker Solar Probe atop a ULA Delta IV Heavy rocket from Cape Canaveral Air Force Station on August 12th, 2018. Credit: Glenn Davis

Over the course of its seven-year mission, the probe will conduct seven gravity-assists with Venus and will make 24 passes of the Sun, gradually tightening its orbit in the process. Eventually, it will reach a distance of roughly 6 million km (3.8 million mi) from the Sun and fly through it’s atmosphere (aka. corona), effectively getting more than seven times closer than any spacecraft in history. In addition, the probe will be traveling at speeds of roughly 692,000 km/h (430,000 mph), which will set the record for the fastest-moving spacecraft in history.

During the first week of its journey, the spacecraft will deploy its high-gain antenna and magnetometer boom, which houses the three instruments it will use to study the Sun’s magnetic field. It will also perform the first of a two-part deployment of its five electric field antennas (aka. the FIELDS instrument suite), which will measure the properties of solar wind and help make a three-dimensional picture of the Sun’s electric fields.

Other instruments aboard the spacecraft include the Wide-Field Imager for Parker Solar Probe (WISPR), the spacecraft’s only imaging instrument. This instrument will take pictures of the large-scale structure of the corona and solar wind before the spacecraft flies through it, capturing such phenomena as coronal mass ejections (CMEs), jets, and other ejecta from the Sun.

There’s also the Solar Wind Electrons Alphas and Protons (SWEAP) investigation instrument, which consists of two other instruments – the Solar Probe Cup (SPC) and the Solar Probe Analyzers (SPAN). These will count the most abundant particles in the solar wind – electrons, protons and helium ions – and measure their velocity, density, temperature, and other properties to improve our understanding of solar wind and coronal plasma.

Then there’s the Integrated Science Investigation of the Sun (ISOIS), which relies on the EPI-Lo and EPI-Hi instruments – Energetic Particle Instruments (EPI). Using these two instruments, ISOIS will measure electrons, protons and ions across a wide range of energies to gain a better understanding of where these particles come from, how they became accelerated, and how they move throughout the Solar System.

In addition to being the first spacecraft to explore the Sun’s corona, the Parker Solar Probe is the first spacecraft named after a living scientist – Eugene Parker, the physicist who first theorized the existence of the solar wind in 1958. As Nicola Fox, the probe’s project scientist at the JHUAPL, indicated:

“Exploring the Sun’s corona with a spacecraft has been one of the hardest challenges for space exploration. We’re finally going to be able to answer questions about the corona and solar wind raised by Gene Parker in 1958 – using a spacecraft that bears his name – and I can’t wait to find out what discoveries we make. The science will be remarkable.”

Dr. Parker was on hand to witness the early morning launch of the spacecraft. In addition to its advanced suite of scientific instruments, the probe also carries a plaque dedicating the mission to Parker. This plaque, which was attached in May, includes a quote from the renowned physicist – “Let’s see what lies ahead” – and a memory card containing more than 1.1 million names submitted by the public to travel with the spacecraft to the Sun.

Instrument testing will begin in early September and last approximately four weeks, after which the Parker Solar Probe can begin science operations. On September 28th, it will conduct its first flyby of Venus and perform its first gravity assist with the planet by early October. This will cause the spacecraft to assume a 180-day orbit of the Sun, which will bring it to a distance of about 24 million km (15 million mi).

In the end, the Parker Solar Probe will attempt to answer several long-standing mysteries about the Sun. For instance, why is the Sun’s corona 300 times hotter than the Sun’s surface, what drives the supersonic solar wind that permeates the entire Solar System, and what accelerates solar energetic particles – which can reach speeds of up to half the speed of light – away from the Sun?

Close up photo of the ULA Delta IV Heavy rocket’s engines as it launches from Cape Canaveral Air Force Station. Credit: Glenn Davis

For sixty years, scientists have pondered these questions, but were unable to answer them since no spacecraft was capable of penetrating the Sun’s corona. Thanks to advances in thermal engineering, the Parker Solar Probe is the first spacecraft that will be able to “touch” the face of the Sun and reveal its secrets. By December, the craft will transmit its first science observations back to Earth.

As Andy Driesman, the project manager of the Parker Probe mission at the Johns Hopkins University Applied Physics Laboratory (JHUAPL), expressed:

“Today’s launch was the culmination of six decades of scientific study and millions of hours of effort. Now, Parker Solar Probe is operating normally and on its way to begin a seven-year mission of extreme science.”

Understanding the dynamics of the Sun is intrinsic to understanding the history of the Solar System and the emergence of life itself. But until now, no mission has been able to get close enough to the Sun to address its greatest mysteries. By the time the Parker Solar Probe’s mission is complete, scientists expect to have learned a great deal about the phenomena that can give rise to life, and disrupt it!

Further Reading: NASA, Youtube

The Black Hole Ultimate Solar System: a Supermassive Black Hole, 9 Stars and 550 Planets

Shortly after Einstein published his Theory of General Relativity in 1915, physicists began to speculate about the existence of black holes. These regions of space-time from which nothing (not even light) can escape are what naturally occur at the end of most massive stars’ life cycle. While black holes are generally thought to be voracious eaters, some physicists have wondered if they could also support planetary systems of their own.

Looking to address this question, Dr. Sean Raymond – an American physicist currently at the University of Bourdeaux – created a hypothetical planetary system where a black hole lies at the center. Based on a series of gravitational calculations, he determined that a black hole would be capable of keeping nine individual Suns in a stable orbit around it, which would be able to support 550 planets within a habitable zone.

He named this hypothetical system “The Black Hole Ultimate Solar System“, which consists of a non-spinning black hole that is 1 million times as massive as the Sun. That is roughly one-quarter the mass of Sagittarius A*, the super-massive black hole (SMBH) that resides at the center of the Milky Way Galaxy (which contains 4.31 million Solar Masses).

Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.

As Raymond indicates, one of the immediate advantages of having this black hole at the center of a system is that it can support a large number of Suns. For the sake of his system, Raymond chose 9, thought he indicates that many more could be sustained thanks to the sheer gravitational influence of the central black hole. As he wrote on his website:

“Given how massive the black hole is, one ring could hold up to 75 Suns! But that would move the habitable zone outward pretty far and I don’t want the system to get too spread out. So I’ll use 9 Suns in the ring, which moves everything out by a factor of 3. Let’s put the ring at 0.5 AU, well outside the innermost stable circular orbit (at about 0.02 AU) but well inside the habitable zone (from about 2.7 to 5.4 AU).”

Another major advantage of having a black hole at the center of a system is that it shrinks what is known as the “Hill radius” (aka. Hill sphere, or Roche sphere). This is essentially the region around a planet where its gravity is dominant over that of the star it orbits, and can therefore attract satellites. According to Raymond, a planet’s Hill radius would be 100 times smaller around a million-sun black hole than around the Sun.

This means that a given region of space could stably fit 100 times more planets if they orbited a black hole instead of the Sun. As he explained:

“Planets can be super close to each other because the black hole’s gravity is so strong! If planets are little toy Hot wheels cars, most planetary systems are laid out like normal highways (side note: I love Hot wheels).  Each car stays in its own lane, but the cars are much much smaller than the distance between them.  Around a black hole, planetary systems can be shrunk way down to Hot wheels-sized tracks.  The Hot wheels cars — our planets — don’t change at all, but they can remain stable while being much closer together. They don’t touch (that would not be stable), they are just closer together.”

This is what allows for many planets to be placed with the system’s habitable zone. Based on the Earth’s Hill radius, Raymond estimates that about six Earth-mass planets could fit into stable orbits within the same zone around our Sun. This is based on the fact that Earth-mass planets could be spaced roughly 0.1 AU from each other and maintain a stable orbit.

Given that the Sun’s habitable zone corresponds roughly to the distances between Venus and Mars – which are 0.3 and 0.5 AU away, respectively – this means there is 0.8 AUs of room to work with. However, around a black hole with 1 million Solar Masses, the closest neighboring planet could be just 1/1000th (0.001) of an AU away and still have a stable orbit.

Doing the math, this means that roughly 550 Earths could fit in the same region orbiting the black hole and its nine Suns. There is one minor drawback to this whole scenario, which is that the black hole would have to remain at its current mass. If it were to become any larger, it would cause the Hill radii of its 550 planets to shrink down further and further.

Once the Hill radius got down to the point where it was the same size as any of the Earth-mass planets, the black hole would begin to tear them apart. But at 1 million Solar masses, the black hole is capable of supporting a massive system of planets comfortably. “With our million-Sun black hole the Earth’s Hill radius (on its current orbit) would already be down to the limit, just a bit more than twice Earth’s actual radius,” he says.

Illustration of tightly-packed orbits of Earth-mass planets in orbit around the Sun (in black) vs. around a supermassive black hole (green). Credit: Sean Raymond

Lastly, Raymond considers the implications that living in such a system would have. For one, a year on any planet within the system’s habitable zone would be much shorter, owing to the fact their orbital periods would be much faster. Basically, a year would last roughly 1.6 days for planets at the inner edge of the habitable zone and 4.6 days for planets at the outer edge of the habitable zone.

In addition, on the surface of any planet in the system, the sky would be a lot more crowded! With so many planets in close orbit together, they would pass very close to one another. That essentially means that from the surface of any individual Earth, people would be able to see nearby Earths as clear as we see the Moon on some days. As Raymond illustrated:

“At closest approach (conjunction) the distance between planets is about twice the Earth-Moon distance. These planets are all Earth-sized, about 4 times larger than the Moon. This means that at conjunction each planet’s closest neighbor appears about twice the size of the full Moon in the sky. And there are two nearest neighbors, the inner and outer one. Plus, the next-nearest neighbors are twice as far away so they are still as big as the full Moon during conjunction. And four more planets that would be at least half the full Moon in size during conjunction.”

He also indicates that conjunctions would occur almost once per orbit, which would mean that every few days, there would be no shortage of giant objects passing across the sky. And of course, there would be the Sun’s themselves. Recall that scene in Star Wars where a young Luke Skywalker is watching two suns set in the desert? Well, it would a little like that, except way more cool!

According to Raymond’s calculations, the nine Suns would complete an orbit around the black hole every three hours. Every twenty minutes, one of these Suns would pass behind the black hole, taking just 49 seconds to do so. At this point, gravitational lensing would occur, where the black hole would focus the Sun’s light toward the planet and distort the apparent shape of the Sun.

To illustrate what this would look like, he provides an animation (shown above) created by – a planet modeller who develops space graphics for Kerbal and other programs – using Space Engine.

While such a system may never occur in nature, it is interesting to know that such a system would be physically possible. And who knows? Perhaps a sufficiently advanced species, with the ability to tow stars and planets from one system and place them in orbit around a black hole, could fashion this Ultimate Solar System. Something for SETI researchers to be on the lookout for, perhaps?

This hypothetical exercise was the second installment in two-part series by Raymond, titled “Black holes and planets”. In the first installment, “The Black Hole Solar System“, Raymond considered what it would be like if our system orbited around a black hole-Sun binary. As he indicated, the consequences for Earth and the other Solar planets would be interesting, to say the least!

Raymond also recently expanded on the Ultimate Solar System by proposing The Million Earth Solar System. Check them all out at his website, PlanetPlanet.net.

Further Reading: PlanetPlanet

Two Spacecraft Will Get Closer to the Sun Than Ever Before

Our understanding of distant stars has increased dramatically in recent decades. Thanks to improved instruments, scientists are able to see farther and clearer, thus learning more about star systems and the planets that orbit them (aka. extra-solar planets). Unfortunately, it will be some time before we develop the necessary technology to explore these stars up close.

But in the meantime, NASA and the ESA are developing missions that will allow us to explore our own Sun like never before. These missions, NASA’s Parker Solar Probe and the ESA’s (the European Space Agency) Solar Orbiter, will explore closer to the Sun than any previous mission. In so doing, it is hoped that they will resolve decades-old questions about the inner workings of the Sun.

These missions – which will launch in 2018 and 2020, respectively – will also have significant implications for life here on Earth. Not only is sunlight essential to life as we know it, solar flares can pose a major hazard for technology that humanity is becoming increasingly dependent on. This includes radio communications, satellites, power grids and human spaceflight.

And in the coming decades, Low-Earth Orbit (LEO) is expected to become increasingly crowded as commercial space stations and even space tourism become a reality. By improving our understanding of the processes that drive solar flares, we will therefore be able to better predict when they will occur and how they will impact Earth, spacecraft, and infrastructure in LEO.

As Chris St. Cyr, the Solar Orbiter project scientist at NASA’s Goddard Space Flight Center, explained in a recent NASA press release:

“Our goal is to understand how the Sun works and how it affects the space environment to the point of predictability. This is really a curiosity-driven science.”

Both missions will focus on the Sun’s dynamic outer atmosphere, otherwise known as the corona. At present, much of the behavior of this layer of the Sun is unpredictable and not well understood. For instance, there’s the so-called “coronal heating problem”, where the corona of the Sun is so much hotter than the solar surface. Then there is the question of what drives the constant outpouring of solar material (aka. solar wind) to such high speeds.

As Eric Christian, a research scientist on the Parker Solar Probe mission at NASA Goddard, explained:

“Parker Solar Probe and Solar Orbiter employ different sorts of technology, but — as missions — they’ll be complementary. They’ll be taking pictures of the Sun’s corona at the same time, and they’ll be seeing some of the same structures — what’s happening at the poles of the Sun and what those same structures look like at the equator.”

Illustration of the Parker Solar Probe spacecraft approaching the Sun. Credits: Johns Hopkins University Applied Physics Laboratory

For its mission, the Parker Solar Probe will get closer to the Sun than any spacecraft in history – as close as 6 million km (3.8 million mi) from the surface. This will replace the previous record of 43.432 million km (~27 million mi), which was established by the Helios B probe in 1976. From this position, the Parker Solar Probe will use its four suites of scientific instruments to image the solar wind and study the Sun’s magnetic fields, plasma and energetic particles.

In so doing, the probe will help clarify the true anatomy of the Sun’s outer atmosphere, which will help us to understand why the corona is hotter than the Sun’s surface. Basically, while temperatures in the corona can reach as high as a few million degrees, the solar surface (aka. photosphere), experiences temperatures of around 5538 °C (10,000 °F).

Meanwhile, the Solar Orbiter will come to a distance of about 42 million km (26 million mi) from the Sun, and will assume a highly-tilted orbit that can provide the first-ever direct images of the Sun’s poles. This is another area of the Sun that scientists don’t yet understand very well, and the study of it could provide valuable clues as to what drives the Sun’s constant activity and eruptions.

Both missions will also study solar wind, which is the Sun’s most pervasive influence on the solar system. This steam of magnetized gas fills the inner Solar System, interacting with magnetic fields, atmospheres and even the surfaces of planets. Here on Earth, it is what is responsible for the Aurora Borealis and Australis, and can also play havoc with satellites and electrical systems at times.

Artist’s impression of a solar flare erupting from the Sun’s surface. Credit: NASA Goddard Space Flight Center

Previous missions have led scientists to believe that the corona contributes to the process that accelerates solar wind to such high speeds. As these charged particles leave the Sun and pass through the corona, their speed effectively triples. By the time the solar wind reaches the spacecraft responsible for measuring it – 148 million km (92 million mi) from the Sun – it has plenty of time to mix with other particles from space and lose some of its defining features.

By being parked so close to the Sun, the Parker Solar Probe will able to measure the solar wind just as it forms and leaves the corona, thus providing the most accurate measurements of solar wind ever recorded. From its perspective above the Sun’s poles, the Solar Orbiter will complement the Parker Solar Probe’s study of the solar wind by seeing how the structure and behavior of solar wind varies at different latitudes.

This unique orbit will also allow the Solar Orbiter to study the Sun’s magnetic fields, since some of the Sun’s most interesting magnetic activity is concentrated at the poles. This magnetic field is far-reaching largely because of solar wind, which reaches outwards to create a magnetic bubble known as the heliosphere. Within the heliosphere, solar wind has a profound effect on planetary atmospheres and its presence protects the inner planets from galactic radiation.

In spite of this, it is still not entirely clear how the Sun’s magnetic field is generated or structured deep inside the Sun. But given its position, the Solar Orbiter will be able to study phenomena that could lead to a better understanding of how the Sun’s magnetic field is generated. These include solar flares and coronal mass ejections, which are due to variability caused by the magnetic fields around the poles.

In this way, the Parker Solar Probe and Solar Orbiter are complimentary missions, studying the Sun from different vantage points to help refine our knowledge of the Sun and heliosphere. In the process, they will provide valuable data that could help scientists to tackle long-standing questions about our Sun. This could help expand our knowledge of other star systems and perhaps even answer questions about the origins of life.

As Adam Szabo, a mission scientist for Parker Solar Probe at NASA Goddard, explained:

“There are questions that have been bugging us for a long time. We are trying to decipher what happens near the Sun, and the obvious solution is to just go there. We cannot wait — not just me, but the whole community.”

In time, and with the development of the necessary advanced materials, we might even be able to send probes into the Sun. But until that time, these missions represent the most ambitious and daring efforts to study the Sun to date. As with many other bold initiatives to study our Solar System, their arrival cannot come soon enough!

Further Reading: NASA

The Future of Our Sun is Still a Bit of a Puzzle. What Will Happen When it Dies?

The life cycle of our Sun began roughly 4.6 billion years ago. In roughly 4.5 to 5.5 billion years, when it depletes its supply of hydrogen and helium, it will enter into its Red Giant Branch (RGB) phase, where it will expand to several times its current size and maybe even consume Earth! And then, when it has reached the end of its life-cycle, it is believed that it will blow off its outer layers and become a white dwarf.

Until recently, astronomers were not certain how this would take place and whether or not our Sun would end up as a planetary nebula (as most other stars in our Universe do). But thanks to a new study by an international team of astronomers, it is now understood that our Sun will end its life-cycle by turning into a massive ring of luminous interstellar gas and dust – known as a planetary nebula.

Their study, titled “The mysterious age invariance of the cut-off the Planetary Nebula Luminosity Function“, was recently published in the scientific journal Nature. The study was led by Krzysztof Gesicki, an astrophysicist from Nicolaus Copernicus University, Poland; and included Albert Zijlstra and M Miller Bertolami – a professor from the University of Manchester and an astronomer the Instituto de Astrofísica de La Plata (IALP), Argentina, respectively.

The life cycle of a Sun-like star, from its birth (left side) to its evolution into a red giant (right side) after billions of years. Credit: ESO/M. Kornmesser

Roughly 90% of all stars end up as a planetary nebula, which traces the transition they go through between being a red giant and a white dwarf. However, scientists were previously unsure if our Sun would follow this same path, as it was thought to not be massive enough to create a visible planetary nebula. To determine if this would be the case, the team developed a new stellar, data-model that predicts the lifecycle of stars.

This model – which they refer to as the Planetary Nebula Luminosity Function (PNLF) -was used to predict the brightness of the ejected envelope for stars of different masses and ages. What they found was that our Sun was just massive enough to end up as a faint nebula. As Prof. Zijlstra explained in a Manchester University press release:

“When a star dies it ejects a mass of gas and dust – known as its envelope – into space. The envelope can be as much as half the star’s mass. This reveals the star’s core, which by this point in the star’s life is running out of fuel, eventually turning off and before finally dying. It is only then the hot core makes the ejected envelope shine brightly for around 10,000 years – a brief period in astronomy. This is what makes the planetary nebula visible. Some are so bright that they can be seen from extremely large distances measuring tens of millions of light years, where the star itself would have been much too faint to see.”

This model also addressed an enduring mystery in astronomy, which is why the brightest nebulae in distant galaxies all appear to have the same luminosity. Roughly 25 years ago, astronomers began to observe this, and found that they could gauge the distance to other galaxies (in theory) by examining their brightest planetary nebulae. However, the model created by Gesicki and his colleagues contradicted this theory.

Four different planetary nebulae from our galaxy. Credit: NASA/Chandra Observatory

In short, the luminosity of a planetary nebula does not come down to the mass of the star creating it, as was previously assumed. “Old, low mass stars should make much fainter planetary nebulae than young, more massive stars,” said Prof. Zijlstra. “This has become a source of conflict for the past for 25 years. The data said you could get bright planetary nebulae from low mass stars like the Sun, the models said that was not possible, anything less than about twice the mass of the sun would give a planetary nebula too faint to see.”

Essentially, the new models demonstrated that after a star ejects its envelope, it will heat up three times faster than what older models indicated – which makes it much easier for low mass stars to form a bright planetary nebula. The new models also indicated that the Sun is almost exactly at the lower cut off for low mass stars that will still produce a visible, though faint, planetary nebula. Anything smaller, Prof. Zijlstra added, will not produce a nebula:

“We found that stars with mass less than 1.1 times the mass of the sun produce fainter nebula, and stars more massive than 3 solar masses brighter nebulae, but for the rest the predicted brightness is very close to what had been observed. Problem solved, after 25 years!”

In the end, this study and the model the team produced has some truly beneficial implications for astronomers. Not only have they indicated with scientific confidence what will happen to our Sun when it dies (for the first time), they have also provided a powerful diagnostic tool for determining the history of star formation for intermediate-age stars (a few billion years old) in distant galaxies.

It’s also good to know that when our Sun does reach the end of lifespan, billions of years from now, whatever progeny we leave behind will be able to appreciate it – even if they are looking across the vast distances of space.

Further Reading: University of Manchester, Nature

Flying Into the Sun? NASA’s Parker Solar Probe Mission

Into The Sun!


If you’ve read enough of our articles, you know I’ve got an uneasy alliance with the Sun. Sure, it provides the energy we need for all life on Earth. But, it’s a great big ongoing thermonuclear reaction, and it’s right there! As soon as we get fusion, Sun, in like, 30 years or so, I tell you, we’ll be the ones laughing.

But to be honest, we still have so many questions about the Sun. For starters, we don’t fully understand the solar wind blasting out of the Sun. This constant wind of charged particles is constantly blowing out into space, but sometimes it’s stronger, and sometimes it’s weaker.

What are the factors that contribute to the solar wind? And as you know, these charged particles are not healthy for the human body, or for our precious electronics. In fact, the Sun occasionally releases enormous blasts that can damage our satellites and electrical grids.

How can we predict the intensity so that we can be better prepared for dangerous solar storms? Especially the Carrington-class events that might take down huge portions of our modern society.

Perhaps the biggest mystery with the Sun is the temperature of its corona. The surface of the Sun is hot, like 5,500 degrees Celsius. But if you rise up into the atmosphere of the Sun, into its corona, the temperature jumps beyond a million degrees.

The list of mysteries is long. And to start understanding what’s going on, we’ll need to get much much closer to the Sun.

Good news, NASA has a new mission in the works to do just that.

The Parker Solar Probe logo. Credit: NASA/JHUAPL

The mission is called the Parker Solar Probe. Actually, last week, it was called the Solar Probe Plus, but then NASA renamed it, and that reminded me to do a video on it.

It’s pretty normal for NASA to rename their spacecraft, usually after a dead astronomer/space scientist, like Kepler, Chandra, etc. This time, though, they renamed it for a legendary solar astronomer Eugene Parker, who developed much of our modern thinking on the Sun’s solar wind. Parker just turned 90 and this is the first time NASA has named it after someone living.

Anyway, back to the spacecraft.

The mission is due to launch in early August 2018 on a Delta IV Heavy, so we’re still more than a year away at this point. When it does, it’ll carry the spacecraft on a very unusual trajectory through the inner Solar System.

The problem is that the Sun is actually a very difficult place to reach. In fact, it’s the hardest place to get to in the entire Solar System.

Remember that the Earth is traveling around the Sun at a velocity of 30 km/s. That’s almost three times the velocity it takes to get into orbit. That’s a lot of velocity.

In order to be able to get anywhere near the Sun, the probe needs to shed velocity. And in order to do this, it’s going to use gravitational slingshots with Venus. We’ve talked about gravitational slingshots in the past, and how you can use them to speed up a spacecraft, but you can actually do the reverse.

The Parker Solar Probe will fall down into Venus’ gravity well, and give orbital velocity to Venus. This will put it on a new trajectory which takes it closer to the Sun. It’ll do a total of 7 flybys in 7 years, each of which will tweak its trajectory and shed some of that orbital momentum.

Parker Solar Probe's trajectory including Venus flybys. Credit: NASA/JHUAPL
Parker Solar Probe’s trajectory including Venus flybys. Credit: NASA/JHUAPL

You know, trying to explain orbital maneuvering is tough. I highly recommend that you try out Kerbal Space Program. I’ve learned more about orbital mechanics by playing that game for a few months than I have in almost 2 decades of space journalism. Go ahead, try to get to the Sun, I challenge you.

Anyway, with each Venus flyby, the Parker Solar Probe will get closer and closer to the Sun, well within the orbit of Mercury. Far closer than any spacecraft has ever gotten to the Sun. At its closest point, it’ll only be 5.9 million kilometers from the Sun. Just for comparison, the Earth orbits at an average distance of about 150 million kilometers. That’s close.

And over the course of its entire mission, the spacecraft is expected to make a total of 24 complete orbits of the Sun, analyzing that plasma ball from every angle.

The orbit is also highly elliptical, which means that it’s going really really fast at its closest point. Almost 725,000 km/h.

In order to withstand the intense temperatures of being this close to the Sun, NASA has engineered the Parker Solar Probe to shed heat. It’s equipped with an 11.5 cm-thick shield made of carbon-composite. For that short time it spends really close to the Sun, the spacecraft will keep the shield up, blocking that heat from reaching the rest of its instruments.

And it’s going to get hot. We’re talking about more than 1,300 degrees Celsius, which is about 475 times as much energy as a spacecraft receives here on Earth. In the outer Solar System, the problem is that there just isn’t enough energy to power solar panels. But where Parker is going, there’s just too much energy.

Now we’ve talked about the engineering difficulties of getting a spacecraft this close to the Sun, let’s talk about the science.

Coronal holes are regions in the sun’s atmosphere or corona where solar plasma can stream directly into space. Often a hole will a couple rotations, inciting repeat auroras approximately every 4 weeks. Credit: NASA

The biggest question astronomers are looking to solve is, how does the corona get so hot. The surface is 5,500 Celsius. As you get farther away from the Sun, you’d expect the temperature to go down. And it certainly does once you get as far as the orbit of the Earth.

But the Sun’s corona, or its outer atmosphere, extends millions of kilometers into space. You can see it during a solar eclipse as this faint glow around the Sun. Instead of dropping, the temperature rises to more than a million degrees.

What could be causing this? There are a couple of ideas. Plasma waves pushed off the Sun could bunch up and release their heat into the corona. You could also get the crisscrossing of magnetic field lines that create mini-flares within the corona, heating it up.

The second great mystery is the solar wind, the stream of charged protons and electrons coming from the Sun. Instead of a constant blowing wind, it can go faster or slower. And when the speed changes, the contents of the wind change too.

There’s the slow wind, that goes a mere 1.1 million km/h and seems to emanate from the Sun’s equatorial regions. And then the fast wind, which seems to be coming out of coronal holes, cooler parts in the Sun’s corona, and can be going at 2.7 million km/h.

Why does the solar wind speed change? Why does its consistency change?

Parker Solar Probe's instruments. Credit: NASA/JHUAPL
Parker Solar Probe’s instruments. Credit: NASA/JHUAPL

The Parker Solar Probe is equipped with four major instruments, each of which will gather data from the Sun and its environment.

The FIELDS experiment will measure the electric and magnetic fields and waves around the Sun. We know that much of the Sun’s behavior is driven by the complex interaction between charged plasma in the Sun. In fact, many physicists agree that magnetohydrodynamics is easily one of the most complicated fields you can get into.

Integrated Science Investigation of the Sun, or ISOIS (which I suspect needs a renaming) will measure the charged particles streaming off the Sun, during regular solar activity and during dangerous solar storms. Can we get any warning before these events occur, giving astronauts more time to protect themselves?

Wide-field Imager for Solar PRobe or WISPR is its telescope and camera. It’s going to be taking close up, high resolution images of the Sun and its corona that will blow our collective minds… I hope. I mean, if it’s just a bunch of interesting data and no pretty pictures, it’s going to be hard to make cool videos showcasing the results of the mission. You hear me NASA, we want pictures and videos. And science, sure.

And then the Solar Wind Electrons Alphas and Protons Investigation, or SWEAP, will measure type, velocity, temperature and density of particles around the Sun, to help us understand the environment around it.

One interesting side note, the spacecraft will be carrying a tiny chip on board with photos of Eugene Parker and a copy of his original 1958 paper explaining the Sun’s solar wind.

The Parker Solar Probe orbiting the Sun. Credit: NASA/JHUAPL
The Parker Solar Probe orbiting the Sun. Credit: NASA/JHUAPL

I know we’re still more than a year away from liftoff, and several years away before the science data starts pouring in. But you’ll be hearing more and more about this mission shortly, and I’m pretty excited about what it’s going to accomplish. So stay tuned, and once the science comes in, I’m sure you’ll hear plenty more about it.

How Far Away is Fusion? Unlocking the Power of the Sun

Best Energy?


I’d like to think we’re smarter than the Sun.

Let’s compare and contrast. Humans, on the one hand, have made enormous advances in science and technology, built cities, cars, computers, and phones. We have split the atom for war and for energy.

What has the Sun done? It’s a massive ball of plasma, made up of mostly hydrogen and helium. It just, kind of, sits there. Every now and then it burps up hydrogen gas into a coronal mass ejection. It’s not a stretch to say that the Sun, and all inanimate material in the Universe, isn’t the sharpest knife in the drawer.

And yet, the Sun has mastered a form of energy that we just can’t seem to wrap our minds around: fusion. It’s really infuriating, seeing the Sun, just sitting there, effortlessly doing something our finest minds have struggled with for half a century.

Why can’t we make fusion work? How long until we can finally catch up technologically with a sphere of ionized gas?

Our Sun in all its intense, energetic glory. When life appeared on Earth, the Sun would have been much different than it is now; a more intense, energetic neighbor. Image: NASA/SDO.
Our Sun in all its intense, energetic glory. Credit: NASA/SDO.

The trick to the Sun’s ability to generate power through nuclear fusion, of course, comes from its enormous mass. The Sun contains 1.989 x 10^30 kilograms of mostly hydrogen and helium, and this mass pushes inward, creating a core heated to 15 million degrees C, with 150 times the density of water.

It’s at this core that the Sun does its work, mashing atoms of hydrogen into helium. This process of fusion is an exothermic reaction, which means that every time a new atom of helium is created, photons in the form of gamma radiation are also released.

The only thing the Sun uses this energy for is light pressure, to counteract the gravity pulling everything inward. Its photons slowly make their way up through the Sun and then they’re released into space. So wasteful.

How can we replicate this on Earth?

Now gathering together a Sun’s mass of hydrogen here on Earth is one option, but it’s really impractical. Where would we put all that hydrogen. The better solution will be to use our technology to simulate the conditions at the core of the Sun.

If we can make a fusion reactor where the temperatures and pressures are high enough for atoms of hydrogen to merge into helium, we can harness those sweet sweet photons of gamma radiation.

Tokamak
Inside a Tokamak. Credit: Princeton Plasma Physics Laboratory

The main technology developed to do this is called a tokamak reactor; it’s a based on a Russian acronym for: “toroidal chamber with magnetic coils”, and the first prototypes were created in the 1960s. There are many different reactors in development, but the method is essentially the same.

A vacuum chamber is filled with hydrogen fuel. Then an enormous amount of electricity is run through the chamber, heating up the hydrogen into a plasma state. They might also use lasers and other methods to get the plasma up to 150 to 300 million degrees Celsius (10 to 20 times hotter than the Sun’s core).

Superconducting magnets surround the fusion chamber, containing the plasma and keeping it away from the chamber walls, which would melt otherwise.

Once the temperatures and pressures are high enough, atoms of hydrogen are crushed together into helium just like in the Sun. This releases photons which heat up the plasma, keeping the reaction going without any addition energy input.

Excess heat reaches the chamber walls, and can be extracted to do work.

The spherical tokamak MAST at the Culham Centre for Fusion Energy (UK). Photo: CCFE

The challenge has always been that heating up the chamber and constraining the plasma uses up more energy than gets produced in the reactor. We can make fusion work, we just haven’t been able to extract surplus energy from the system… yet.

Compared to other forms of energy production, fusion should be clean and safe. The fuel source is water, and the byproduct is helium (which the world is actually starting to run out of). If there’s a problem with the reactor, it would cool down and the fusion reaction would stop.

The high energy photons released in the fusion reaction will be a problem, however. They’ll stream into the surrounding fusion reactor and make the whole thing radioactive. The fusion chamber will be deadly for about 50 years, but its rapid half-life will make it as radioactive as coal ash after 500 years.

External view of Princeton’s Tokamak Fusion Test Reactor which operated from 1982 to 1997. Credit: Princeton Plasma Physics Laboratory (CC BY 3.0)

Now you know what fusion power is and how it works, what’s the current state, and how long until fusion plants give us unlimited cheap safe power, if ever?

Fusion experiments are measured by the amount of energy they produce compared to the amount of energy you put into them. For example, if a fusion plant required 100MW of electrical energy to produce 10 MW of output, it would have an energy ratio of 0.1. You want at least a ratio of 1. That means energy in equals energy out, and so far, no experiment has ever reached that ratio. But we’re close.

The EAST facility’s tokamak reactor, part of the Institute of Physical Science in Hefei. Credit: ipp.cas.cn

The Chinese are building the Experimental Advanced Superconducting Tokamak, or EAST. In 2016, engineers reported that they had run the facility for 102 seconds, achieving temperatures of 50 million C. If true, this is an enormous advancement, and puts China ahead in the race to create stable fusion. That said, this hasn’t been independently verified, and they only published a single scientific paper on the milestone.

Karlsruhe Institute of Technology’s Wendelstein 7-X (W7X) stellarator. Credit: Max-Planck-Institut für Plasmaphysik, Tino Schulz (CC BY-SA 3.0)

Researchers at the Karlsruhe Institute of Technology (KIT) in Germany recently announced that their Wendelstein 7-X (W7X) stellarator (I love that name), heated hydrogen gas to 80 million C for only a quarter of a second. Hot but short. A stellarator works differently than a tokamak. It uses twisted rings and external magnets to confine the plasma, so it’s good to know we have more options.

The biggest, most elaborate fusion experiment going on in the world right now is in Europe, at the French research center of Cadarache. It’s called ITER, which stands for the International Thermonuclear Experimental Reactor, and it hopes to cross that magic ratio.

The ITER Tokamak Fusion Reactor. Credits: ITER, Illus. T.Reyes

ITER is enormous, measuring 30 meters across and high. And its fusion chamber is so large that it should be able to create a self-sustaining fusion reaction. The energy released by the fusing hydrogen keeps the fuel hot enough to keep reacting. There will still be energy required to run the electric magnets that contain the plasma, but not to keep the plasma hot.

And if all goes well, ITER will have a ratio of 10. In other words, for every 10 MW of energy pumped in, it’ll generate 100 MW of usable power.

ITER is still under construction, and as of June 2015, the total construction costs had reached $14 billion. The facility is expected to be complete by 2021, and the first fusion tests will begin in 2025.

So, if ITER works as planned, we are now about 8 years away from positive energy output from fusion. Of course, ITER will just be an experiment, not an actual powerplant, so if it even works, an actual fusion-based energy grid will be decades after that.

At this point, I’d say we’re about a decade away from someone demonstrating that a self-sustaining fusion reaction that generates more power than it consumes is feasible. And then probably another 2 decades away from them supplying electricity to the power grid. By that point, our smug Sun will need to find a new job.

What Constellation is the Sun in?

Since ancient times, astronomers have organized the stars into various constellations. We have the Big Dipper (Ursa Major), Orion the Hunter, and his “Greater Dog” and “Lesser Dog”(Canis Major and Canis Minor). And those are just some of the better-known ones. But have you ever wondered if the Sun belongs to one of these collections of stars?

The simple answer is that – in accordance with both ancient astrological tradition and modern astronomy – the Sun technically has no constellation. But if you were to change locations and travel to a new star system, you would then be able to view the Sun as we do other distant collection of stars. Unfortunately, depending on where you are, the answer would change.

The Zodiac:

First, let us consider the astrological answer to this question. Unless you were born prior to the Scientific Revolution – during which time Nicolaus Copernicus proposed the heliocentric model of the Solar System – you know that the Earth revolves around the Sun. Over the course of a year, the position of the stars changes as the Earth’s position relative to the Sun changes.

A chart of the constellations and signs that make up the zodiac. Credit: NASA

During the year, the Sun passes through each of the constellations of the Zodiac. For example, in August, the Sun is in Leo, and then in September, the Sun is in Virgo. Your astrological sign is based on this. What this means is that the Sun is part of each constellation of the Zodiac over the course of a single year, so it can’t be said to be in any single constellation.

However, astrology is an obsolete and entirely unscientific practice. And if someone were to ask which constellation the Sun is in, surely they are seeking an answer that was astronomical (and not astrological) in nature. For that, we must consider what the constellations are in scientific terms.

The 88 Constellations:

Since ancient times, astronomers and scholars have been keeping track of “asterisms” (aka. constellations) in the night sky. By definition, these are collections of stars that, when viewed from Earth, appear in the same general area as each other night after night. In reality, they are actually located in very different locations, and can sometimes be up to thousands of light-years away from each other.

During the 2nd century CE, Hellenistic astronomer Claudius Ptolemaeus (Ptolemy) organized the constellations into a single treatise. This treatise, known as the Almagest, was the definitive source on Greek astronomy, and contained the names and meanings of the then-known 48 constellations. For over a thousand years, this work would remain canon for European and Islamic Astronomers.

The modern constellations. color-coded by family, with a dotted line denoting the ecliptic. Credit: NASA/Scientific Visualization Studio

Thanks to the Scientific Revolution and “Age of Exploration” – ca. 15th to 18th centuries CE – astronomers became aware of many more constellations. This was due to extensive overseas exploration, which brought European traders, explorers and waves of colonization to the Southern Hemisphere, East Asia and the Americas.

By 1922, the International Astronomical Union (IAU) officially divided the celestial sphere into 88 constellations. Of these, 36 lie predominantly in the northern sky while the other 52 lie predominantly in the southern. While it would take years to work out the exact delineation between these constellations, and many corresponded to their Greco-Roman predecessors, these 88 modern constellations would remain in use until this day.

However, these constellations divide up the night sky based on how it is viewed from Earth. Once again, our Sun cannot be considered to lie in any one of them because – relative to the Earth-bound observer – it passes through them. Alas, the only way to answer this question is to change our perspective.

From Other Star Systems:

If you could move away to another star, then our Sun would indeed appear to be part of the background stars. For example, if you were to travel to a planet orbiting the nearest star to the Solar System – Alpha Centauri (aka. Rigil Kentaurus) – then the Sun would indeed appear to be part of a constellation.

Artist’s impression of the Earth-like exoplanet orbiting Alpha Centauri B Credit: ESO

To be scientifically accurate, let us consider a planet that we actually know of. This would be the rocky extrasolar planet recently discovered around Proxima Centauri, which is known as Proxima b. Viewed from the surface of this planet, the Sun would appear to be part of the Cassiopeia constellation. However, rather than forming a W shape, our Sun would form a sixth point on its “western” end, making it look like a mountain chain (or a scribbled line).

But if you went to a different star system, the Sun’s position would change, depending on the direction. As such, the Sun really isn’t in any constellation per se. But then again, none of the other stars that make up the Milky Way are either. Much like what Einstein’s Theory of Relativity teaches us about space and time, the constellations themselves are relative to the observer.

We have written many interesting articles about the Sun and the constellations here at Universe Today. Here’s What are the Constellations?, Zodiac Signs and their Dates?, Where is the Sun?, and Earth’s Orbit Around the Sun.

For more information on how our Sun looks from Alpha Centauri, be sure to check out this page from Learn Astronomy. SAnd here’s an article about all 88 recognized constellations.

Astronomy Cast also has episodes on the subject. Here’s Episode 30: The Sun, Spots and All and Episode 157: Constellations.

Sources:

Solar Probe Plus Will ‘Touch’ The Sun

Coronal Mass Ejections (aka. solar flares) are a seriously hazardous thing. Whenever the Sun emits a burst of these charged particles, it can play havoc with electrical systems, aircraft and satellites here on Earth. Worse yet is the harm it can inflict on astronauts stationed aboard the ISS, who do not have the protection of Earth’s atmosphere. As such, it is obvious why scientists want to be able to predict these events better.

For this reason, the Smithsonian Astrophysical Observatory and the Charles Stark Draper Laboratory – a Cambridge, Massachusetts-based non-profit engineering organization – are working to develop specialized sensors for NASA’s proposed solar spacecraft. Launching in 2018, this spacecraft will fly into the Sun atmosphere and “touch” the face of the Sun to learn more about its behavior.

This spacecraft – known as the Solar Probe Plus (SPP) – is currently being designed and built by the Johns Hopkins University Applied Physics Laboratory. Once it is launched, the SPP will use seven Venus flybys over nearly seven years to gradually shrink its orbit around the Sun. During this time, it will conduct 24 flybys of the Sun and pass into the Sun’s upper atmosphere (corona), passing within 6.4 million km (4 million mi) of its surface.

At this distance, it will have traveled 37.6 million km (23.36 million mi) closer to the Sun than any spacecraft in history. At the same time, it will set a new record for the fastest moving object ever built by human beings – traveling at speeds of up to 200 km/sec (124.27 mi/s). And last but not least, it will be exposed to heat and radiation that no spacecraft has ever faced, which will include temperatures in excess of 1371 °C (2500 °F).

As Seamus Tuohy, the Director of the Space Systems Program Office at Draper, said in a CfA press release:

“Such a mission would require a spacecraft and instrumentation capable of withstanding extremes of radiation, high velocity travel and the harsh solar condition—and that is the kind of program deeply familiar to Draper and the Smithsonian Astrophysical Observatory.”

In addition to being an historic first, this probe will provide new data on solar activity and help scientists develop ways of forecasting major space-weather events – which impact life on Earth. This is especially important in an age when people are increasingly reliant on technology that can be negatively impacted by solar flares – ranging from aircraft and satellites to appliances and electrical devices.

According to a recent study by the National Academy of Sciences, it is estimated that a huge solar event today could cause two trillion dollars in damage in the US alone – and places like the eastern seaboard would be without power for up to a year. Without electricity to provide heating, utilities, light, and air-conditioning, the death toll from such an event would be significant.

As such, developing advanced warning systems that could reliably predict when a coronal mass ejection is coming is not just a matter of preventing damage, but saving lives. As Justin C. Kasper, the principal investigator at the Smithsonian Astrophysical Observatory and a professor in space science at the University of Michigan, said:

“[I]n addition to answering fundamental science questions, the intent is to better understand the risks space weather poses to the modern communication, aviation and energy systems we all rely on. Many of the systems we in the modern world rely on—our telecommunications, GPS, satellites and power grids—could be disrupted for an extended period of time if a large solar storm were to happen today. Solar Probe Plus will help us predict and manage the impact of space weather on society.”

To this end, the SPP has three major scientific objectives. First, it will seek to trace the flow of energy that heats and accelerates the solar corona and solar wind. Second, its investigators will attempt to determine the structure and dynamics of plasma and magnetic fields as the source of solar wind. And last, it will explore the mechanisms that accelerate and transport energetic particles – specifically electrons, protons, and helium ions.

To do this, the SPP will be equipped with an advanced suite of instruments. One of the most important of these is the one built by the Smithsonian Astrophysical Observatory with technical support from Draper. Known as the Faraday Cup – and named after famous electromagnetic scientists Michael Faraday – this device will be operated by SAO and the University of Michigan in Ann Arbor.

Designed to withstand interference from electromagnetic radiation, the Farady Cup will measure the velocity and direction of the Sun’s charged particles, and will be only two positioned outside of the SPP’s protective sun shield – another crucial component. Measuring 11.43 cm (4.5 inches) thick, this carbon composition shield will ensure that the probe can withstand the extreme conditions as it conducts its many flybys through the Sun’s corona.

Naturally, the mission presents several challenges, not the least of which will be capturing data while operating within an extreme environment, and while traveling at extreme speeds. But the payoff is sure to be worth it. For years, astronomers have studied the Sun, but never from inside the Sun’s atmosphere.

By flying through the birthplace of the highest-energy solar particles, the SPP is set to advance our understanding of the Sun and the origin and evolution of the solar wind. This knowledge could not only help us avoid a natural catastrophe here on Earth, but help advance our long-term goal of exploring (and even colonizing) the Solar System.

Further Reading: CfA

What is an Astronomical Unit?

Apsis

When it comes to dealing with the cosmos, we humans like to couch things in familiar terms. When examining exoplanets, we classify them based on their similarities to the planets in our own Solar System – i.e. terrestrial, gas giant, Earth-size, Jupiter-sized, Neptune-sized, etc. And when measuring astronomical distances, we do much the same.

For instance, one of the most commonly used means of measuring distances across space is known as an Astronomical Unit (AU). Based on the distance between the Earth and the Sun, this unit allows astronomers to characterize the vast distances between the Solar planets and the Sun, and between extra-solar planets and their stars.

Definition:

According to the current astronomical convention, a single Astronomical Unit is equivalent to 149,597,870.7 kilometers (or 92,955,807 miles). However, this is the average distance between the Earth and the Sun, as that distance is subject to variation during Earth’s orbital period. In other words, the distance between the Earth and the Sun varies in the course of a single year.

Earth’s orbit around the Sun, showing its average distance (or 1 AU). Credit: Huritisho/Wikipedia Commons

During the course of a year, the Earth goes from distance of 147,095,000 km (91,401,000 mi) from the Sun at perihelion (its closest point) to 152,100,000 km (94,500,000 mi) at aphelion (its farthest point) – or from a distance of 0.983 AUs to 1.016 AUs.

History of Development:

The earliest recorded example of astronomers estimating the distance between the Earth and the Sun dates back to Classical Antiquity. In the 3rd century BCE work, On the Sizes and Distances of the Sun and Moon – which is attributed to Greek mathematician Aristarchus of Samos – the distance was estimated to be between 18 and 20 times the distance between the Earth and the Moon.

However, his contemporary Archimedes, in his 3rd century BCE work Sandreckoner, also claimed that Aristarchus of Samos placed the distance of 10,000 times the Earth’s radius. Depending on the values for either set of estimates, Aristarchus was off by a factor of about 2 (in the case of Earth’s radius) to 20 (the distance between the Earth and the Moon).

The oldest Chinese mathematical text – the 1st century BCE treatise known as Zhoubi Suanjing – also contains an estimate of the distance between the Earth and Sun. According to the anonymous treatise, the distance could be calculated by conducting geometric measurements of the length of noontime shadows created by objects spaced at specific distances. However, the calculations were based on the idea that the Earth was flat.

Illustration of the Ptolemaic geocentric conception of the Universe, by Bartolomeu Velho (?-1568), from his work Cosmographia, made in France, 1568. Credit: Bibilotèque nationale de France, Paris

Famed 2nd century CE mathematician and astronomer Ptolemy relied on trigonometric calculations to come up with a distance estimate that was equivalent to 1210 times the radius of the Earth. Using records of lunar eclipses, he estimated the Moon’s apparent diameter, as well as the apparent diameter of the shadow cone of Earth traversed by the Moon during a lunar eclipse.

Using the Moon’s parallax, he also calculated the apparent sizes of the Sun and the Moon and concluded that the diameter of the Sun was equal to the diameter of the Moon when the latter was at it’s greatest distance from Earth. From this, Ptolemy arrived at a ratio of solar to lunar distance of approximately 19 to 1, the same figure derived by Aristarchus.

For the next thousand years, Ptolemy’s estimates of the Earth-Sun distance (much like most of his astronomical teachings) would remain canon among Medieval European and Islamic astronomers. It was not until the 17th century that astronomers began to reconsider and revise his calculations.

This was made possible thanks to the invention of the telescope, as well as Kepler’s Three Laws of Planetary Motion, which helped astronomers calculate the relative distances between the planets and the Sun with greater accuracy. By measuring the distance between Earth and the other Solar planets, astronomers were able to conduct parallax measurements to obtain more accurate values.

With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

By the 19th century, determinations of about the speed of light and the constant of the aberration of light resulted in the first direct measurement of the Earth-Sun distance in kilometers.  By 1903, the term “astronomical unit” came to be used for the first time. And throughout the 20th century, measurements became increasingly precise and sophisticated, thanks in part to accurate observations of the effects of Einstein’s Theory of Relativity.

Modern Usage:

By the 1960s, the development of direct radar measurements, telemetry, and the exploration of the Solar System with space probes led to precise measurements of the positions of the inner planets and other objects. In 1976, the International Astronomical Union (IAU) adopted a new definition during their 16th General Assembly. As part of their System of Astronomical Constants, the new definition stated:

“The astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time. The dimensions of k² are those of the constant of gravitation (G), i.e., L³M-1T2. The term “unit distance” is also used for the length A.”

In response to the development of hyper-precise measurements, the International Committee for Weights and Measures (CIPM) decided to modify the the International System of Units (SI) in 1983. Consistent with this, they redefined the meter to be measured in terms of the speed of light in vacuum.

Infographic comparing the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO

However, by 2012, the IAU determined that the equalization of relativity made the measurement of AUs too complex, and redefined the astronomical unit in terms of meters. In accordance with this, a single AU is equal to 149597870.7 km exactly (92.955807 million miles), 499 light-seconds, 4.8481368×10-6 of a parsec, or 15.812507×10-6 of a light-year.

Today, the AU is used commonly to measure distances and create numerical models for the Solar System. It is also used when measuring extra-solar systems, calculating the extent of protoplanetary clouds or the distance between extra-solar planets and their parent star. When measuring interstellar distances, AUs are too small to offer convenient measurements. As such, other units – such as the parsec and the light year – are relied upon.

The Universe is a huge place, and measuring even our small corner of it producing some staggering results. But as always, we prefer to express them in ways that are as relatable and familiar.

We’ve written many interesting articles about distances in the Solar System here at Universe Today. Here’s How Far are the Planets from the Sun?, How Far is Mercury from the Sun?, How Far is Venus from the Sun?, How Far is Earth from the Sun?, How Far is Mars from the Sun?, How Far is Jupiter from the Sun?, How Far is Saturn from the Sun?, How Far is Uranus from the Sun?, How Far is Neptune from the Sun?, How Far is Pluto from the Sun?

If you’d like more information about the Earth’s orbit, check out NASA’s Solar System Exploration page.

We’ve also recorded an episode of Astronomy Cast dedicated to the measurement of distances in astronomy. Listen here, Episode 10: Measuring Distance in the Universe.

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