This week, we return to our starting point, where Astronomy Cast began: Pluto. 11 years on, we have a whole new appreciate for the dwarf planet Pluto. We’ve visited it, probed it and taken pictures. It’s time for an update.
Since it made its historic flyby of Pluto in July of 2015, the New Horizons mission has been venturing farther into the outer Solar System. With the spacecraft still healthy and its system in working order, the mission was extended to include the exploration of additional Kuiper Belt Objects (KBOs). The first target for this part of its mission is the KBO known as 2014 MU69, which New Horizons is currently making its way towards.
In the past, NASA believed this object was a spherical chunk of ice and rock measuring 18–41 km (10–30 mi) in diameter. However, a more recent occultation observation has led the New Horizon‘s team to conclude that MU69 may actually be a large object with a chunk taken out of it (an “extreme prolate spheroid”) or two objects orbiting very closely together or touching – aka. a close or contact binary.
In 2015, MU69 was identified as one of two potential destinations for New Horizons and was recommended to NASA by the mission science team. It was selected because of the immense opportunities for research it presented. As Alan Stern, the Principle Investigator (PI) for the New Horizons mission at the Southwest Research Institute (SwRI), indicated at the time:
“2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by. Moreover, this KBO costs less fuel to reach [than other candidate targets], leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen.”
The most recent observation of the KBO took place on July 17th, 2017, when the object passed in front of a star. This provided the New Horizon’s team with an opportunity to measure the resulting dip in the star’s luminosity – aka. an occultation – using a series of telescopes that they had deployed to a remote part of Patagonia, Argentina. These sorts of observations are performed regularly in order to obtain estimates of an asteroid’s size and position.
In the case of MU69’s occulation, the New Horizons team was able to obtain vital data that will help the mission planners to plot the trajectory of their flyby. In addition, the data revealed things about MU69’s size, shape, orbit, and the environment that surrounds it. It was because of this that the team began to question earlier estimates on the object’s size and shape.
Based on their new observations, they are confident that the object is no more than 30 km (20 mi) long, if it is an extreme prolate spheroid. If, however, it is a binary, the two objects that compose it are believed to measure about 15-20 km (9-12 mi) in diameter each. Alan Stern expanded on these new findings in a recent NASA press statement, saying:
“This new finding is simply spectacular. The shape of MU69 is truly provocative, and could mean another first for New Horizons going to a binary object in the Kuiper Belt. I could not be happier with the occultation results, which promise a scientific bonanza for the flyby.”
The recent stellar occulation was the third of three observations conducted for the New Horizons mission. To prepare for the event, the New Horizons team traveled to Argentina and South Africa on June 3rd. On July 10th, a week before the occultation, NASA’s airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) provided support by studying the space around MU69.
Using its 2.5 m (100-inch) telescope, SOFIA was looking for debris that might present a hazard to New Horizons spacecraft as it makes its flyby less than 17 months from now. Last, but certainly not least, the team also relied on data provided by NASA’s Hubble Space Telescope and the ESA’s Gaia satellite to calculate and pinpoint where MU69 would cast its shadow on Earth’s surface.
Thanks to their assistance, the New Horizons team knew exactly where the occultation shadow would be and set up their “fence line” of small, mobile telescopes accordingly. Marc Buie – the New Horizons co-investigator – was responsible for leading the observation campaign. As he explained, the data it yielded will be of great help in the planning the flyby, but also indicated that their could be some surprises in the future:
“These exciting and puzzling results have already been key for our mission planning,” he said, “but also add to the mysteries surrounding this target leading into the New Horizons encounter with MU69, now less than 17 months away.”
The flyby with MU69 is scheduled to take place on Jan. 1st, 2019, and will be the most distant flyby in the history of space exploration. In addition to being 1.6 billion km (1 billion mi) from Pluto, the New Horizons spacecraft will be 6.5 billion km (4 billion mi) from Earth! What’s more, the first-ever study of a KBO is expected to yield some fantastic scientific data, and tell us much about the formation and evolution of our Solar System.
On July 14th, 2015, the New Horizons mission made history when it became the first spacecraft to conduct a flyby of Pluto and its moons. In the course of making its way through this system, the probe gathered volumes of data on Pluto and its many satellites using a sophisticated suite of instruments. These included the first detailed images of what Pluto and its largest moon (Charon) look like up close.
And while scientists are still analyzing the volumes of data that the probe has sent home (and probably will be for years to come), the New Horizons mission team has given us plenty of discoveries to mull over in the meantime. For instance, using the many images taken by the mission, they recently created a series of high-quality, highly-detailed global maps of Pluto and Charon.
The maps were created thanks to the plethora of images that were taken by New Horizons’ Long-Range Reconnaissance Imager (LORRI) and its Multispectral Visible Imaging Camera (MVIC). Whereas LORRI is a telescopic camera that was responsible for obtaining encounter and high-resolution geologic data of Pluto at long distances, the MVIC is an optical and infrared instrument that is part of the Ralph instrument – the main imaging device of the probe.
Dr. Stern, who is also the PI of the New Horizons mission, commented on the release of the maps in a recent NASA press statement. As he stated, they are just the latest example of what the New Horizons mission accomplished during its historic mission:
“The complexity of the Pluto system — from its geology to its satellite system to its atmosphere— has been beyond our wildest imagination. Everywhere we turn are new mysteries. These new maps from the landmark exploration of Pluto by NASA’s New Horizons mission in 2015 will help unravel these mysteries and are for everyone to enjoy.”
And these were not the only treats to come from the New Horizons team in recent days. In addition, the mission scientists used actual New Horizons data and digital elevation models to create flyover movies that show what it would be like to pass over Pluto and Charon. These videos offer a new perspective on the system and showcase the many unusual features that were discovered on both bodies.
The video of the Pluto flyover (shown above) begins over the highlands that are located to the southwest of Sputnik Planitia – the nitrogen ice basin that measures some 1,050 by 800 km (650 by 500 mi) in size. These plains constitute the western lobe of the feature known as Tombaugh Regio, the heart-shaped region that is named after the man who discovered Pluto in 1930 – Clyde Tombaugh.
The flyover also passes by cratered terrain of Cthulhu Macula before moving north past the highlands of Voyager Terra. It then turns south towards the pitted region known as Pioneer Terra before concluding over Tartarus Dorsa, a mountainous region that also contains bowl-shaped ice and snow features called penitentes (which are found on Earth and are formed by erosion).
The flyover video of Charon begins over the hemisphere that the New Horizons mission saw during its closest approach to the moon. The view then descends over Serenity Chasma, the wide and deep canyon that is named after the ship from the sci-fi series Firefly. This feature is part of the vast equatorial belt of chasms on Charon, which is one of the longest in the Solar System – 1,800 km (1,100 mi) long 7.5 km (4.5 mi) deep.
The view then moves north, passing over the Dorothy Gale crater and the dark polar region known as Mordor Macula (appropriately named after the domain of the Dark Lord Sauron in The Lord of the Rings). The video then turn south to fly over the northern terrain known as Oz Terra before finishing over the equatorial plans of Vulcan Planum and the mountain of Clarke Montes.
These videos were color-enhanced in order to bring out the surface details, and the topographic relief was exaggerated by a factor or two to three to emphasize the topography of Pluto and its largest moon. Digital mapping and rendering of these videos was performed by Paul Schenk and John Blackwell of the Lunar and Planetary Institute (LPI) in Houston.
It may be many years before another mission is able to travel to the Trans-Neptunian region and Kuiper Belt. As a result, the maps and videos and images that were taken by the New Horizons mission may the last glimpse some us get of the Pluto system. Luckily, the New Horizons mission has provided scientists and the general public with enough information to keep them busy and fascinated for years!
It was two years ago this morning that we awoke to see the now iconic image of Pluto that the New Horizons spacecraft had sent to Earth during the night. You, of course, know the picture I’m talking about – the one with a clear view of the giant heart-shaped region on the distant, little world (see above).
This image was taken just 16 hours before the spacecraft would make its closest approach to Pluto. Then, during that seemingly brief flyby (after traveling nine-and-a-half years and 3 billion miles to get there), the spacecraft gathered as much data as possible and we’ve been swooning over the images and pondering the findings from New Horizons ever since.
“This is what we came for – these images, spectra and other data types that are helping us understand the origin and the evolution of the Pluto system for the first time,” New Horizons principal investigator Alan Stern told me last year. “We’re seeing that Pluto is a scientific wonderland. The images have been just magical. It’s breathtaking.”
See a stunning new video created from flbyby footage in honor of the two-year anniversary of the flyby:
All the images have shown us that Pluto is a complex world with incredible diversity, in its geology and also in its atmosphere.
While the iconic “heart” image shows a clear and cloudless view of Pluto, a later image showed incredible detail of Pluto’s hazy atmosphere, with over two dozen concentric layers that stretches more than 200 km high in Pluto’s sky.
With all those layers and all that haze, could there be clouds on Pluto too?
This is a question Stern and his fellow scientists have been asking for a long time, actually, as they have been studying Pluto for decades from afar. Now with data from New Horizons, they’ve been able to look closer. While Stern and his colleagues have been discussing how they found possible clouds on Pluto for a few months, they have now detailed their findings in a paper published last month.
“Numerous planets in our solar system, including Venus, Earth, Mars, Titan, and all four of the giant planets possess atmospheres that contain clouds, i.e., discrete atmospheric condensation structures,” the team wrote in their paper. “This said, it has long been known that Pluto’s current atmosphere is not extensively cloudy at optical or infrared wavelengths.”
They explained that evidence for this came primarily from the “high amplitude and temporal stability of Pluto’s lightcurve,” however, because no high spatial resolution imagery of Pluto was possible before New Horizons, it remained to be seen if clouds occur over a small fraction of Pluto’s surface area.
But now with flyby images in hand, the team set out to do searches for clouds on Pluto, looking at all available imagery from the Long Range Reconnaissance Imager and the Multispectral Visible Imaging Camera, looking at both the disk of Pluto and near and on the limb. Since an automated cloud search was nearly impossible, it was all done by visual inspection of the images by the scientists.
They looked for features in the atmosphere that including brightness, fuzzy or fluffy-looking edges and isolated borders.
In all, they found seven bright, discrete possible cloud candidates. The seven candidates share several different attributes including small size, low altitude, they all were visible either early or late in the day local time, and were only visible at oblique geometry – which is basically a sideways look from the spacecraft.
Also, several cloud candidates also coincided with brighter surface features below, so the team is still pondering the correlation.
“The seven candidates are all similar in that they are very low altitude,” Stern said last fall at the Division of Planetary Sciences meeting, “and they are all low-lying, isolated small features, so no broad cloud decks or fields. When we map them over the surface, they all lie near the terminator, so they occur near dawn or dusk. This is all suggestive they are clouds because low-lying regions and dawn or dusk provide cooler conditions where clouds may occur.”
While haze was detected as high as 220 km, the possible clouds were found at very low altitudes. Stern told Universe Today that these possible, rare condensation clouds could be made of ethane, acetylene, hydrogen cyanide or methane under the right conditions. Stern added these clouds are probably short-lived phenomena – again, likely occurring only at dawn or dusk. A day on Pluto is 6.4 days on Earth.
But all in all, they concluded that at the current time Pluto’s atmosphere is almost entirely free of clouds – in fact the dwarf planet’s sky was 99% cloud free the day that New Horizons whizzed by.
“But if there are clouds, it would mean the weather on Pluto is even more complex than we imagined,” Stern said last year.
The seven cloud candidates cannot be confirmed as clouds because none are in the region where there was stereo imaging or other available ways to cross-check it. They concluded that further modeling would be needed, but specifically a Pluto orbiter mission would be the only way to “search for clouds more thoroughly than time and space and was possible during the brief reconnaissance flyby by New Horizons.”
While the New Horizons spacecraft was heading to Pluto, scientists from the mission used Hubble and other telescopes to try and find out more about the environment their spacecraft would be flying through. No one wanted New Horizons to run into unexpected dust or debris.
And now, as New Horizons prepares to fly past its next target, the Kuiper Belt Object known as 2014 MU69, mission scientists are using every tool at their disposal to examine this object and the surrounding region. The flyby will take place on January 1, 2019.
They’ve already uncovered some surprises.
On June 3, 2017, 2014 MU69 passed in front of a star – in an event called an occultation – providing a two-second glimpse of the object’s shadow.
More than 50 mission team members and collaborators traveled to South Africa and Argentina to catch the occultation, setting up telescopes to capture the event. They are now looking through more than 100,000 images of the occultation star that can be used to assess the environment around this Kuiper Belt object (KBO). In addition, the Hubble Space Telescope and Gaia, a space observatory of the European Space Agency (ESA) also observed the event.
The team said that while MU69 itself eluded direct detection, the June 3 data provided valuable and unexpected insights that have already helped New Horizons.
“These results are telling us something really interesting,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute. “The fact that we accomplished the occultation observations from every planned observing site but didn’t detect the object itself likely means that either MU69 is highly reflective and smaller than some expected, or it may be a binary or even a swarm of smaller bodies left from the time when the planets in our solar system formed.”
Mission scientist Simon Porter said on Twitter, “The upshot is that MU69 is probably not as big and dark as it could have been, and (more importantly) doesn’t seem to have rings or a dust cloud,” adding later that the “lack of dust was reassuring.”
Again, no one wants to New Horizons to run into any surprising dust or debris.
The team will be observing two more occultation events on July 10 and July 17, and Porter said they should get even better constraints from these next two events.
On July 10, NASA’s airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) will use its 100-inch (2.5-meter) telescope to probe the space around MU69 for debris that might present a hazard to New Horizons as it flies by in 18 months.
On July 17, the Hubble Space Telescope also will check for debris around MU69, while team members set up another ground-based “fence line” of small mobile telescopes along the predicted ground track of the occultation shadow in southern Argentina to try to better constrain, or even determine, the size of MU69.
Initial estimates of MU69’s diameter, based primarily on data taken by the Hubble Space Telescope since the KBO’s discovery in 2014, fall in the 12-25-mile (20-40-kilometer) range. However, the latest data from the June occultation seem to imply it’s at or even below the smallest estimated sizes.
“2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by,” Stern said back in August 2015 when the target was announced. “Moreover, this KBO costs less fuel to reach [than other candidate targets], leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen.”
You can see the star brightness, predicted shadow path and other tech specs for the July 10 and July 17 occultation events at the embedded links.
Once held to be the outermost planet of the Solar System, Pluto‘s designation was changed by the International Astronomical Union in 2006, owing to the discovery of many new Kuiper Belt Objects that were comparable in size. In spite of this, Pluto remains a source of fascination and a focal point of much scientific interest. And even after the historic flyby conducted by the New Horizons probe in July of 2015, many mysteries remain.
What’s more, ongoing analysis of the NH data has revealed new mysteries. For instance, a recent study by a team of astronomers indicated that a survey by the Chandra X-ray Observatory revealed the presence of some rather strong x-rays emissions coming from Pluto. This was unexpected, and is causing scientists to rethink what they thought they knew about Pluto’s atmosphere and its interaction with solar wind.
In the past, many Solar bodies have been observed emitting x-rays, which were the result of interaction between solar wind and neutral gases (like argon and nitrogen). Such emissions have been detected from planets like Venus and Mars (due to the presence of argon and/or nitrogen in their atmospheres), but also with smaller bodies like comets – which acquire halos due to outgassing.
Ever since the NH probe conducted its flyby of Pluto in 2015, astronomers have been aware that Pluto has an atmosphere which changes size and density with the seasons. Basically, as the planet reaches perihelion during its 248 year orbital period – a distance of 4,436,820,000 km, 2,756,912,133 mi from the Sun – the atmosphere thickens due to the sublimation of frozen nitrogen and methane on the surface.
The last time Pluto was at perihelion was on September 5th, 1989, which means that it was still experiencing summer when NH made its flyby. While studying Pluto, the probe detected an atmosphere that was primarily composed of nitrogen gas (N²) along with methane (CH4) and carbon dioxide (CO²). Astronomers therefore decided to look for signs of x-ray emissions coming from Pluto’s atmosphere using the Chandra X-ray Observatory.
Prior to the NH mission’s flyby, most models of Pluto’s atmosphere expected it to be quite extended. However, the probe found that the atmosphere was less extended and that its rate of loss was hundreds of times lower than what these models predicted. Therefore, as the team indicated in their study, they expected to find x-ray emissions that were consistent with what the NH flyby observed:
“Given that most pre-encounter models of Pluto’s atmosphere had predicted it to be much more extended, with an estimated loss rate to space of ~1027 to 1028 mol/sec of N² and CH4… we attempted to detect X-ray emission created by [solar wind] neutral gas charge exchange interactions in the low density neutral gas surrounding Pluto,” they wrote.
However, after consulting data from the Advanced CCD Imaging Spectrometer (ACIS) aboard Chandra, they found that x-ray emissions coming from Pluto were greater than what this would allow for. In some cases, strong x-ray emissions have been noted coming from other smaller objects in the Solar System, which is due to the scattering of solar x-rays by small dust grains composed of carbon, nitrogen and oxygen.
But the energy distribution they noted with Pluto’s x-rays were not consistent with this explanation. Another possibility that the team offered is that they could be due to some process (or processes) that focus the solar wind near Pluto, which would enhance the effect of its modest atmosphere. As they indicate in their conclusions:
“The observed emission from Pluto is not aurorally driven. If due to scattering, it would have to be sourced by a unique population of nanoscale haze grains composed of C, N, and O atoms in Pluto’s atmosphere resonantly fluorescing under the Sun’s insolation. If driven by charge exchange between [solar wind] minor ions and neutral gas species (mainly CH4) escaping from Pluto, then density enhancement and adjustment of the [solar wind] minor ion relative abundance in the interaction region near Pluto is required versus naïve models.”
For the time being, the true cause of these x-ray emissions is likely to remain a mystery. They also highlight the need for more research when it comes to this distant and most massive of Kuiper Belt Objects. Luckily, the data provided by the NH mission is likely to be poured over for decades, revealing new and interesting things about Pluto, the outer Solar System, and how the most distant worlds from our Sun behave.
For decades, we could only imagine what the view of Pluto’s surface might be. Now, we have the real thing.
The images and data from the New Horizons’ mission flyby of Pluto in July 2015 showed us an unexpectedly stunning and geologically active world. Scientists have used words like ‘magical,’ ‘breathtaking’ and ‘scientific wonderland’ to describe the long-awaited close-up views of distant Pluto.
Even though scientists are still analyzing the data from New Horizons, ideas are starting to formulate about sending another spacecraft to Pluto, but with a long-term orbiter mission instead of a quick flyby.
“The next appropriate mission to Pluto is an orbiter, maybe equipped with a lander if we had enough funding to do both,” New Horizons’ principal investigator Alan Stern told Universe Today in March.
This week, Stern has shared on social media that the New Horizons’ science team is meeting. But, separately, another group is starting to talk about a possible next mission to Pluto.
Getting a spacecraft to the outer regions of our solar system as fast as possible provides challenges, particularly in being able to slow down enough to enable going into orbit around Pluto. For the speedy and lightweight New Horizons, an orbital mission was impossible.
What propulsion system might make a Pluto orbiter and/or lander mission possible?
A few ideas are being tossed around.
Space Launch System
One concept takes advantage of NASA’s big, new Space Launch System (SLS), currently under development to enable human missions to Mars. NASA describes the SLS as “designed to be flexible and evolvable and will open new possibilities for payloads, included robotic scientific missions.” Even the first Block 1 version can launch 70 metric tons (later versions might be able to lift up to 130 metric tons.) Block 1 will be powered by twin five-segment solid rocket boosters and four liquid propellant engines, with a proposed 15% more thrust at launch than the Saturn V rockets that sent astronauts to the Moon.
But an orbiter mission to Pluto might not be the best use of the SLS alone.
It takes a lot of fuel to accelerate a vehicle to fast enough speed to get to Pluto in a reasonable amount of time. For example, New Horizons was the fastest spacecraft ever launched, using a souped-up Atlas V rocket with extra boosters, it performed a big burn when New Horizons departed Earth orbit. The lightweight spacecraft sped away from the Earth at 36,000 miles per hour (about 58,000 km/ hour), then used a gravity assist from Jupiter to boost New Horizons’ speed to 52,000 mph (83,600 km/h), traveling nearly a million miles (1.5 million km) a day in its 3 billion mile (4.8 billion km) journey to Pluto. The flight took nine and a half years.
“To enter Pluto orbit, a vehicle [like SLS] would have to boost up to that same speed, then turn around and decelerate for half the trip to arrive at Pluto with a net velocity of zero relative to the planet,” explained Stephen Fleming, an investor in several alt-space startups including XCOR Aerospace, Planetary Resources and NanoRacks. “Unfortunately, due to the tyranny of the rocket equation, you would have to carry all the fuel/propellant to decelerate with you at launch … which means accelerating the orbiter AND all that fuel in the initial phase. That requires logarithmically more fuel for the initial burn, and it turns out to be a LOT of fuel.”
Fleming told Universe Today that using the multi-billion dollar SLS to launch a Pluto orbiter, you would wind up launching an entire payload full of propellant just to accelerate and decelerate a tiny Pluto orbiter.
“That’s an extraordinarily expensive mission,” he said.
A better option might be to use a propulsion system of combined technologies. Stern mentioned a NASA study that looked at using the SLS as the launch vehicle and to boost the spacecraft towards Pluto, but then using an RTG (Radioisotope Thermoelectric Generator) powered ion engine to later brake for an orbital arrival.
An RTG produces heat from the natural decay of non-weapons-grade plutonium-238, and the heat is converted into electricity. An RTG ion engine would be a more powerful ion propulsion system than the current solar electric ion engine on the Dawn spacecraft, now orbiting Ceres, in the asteroid belt, plus it would enable operation in the outer solar system, far from the Sun. This nuclear powered ion engine would enable a speeding spacecraft to slow down and go into orbit.
“The SLS would boost you to fly out to Pluto,” Stern said, “and it would actually take two years to do the braking with ion propulsion.”
Stern said the flight time for such a mission to Pluto would be seven and a half years, two years faster than New Horizons.
The proposal uses a Direct Fusion Drive (DFD) engine that has propulsion and power in one integrated device. DFD provides high thrust to allow for a flight time of about 4 years to Pluto, plus being able to send substantial mass to orbit, perhaps between 1000 to 8000 kg.
If this propulsion system works as planned, it could launch a Pluto orbiter and a lander (or possibly a rover), and provide enough power to maintain an orbiter and all its instruments, as well as beam a lot of power to a lander. That would enable the surface vehicle to beam back video to the orbiter because it would have so much power, according to Stephanie Thomas from Princeton Satellite Systems, Inc., who is leading the NIAC study.
“Our concept is generally received as, ‘wow, that sounds really cool! When can I get one?’” Thomas told Universe Today. She said her and her team chose a prototype Pluto orbiter and lander mission in their proposal because it’s a great example of what can be done with a fusion rocket.
Their fusion system uses a small linear array of solenoid coils, and their fuel of choice is deuterium helium 3, which has very low neutron production.
“It fits on a spacecraft, it fits on a launch vehicle,” Thomas explained in a NIAC symposium talk (her talk starts about 17:30 in the linked video). “There’s no lithium, or other dangerous materials, it produces very few damaging particles. It’s about the size of a minivan or small truck. Our system is cheaper and faster to develop than other fusion proposals.”
The Princeton team has been able to produce 300 millisecond pulses with their plasma heating experiment, orders of magnitude better than any other system.
“The biggest hurdle is the fusion itself,” she said. “We need to build a bigger experiment to finish proving the new heating method, which will require an order of magnitude more resources than the project has been receiving from the Department of Energy so far,” Thomas said via email. “However, it’s still small in the grand scheme of advanced technology projects, about $50 million.”
Thomas said that DARPA has spent much more on many technology initiatives that ended up canceled. And it’s also much less than other fusion technologies require for the same stage of research, since our machine is so small and has a simple coil configuration.” (Thomas said have a look at the budget for ITER, the international nuclear fusion research and engineering megaproject, currently running over $20 billion).
“To put it simply, we know our method heats electrons really well and can extrapolate to heating ions, but we need to build it and prove it,” she said.
Thomas and her team are currently working on the “balance of plant” technology – the subsystems that will be required to operate the engine in space, assuming the heating method works as currently predicted.
In terms of the Pluto mission itself, Thomas said there aren’t any particular hurdles on the orbiter itself, but it would involve scaling up a few technologies to take advantage of the very large amount of power available, such as the optical communications.
“We could dedicate tens or more kW of power to the communication laser, not 10 watts, [like current missions]” she said. “Another unique feature of our concept is being able to beam a lot of power to a lander. This would enable new classes of planetary science instruments like powerful drills. The technology to do this exists but the specific instruments need to be designed and built. Additional technology that will be needed that is under development in various industries are lightweight space radiators, next-generation superconducting wires, and long-term cryogenic storage for the deuterium fuel.”
Thomas said their NIAC research is going well.
“We were selected for the NIAC Phase II study, and are in contract negotiations now,” she said. “We are busy working on higher fidelity models of the engine’s thrust, designing components of the trajectory, and sizing the various subsystems, including the superconducting coils,” she said. “Our current estimates are that a single 1 to 10 MW engine will produce between 5 and 50 N thrust, at about 10,000 sec specific impulse.”
Laser Zapping to Pluto
Another futuristic propulsion possibility is the laser-based systems proposed by Yuri Milner for his Breakthrough Starshot proposal, where small cubesats could be zapped by lasers on Earth, basically “bug zapping” spacecraft to reach incredible speeds (possibly millions of miles/km per hour) to visit the outer solar system or beyond.
“It’s not really in the cards for us to use this kind of technology, because we’d have to wait decades just for this to be developed,” Stern said. “But if you could send lightweight, inexpensive spacecraft at speeds like one-10th the speed of light based on lasers from Earth. We could send these small spacecraft to hundreds or thousands of objects in the Kuiper Belts, and you’d be out there in a matter of two-and-a-half days. You could send a spacecraft past Pluto every day. That would be really game changing.”
The Realistic Future
But even if everyone agrees a Pluto orbiter should be done, the earliest possible date for such a mission is sometime between the early 2020s and the early 2030s. But it all depends on the recommendations put forth by the scientific community’s next decadal survey, which will suggest the most top-priority missions for NASA’s Planetary Science Division.
These Decadal Surveys are 10-year “roadmaps” that set science priorities and provide guidance on where NASA should send spacecraft and what types of missions they should be. The last Decadal Survey was published in 2011, and that set planetary science priorities through 2022. The next one, for 2023-2034, will likely be published in 2022.
The New Horizons mission was the result of the suggestions from the 2003 planetary science Decadal Survey, where scientists said visiting the Pluto system and worlds beyond was a top-priority destination.
So, if you’re dreaming of a Pluto orbiter, keep talking about it.
The New Horizons probe made history in July of 2015, being the first mission to ever conduct a close flyby of Pluto. In so doing, the mission revealed some never-before-seen things about this distant world. This included information about its many surface features, it’s atmosphere, magnetic environment, and its system of moons. It also provided images that allowed for the first detailed maps of the planet.
Having completed its rendezvous with Pluto, the probe has since been making its way towards its first encounter with a Kuiper Belt Object (KBO) – known as 2014 MU69. And in the meantime, it has been given a special task to keep it busy. Using archival data from the probe’s Long Range Reconnaissance Imager (LORRI), a team of scientists is taking advantage of New Horizon‘s position to conduct measurements of the Cosmic Optical Background (COB).
The COB is essentially the visible light from other galaxies which shines beyond the edge of the Milky Way. By measuring this light, astronomers are able to learn a great deal about the locations of stars, the size and density of galaxies, and test theories about the structure and formation of the Universe. This is no easy task, mind you, as any measurements conducted from inside the Solar System are subject to interference.
Whereas Earth-based telescopes experience interference from our atmosphere, space-based telescopes have to contend with the brightness of our Sun. In addition, interplanetary dust (IPD) has the effect of scattering light in the Solar System (known as Zodiacal Light) which can also obscure light coming from distant sources. But a probe like New Horizons, which is well into the outer Solar System, is not subject to such interference.
For the sake of this study, the team analyzed LORRI data obtained during NH’s cruise phase between Jupiter and Uranus. After using data from four different isolated fields in the sky (captured between 2007 and 2010), the team was able to obtain a statistical upper limit on the optical background’s brightness.
“This result shows some of the promise of doing astronomy from the outer solar system. What we’re seeing is that the optical background is completely consistent with the light from galaxies and we don’t see a need for a lot of extra brightness; whereas previous measurements from near the Earth need a lot of extra brightness. The study is proof that this kind of measurement is possible from the outer solar system, and that LORRI is capable of doing it.”
Their results also showed that earlier measurements conducted by Hubble’s Wide Field Planetary Camera 2 were excessively bright (owing to interference). However, their results were consistent with previous measurements that were based on data obtained by the Pioneer 10 and 11 missions. Back in the 1970s, these probes managed to gather data on the Universe while swinging past Jupiter and exploring the outer Solar System.
By showing consistency with these results (and other measurements from over the years), the team demonstrated just how valuable missions like New Horizons are. It is hoped that before it wraps up in 2021, that scientists will have a chance to conduct more measurements of the COB. Considering how rare missions to the outer Solar System are, it is understandable why Zemcov and his colleagues want to take full advantage of this opportunity.
“NASA sends missions to the outer Solar System once a decade or so,” he said. “What they send is typically going to planets and the instruments onboard are designed to look at them, not to do astrophysics. Measurements could be designed to optimize this technique while LORRI is still functioning… With a carefully designed survey, we should be able to produce a definitive measurement of the diffuse light in the local universe and a tight constraint on the light from galaxies in the optical wavebands.”
In other mission-related news, New Horizons probe will be taking a nap as it approaches its next destination – 2014 MU69. On Friday, April 7th, at 15:32 EDT, mission controllers at the John Hopkins University APL verified that the probe had entered hibernation. It will remain in this state for the next 157 days, waking up again on September 11th, 2017, as it makes its approach to 2014 MU69.
Originally, the New Horizons mission was scheduled to end after its historic encounter with Pluto. However, the mission was extended shortly thereafter to 2021 so the probe would also be able to make some more historic encounters. If, in the meantime, this probe can also shed new light on the mysteries of the Universe, it will surely be remembered as one of the most groundbreaking missions of all time.
Discovered in 1930 by Clyde Tombaugh, Pluto was once thought to be the ninth and outermost planet of the Solar System. However, due to the formal definition adopted in 2006 at the 26th General Assembly of the International Astronomical Union (IAU), Pluto ceased being the ninth planet of the Solar System and has become alternately known as a “Dwarf Planet”, “Plutiod”, Trans-Neptunian Object (TNO) and Kuiper Belt Object (KBO).
Despite this change of designation, Pluto remains one of the most fascinating celestial bodies known to astronomers. In addition to having a very distant orbit around the Sun (and hence a very long orbital period) it also has the most eccentric orbit of any planet or minor planet in the Solar System. This makes for a rather long year on Pluto, which lasts the equivalent of 248 Earth years!
With an extreme eccentricity of 0.2488, Pluto’s distance from the Sun ranges from 4,436,820,000 km (2,756,912,133 mi) at perihelion to 7,375,930,000 km (4,583,190,418 mi) at aphelion. Meanwhile, it’s average distance (semi-major axis) from the Sun is 5,906,380,000 km (3,670,054,382 mi). Another way to look at it would be to say that it orbits the Sun at an average distance of 39.48 AU, ranging from 29.658 to 49.305 AU.
At its closest, Pluto actually crosses Neptune’s orbit and gets closer to the Sun. This orbital pattern takes place once every 500 years, after which the two objects then return to their initial positions and the cycle repeats. Their orbits also place them in a 2:3 mean-motion resonance, which means that for every two orbits Pluto makes around the Sun, Neptune makes three.
The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years. The last time this cycle took place was between 1979 to 1999, when Neptune was farther from the Sun than Pluto. Pluto reached perihelion in this cycle – i.e. its closest point to the Sun – on September 5th, 1989. Since 1999, Pluto returned to a position beyond that of Neptune, where it will remain for the following 228 years – i.e. until the year 2227.
Sidereal and Solar Day:
Much like the other bodies in our Solar System, Pluto also rotates on its axis. The time it takes for it to complete a single rotation on its axis is known as a “Sidereal Day”, while the amount of time it takes for the Sun to reach the same point in the sky is known as a “Solar Day”. But due to Pluto’s very long orbital period, a sidereal day and a solar day on Pluto are about the same – 6.4 Earth days (or 6 days, 9 hours, and 36 minutes).
It is also worth noting that Pluto and Charon (its largest moon) are actually more akin to a binary system rather than a planet-moon system. This means that the two worlds orbit each other, and that Charon is tidally locked around Pluto. In other words, Charon takes 6 days and 9 hours to orbit around Pluto – the same amount of time it takes for a day on Pluto. This also means that Charon is always in the same place in the sky when seen from Pluto.
In short, a single day on Pluto lasts the equivalent of about six and a half Earth days. A year on Pluto, meanwhile, lasts the equivalent of 248 Earth years, or 90,560 Earth days! And for the entire year, the moon is hanging overhead and looming large in the sky. But factor in Pluto’s axial tilt, and you will come to see just how odd an average year on Pluto is.
It has been estimated that for someone standing on the surface of Pluto, the Sun would appear about 1,000 times dimmer than it appears from Earth. So while the Sun would still be the brightest object in the sky, it would look more like a very bright star that a big yellow disk. But despite being very far from the Sun at any given time, Pluto’s eccentric orbit still results in some considerable seasonal variations.
On the whole, the surface temperature of Pluto does not change much. It’s surface temperatures are estimated to range from a low of 33 K (-240 °C; -400 °F ) to a high of 55 K (-218 °C; -360°F) – averaging at around 44 K (-229 °C; -380 °F). However, the amount of sunlight each side receives during the course of a year is vastly different.
Compared to most of the planets and their moons, the Pluto-Charon system is oriented perpendicular to its orbit. Much like Uranus, Pluto’s very high axial tilt (122 degrees) essentially means that it is orbiting on its side relative to its orbital plane. This means that at a solstice, one-quarter of Pluto’s surface experiences continuous daylight while the other experiences continuous darkness.
This is similar to what happens in the Arctic Circle, where the summer solstice is characterized by perpetual sunlight (i.e. the “Midnight Sun”) and the winter solstice by perpetual night (“Arctic Darkness”). But on Pluto, these phenomena affect nearly the entire planet, and the seasons last for close to a century.
Even if it is no longer considered a planet (though this could still change) Pluto still has some very fascinating quarks, all of which are just as worthy of study as those of the other eight planets. And the time it takes to complete a full year on Pluto, and all the seasonal changes it goes through, certainly rank among the top ten!
When Pluto was first discovered by Clybe Tombaugh in 1930, astronomers believed that they had found the ninth and outermost planet of the Solar System. In the decades that followed, what little we were able to learn about this distant world was the product of surveys conducted using Earth-based telescopes. Throughout this period, astronomers believed that Pluto was a dirty brown color.
In recent years, thanks to improved observations and the New Horizons mission, we have finally managed to obtain a clear picture of what Pluto looks like. In addition to information about its surface features, composition and tenuous atmosphere, much has been learned about Pluto’s appearance. Because of this, we now know that the one-time “ninth planet” of the Solar System is rich and varied in color.
With a mean density of 1.87 g/cm3, Pluto’s composition is differentiated between an icy mantle and a rocky core. The surface is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. Scientists also suspect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice.
The diameter of the core is believed to be approximately 1700 km, which accounts for 70% of Pluto’s total diameter. Thanks to the decay of radioactive elements, it is possible that Pluto contains a subsurface ocean layer that is 100 to 180 km thick at the core–mantle boundary.
Pluto has a thin atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. However, the planet is so cold that during part of its orbit, the atmosphere congeals and falls to the surface. The average surface temperature is 44 K (-229 °C), ranging from 33 K (-240 °C) at aphelion to 55 K (-218 °C) at perihelion.
Pluto’s surface is very varied, with large differences in both brightness and color. Pluto’s surface also shows signs of heavy cratering, with ones on the dayside measuring 260 km (162 mi) in diameter. Tectonic features including scarps and troughs has also been seen in some areas, some as long as 600 km (370 miles).
Mountains have also been seen that are between 2 to 3 kilometers (6500 – 9800 ft) in elevation above their surroundings. Like much of the surface, these features are believed to be composed primarily of frozen nitrogen, carbon monoxide, and methane, which are believed to sit atop a “bedrock” of frozen water ice.
The surface also has many dark, reddish patches due to the presence of tholins, which are created by charged particles from the Sun interacting with mixtures of methane and nitrogen. Pluto’s visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. In other words, the planet has a range of colors, including pale sections of off-white and light blue, to streaks of yellow and subtle orange, to large patches of deep red.
Overall, its appearance could be described as “ruddy”, given that the combination can lend it a somewhat brown and earthy appearance from a distance. In fact, prior to the New Horizon‘s mission, which provided the first high-resolution, close-up images of the planet, this is precisely what astronomers believed Pluto looked like.
Major Surface Features:
Several different regions (“regio”) have been characterized based on the notable features they possess. Perhaps the best known is the large, pale area nicknamed the “Heart” – aka. Tombaugh Regio (named after Pluto’s founder). This large bright area is located on the side of Pluto that lies opposite the side that faces Charon, and is named because of its distinctive shape.
Tombaugh Regio is about 1,590 km (990 mi) across and contains 3,400 m (11,000 ft) mountains made of water ice along its southwestern edge. The lack of craters suggests that its surface is relatively young (about 100 million years old) and hints at Pluto being geologically active. The Heart can be subdivided into two lobes, which are distinct geological features that are both bright in appearance.
The western lobe, Sputnik Planitia, is vast plain of nitrogen and carbon monoxide ices measuring 1000 km in width. It is divided into polygonal sections that are believed to be convection cells, which carry blocks of water ice and sublimation pits along towards the edge of the plain. This region is especially young (less than 10 million years old), which is indicated by its lack of cratering.
Then there is the large, dark area on the trailing hemisphere known as Cthulhu Regio (aka.the “Whale”). Named for its distinctive shape, this elongated, dark region along the equator is the largest dark feature on Pluto – measuring 2,990 km (1,860 mi) in length. The dark color is believed to be the result methane and nitrogen in the atmosphere interacting with ultraviolet light and cosmic rays, creating the dark particles (“tholins”) common to Pluto.
And then there are the “Brass Knuckles”, a series of equatorial dark areas on the leading hemisphere. These features average around 480 km (300 mi) in diameter, and are located along the equator between the Heart and the tail of the Whale.
New Horizons Mission:
The NH mission launched from Cape Canaveral Air Force Station in Florida on January 19th, 2006. After swinging by Jupiter for a gravity boost and to conduct some scientific studies in February of 2007, it reached Pluto in the summer of 2015. Once there, it conducted a six month-long reconnaissance flyby of Pluto and its system of moons, culminating with a closest approach that occurred on July 14th, 2015.
The first images of Pluto acquired by NH were taken on September 21st to 24th, 2006, during a test of the Long Range Reconnaissance Imager (LORRI). At the time, the probe was still at a distance of approximately 4.2 billion km (2.6 billion mi) or 28 AU, and the photos were released on November 28th, 2006. Between July 1st and 3rd, the first images were taken that were able to resolved Pluto and its largest moon, Charon, as separate objects.
Between July 19th–24th, 2014, the probe snapped 12 images of Charon revolving around Pluto, covering almost one full rotation at distances ranging from 429 to 422 million kilometers (267,000,000 to 262,000,000 mi). After a brief hibernation during its final approach, New Horizons “woke up” on Dec. 7th, 2014. Distant-encounter operations began on January 4th, 2015, and NH began taking images of Pluto as it grew closer.
During its closest approach (July 14th, 2015, at at 11:50 UTC), the NH probe passed within 12,500 km (7,800 mi) of Pluto. About 3 days before making its closest approach, long-range imaging of Pluto and Charon took place that were 40 km (25 mi) in resolution, which allowed for all sides of both bodies to be mapped out.
Close-range imaging also took place twice a day during this time to search for any indication of surface changes. The NH probe also analyzed Pluto’s atmosphere using its suite of scientific instruments. This included it’s ultraviolet imaging spectrometer (aka. Alice) and the Radio Science EXperiment (REX), which analyzed the composition and structure of Pluto’s atmosphere.
It’s Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) examined the interaction of Pluto’s high atmosphere with solar wind. Pluto’s diameter was also resolved by measuring the disappearance and reappearance of the radio occultation signal as the probe flew by behind Pluto. And the gravitational tug on the probe were used to determine Pluto’s mass and mass distribution.
All of this information has helped astronomers to make the first detailed maps of Pluto, and led to numerous discoveries about Pluto’s structure, composition, and the kinds of forces that actively shape its surface. The mission also led to the first true images of what Pluto looks like up close, revealing its true colors, it’s famous “Heart” region, and the many other now-famous features.