Planetoid is another term for asteroids, which are also called minor planets. Planetoids are small celestial bodies that orbit the Sun. Planets are simply defined as asteroids, but the term asteroid is not well defined either. In 2006, The International Astronomical Union (IAU) defined it as a “small Solar System body” (SSSB), which does not really tell us anything either. Webster’s Dictionary defines an asteroid as, “any of the thousands of small planets ranging from 1,000 km (621 mi) to less than one km (0.62 mi) in diameter, with orbits usually between those of Mars and Jupiter; minor planet; planetoid.”
Asteroids – planetoids – were first discovered in 1801, and many more have been discovered since then. Up until 1977, almost all the asteroids discovered were near Jupiter. However, then astronomers began to discover planetoids even farther out and started calling them centaurs and trans-Neptunian objects (TNOs). When a region of space in the outer Solar System filled with celestial bodies was discovered, it was called the Kuiper Belt and the objects in it were called Kuiper Belt Objects (KBOs). The large number of synonyms for planetoids is one reason why keeping these terms straight is so difficult.
Some of the largest planetoids are spherical and look like tiny versions of planets. The smaller ones are irregular in shape though. The objects range in size from around ten meters to hundreds of kilometers in diameter. Objects smaller than ten meters are called meteoroids. Unfortunately, astronomers do not know that much about the materials that make up planetoids. They are believed to be composed of various materials including ice, rock, and different metals.
Most planetoids are in a region called the asteroid belt, which is situated between Mars and Jupiter. There are millions of planetoids in this region. Despite the millions of objects, all of them combined are believed to have a mass of only about 4% of the Moon’s mass. After being discovered, the planetoids are given a temporary designation. If they are officially recognized, they are given a number and maybe a name. The first few planetoids were given symbols just like the planets. All except one of the first fifteen asteroids were given extremely complex symbols. For example, one symbol was a star with a plant growing out of it. However, that soon ended when astronomers realized that there were many more planetoids. Planetoids, and other celestial bodies, are a subject of study by astronomers who hope to learn more about how the universe was formed from these ancient rocks.
Trojan asteroids sharing the orbits of Jupiter and Neptune. Image credit: Scott Sheppard.
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A planetesimal is an object formed from dust, rock, and other materials. The word has its roots in the concept infinitesimal, which indicates an object too small to see or measure. Planetesimals can be anywhere in size from several meters to hundreds of kilometers. The term refers to small celestial bodies formed during the creation of planets. One way to think of them is as small planets, but they are much more than that.
The planetesimal theory was suggested by the Russian astronomer Viktor Safronov. The planetesimal theory is a theory on how planets form. According to the planetesimal hypothesis, when a planetary system is forming, there is a protoplanetary disk with materials from the nebulae from which the system came. This material is gradually pulled together by gravity to form small chunks. These chunks get larger and larger until they form planetesimals. Many of the objects break apart when they collide, but some continue to grow. Some of these planetesimals go on to become planets and moons. Since the gas giants are balls of gas with liquid cores, it may seem impossible that an asteroid-like object formed them. The planetesimals formed the core of these gaseous planets, which turned molten when it enough heat was created.
Other planetesimals turn into comets, Kuiper Belt Objects (KBOs), and trojan asteroids. There is some debate as to whether KBOs and asteroids can be called planetesimals. This is one reason why nomenclature of celestial objects is so difficult. The planetesimal theory is not universally accepted though. Like many theories, there are some observations that cannot be explained, but the planetesimal theory is still very popular.
Many people think that around 3.8 billion years ago, many of the planetesimals were thrown into far away regions, such as the Oort cloud or the Kuiper Belt. Other objects collided with other objects after being affected by gas giants. Phobos and Deimos are believed to be planetesimals that were captured by Mars’ gravity and became satellites. Many of Jupiter’s moons are believed to be planetesimals as well.
Planetesimals are very valuable to scientists because they can provide information about the creation of our Solar System. The exterior of planetesimals have been bombarded with solar radiation, which can change their chemistry, for billions of years. Inside though, there is material that has been untouched since the object was first formed. Using this material, astronomers hope to learn about the condition of the nebulae from which our Solar System was formed.
Ceres, the recently promoted dwarf planet in the asteroid belt is still too small to be easily seen by Hubble credit: NASA/ESA/STScI
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Minor planet is a term used to refer to a celestial object – that is not a planet or comet – which orbits the Sun. Found in 1801, Ceres, also known as a dwarf planet, was the first minor planet discovered. The term minor planet has been in use since the 1800’s. Planetoids, asteroids, and minor planets have all been used interchangeably, but the situation became even more confusing when the International Astronomical Union (IAU) committee reclassified minor planets and comets into the new categories of dwarf planets and small solar system bodies. At the same time, the IAU created a new definition of what a planet is, and Pluto was reclassified as a dwarf planet. Hydrostatic equilibrium – the ability to maintain a roughly spherical shape – is what separates dwarf planets from the more irregularly shaped small solar system bodies. The names become even more confusing because the IAU still recognizes the use of the term minor planets.
Minor planets are extremely common with over 400,000 registered and thousands more found each month. Approximately 15,000 minor planets have been given official names while the rest are numbered. When asteroids were first discovered, they were named after characters from Greek and Roman mythology like Ceres was. At first, astronomers thought that the asteroids, especially Ceres and Pallas were actually planets. Astronomers also created symbols for the first asteroids found. There were symbols created for 14 asteroids and some of them were very complex like Victoria’s symbol, which looks like a plant with three leaves growing out of an off center starburst. Soon, astronomers ran out of mythological names and started christening asteroids after television characters, famous people, and relatives of discoverers. Most names were feminine, attesting to an unnamed tradition. As the numbers ran into the thousands, scientists started using their pets as inspiration. After an asteroid was named 2309 Mr. Spock, pet’s names were banned. That did not stop the oddness though because names such as 9007 James Bond and 6402 Chesirecat have been suggested and actually accepted.
There are a number of different categories that minor planets fall into including asteroids, Trans-Neptunian objects, and centaurs. There are various types of asteroids, although most of them can be found in the asteroid belt, which is the region of space between Mars and Jupiter. Trans-Neptunian objects are celestial bodies found orbiting beyond Neptune, and centaurs are celestial bodies with unstable orbits located between Jupiter and Neptune. The categories also overlap, making classifying things a nightmare. For example, Ceres is a dwarf planet and minor planet, additionally it can also be classified as an asteroid.
Ever since our recent encounter with asteroid 2008 TC3 — the first asteroid that was correctly predicted to hit our planet — I’ve had impact craters on the brain. Earth has about 175 known impact craters, but surely our planet has endured more bashing than that in its history. All the other terrestrial planets and moons in our solar system are covered by impact craters. Just look at our Moon through a telescope or binoculars, or check out the recent images of Mercury sent back by the MESSENGER spacecraft, or pictures of Mars from the armada of spacecraft orbiting the Red Planet, and you’ll see that impact craters are the most common landforms in our solar system.
But since two-thirds of Earth is covered by water, any asteroid impacts occurring in the oceans are difficult to find. And even though Earth’s atmosphere protects us from smaller asteroids, just like in the case of 2008 TC3, which broke up high in the atmosphere, weathering, erosion and the tectonic cycling of Earth’s crust have erased much of the evidence of Earth’s early bombardment by asteroids and comets. Most of Earth’s impact craters have been discovered since the dawn of the space age, from satellite imaging. In fact, a geologist recently discovered an impact crater using Google Earth!
Here’s my list of Earth’s Ten Most Impressive Impact Craters, starting with #1. the largest and oldest known impact crater, Vredefort Crater, shown above, located in South Africa. It is approximately 250 kilometers in diameter and is thought to to be about two billion years old. The Vredefort Dome can be seen in this satellite image as a roughly circular pattern. What an impact that must have been!
Manicouagan Reservoir. Credit: NASA 2. Manicouagan Crater: fifth largest known impact crater. This crater is located in Quebec, Canada. It was created about 212 million years ago. Now, it is an ice-covered lake about 70 km across. This image, taken by space shuttle astronauts, shows an outer ring of rock. Close up, the rock reveals clear signs of having been melted and altered by a violent collision. The original rim of the crater, though now eroded away, is thought to have had a diameter of about 100 km.
Chicxulub Crater. 3. Chicxulub Crater, third largest and possible dinosaur killer. The third largest impact crater lies mostly underwater and buried underneath the Yucatán Peninsula in Mexico. At 170km (105 miles) in diameter, the impact is believed to have occurred roughly 65 million years ago when a comet or asteroid the size of a small city crashed, unleashing the equivalent to 100 teratons of TNT. Likely, it caused destructive tsunamis, earthquakes and volcanic eruptions around the world, and is widely believed to have led to the extinction of dinosaurs, because the impact probably created a global firestorm and/or a widespread greenhouse effect that caused long-term environmental changes.
Aorounga Crater. Credit: NASA 4. Aorounga Crater: possible triple crater. The main Aorounga Crater in Chad, Africa, visible in this radar image from space, shows a concentric ring structure that is about 17 kilometers wide. But, this crater might have been formed as the result of a multiple impact event. A second crater, similar in size to the main crater, appears as a circular trough in the center of the image. And a third structure, also about the same size, is seen as a dark, partial circular trough on the right side of the image. The proposed crater “chain” could have formed when a 1 km to 2 km (0.5 mile to 1 mile) diameter object broke apart before impact. Ouch! Clearwater craters. Credit: NASA 5. Clearwater Craters: two for the price of one. Twin, lake-filled impact craters in Quebec, Canada were probably formed simultaneously, about 290 million years ago, by two separate but probably related meteorite impacts. The larger crater, Clearwater Lake West has a diameter of 32 km, and Clearwater Lake East is 22 km wide.
Barringer Crater. 6. Barringer Crater: well preserved. While this crater isn’t all that big, what’s most impressive about Barringer Crater in Arizona (USA) is how well preserved it is. Measuring 1.2 km across and 175 m deep, Barringer Crater was formed about 50,000 years ago by the impact of an iron meteorite, probably about 50 m across and weighing several hundred thousand tons. Most of the meteorite was vaporized or melted, leaving only numerous, mostly small fragments with in the crater and scattered up to 7 km from the impact site. Only about 30 tons, including a 693-kg sample, are known to have been recovered. Wolfe Creek Crater 7. Wolfe Creek Crater, well preserved, too. Another relatively well-preserved meteorite crater is found in the desert plains of north-central Australia. Wolfe Creek crater is thought to be about 300,000 years old and is 880 meters across and and about 60 meters deep. It’s partially buried under the wind-blown sand of the region, and although the unusual landform was well-known to the locals, scientists didn’t find the crater until 1947. Deep Bay Crater. Credit: NASA 8. Deep Bay Crater: deep and cold. Deep Bay crater is located in Saskatchewan, Canada. The bay is a strikingly circular 13 km wide impact crater and is also very deep (220 m). It is part of an otherwise irregular and shallow lake. The age of the crater is estimated to be 99 million years old.
Kara-Kul Crater. Credit: NASA 9. Kara-Kul Crater: high altitude crater. This crater was formed about 10 million years ago, and is located in Tajikistan, near the Afghan border. In total, the crater is about 45 km in diameter and is partially filled with a 25 km-wide lake. This might be the “highest” impact crater, almost 6,000 m above sea-level in the Pamir Mountain Range. It was found only recently from satellite images.
Bosumtwi Crater. 10. Bosumtwi Crater: built of bedrock. The last crater on our tour of impressive impact craters is this located in Ghana, Africa. It is about 10.5 km in diameter and about 1.3 million years old. The crater is filled almost entirely by water, creating Lake Bosumtwi. The lakebed is made of crystalline bedrocks.
Although the chances of an asteroid hitting Earth appear to be small for any given year, the consequences of such an event would be monumental. The science community has come up with some ideas and proposals for ways to mitigate the threat of an incoming asteroid hitting the Earth. Some proposals suggest almost Hollywood type theatrics of launching nuclear weapons to destroy the asteroid, or slamming a spacecraft into a Near Earth Object to blow it apart. But other ideas employ more simple and elegant propositions to merely alter the trajectory of the space rock. One such plan uses a two-piece solar sail called a solar photon thruster that draws on solar energy and resources from the asteroid itself.
Physicist Gregory Matloff has been working with NASA’s Marshall Spaceflight Center to study the two-sail solar photon thruster which uses concentrated solar energy. One of the sails, a large parabolic collector sail would constantly face the sun and direct reflected sunlight onto a smaller, moveable second thruster sail that would beam concentrated sunlight against the surface of an asteroid. In theory, the beam would vaporize an area on the surface to create a ‘jet’ of materials that would serve as a propulsion system to alter the trajectory of the Near Earth Object (NEO.)
Changing the trajectory of a NEO exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time. Since the Earth is approximately 12,750 km in diameter and moves at about 30 km per second in its orbit, it travels a distance of one planetary diameter in about seven minutes. The course of the object would be altered, or either delayed or advanced and cause it to miss the Earth.
But of course, the arrival time of the impactor must be known very accurately in order to forecast the impact at all, and to determine how to affect its velocity.
Additionally, the solar photon thruster’s performance would vary depending on the unique makeup of each NEO. For example, asteroids with a greater density, radius or rate of rotation would cause decreased performance of the solar photon thruster in acceleration and deflection.
Even though the solar photon thruster appears to be efficient in its performance, Matloff said that more than half of the solar energy delivered to the “hotspot” on the NEO would not be available to vaporize and accelerate the jet due to other thermodynamic processes such as conduction, convection, and radiation. As expected, a larger collector sail radius would increase the amount of energy available, and would increase acceleration of the NEO. Matloff said this system allows the sail craft to “tack” against the solar-photon breeze at a larger angle than conventional single solar sails can achieve.
This system of sails would not be attached to the NEO, but would be kept nearby the NEO “on station” either with its own thrusting capability or by auxiliary electric propulsion. More studies would be needed to ascertain if a supplementary propulsion system would be necessary.
The sails used in the study were both inflatable. However, Matloff believes it might be worth considering a small rigid thruster sail, which might simplify deployment and reduce occultation.
Said Matloff, “Hopefully, future design studies will resolve these uncertainties before application of NEO-diversion technology becomes necessary.”
On Tuesday, February 5, 2008 an SUV sized asteroid passed between the Earth and the moon. Asteroid 2008 CT1 came within 135,000 kilometers ( 84,000 miles) of Earth, only a third of the distance to the moon. The asteroid was discovered only two days before its close approach to Earth, spotted by the Lincoln Near Earth Asteroid Research (LINEAR) project, using robotic telescopes located at New Mexico’s White Sands Missile Range. The asteroid’s size is estimated between 8 – 15 meters.
While this asteroid seems small, we know that even small rocks can be devastating. Last September, a meteorite estimated at .2 – 2 meters wide created a crater 13 meters wide in Peru. The cause of the Tunguska Event of the early 20th Century is now believed to be a 35m rock that never even touched the ground. It’s believed that it exploded a few miles above the ground, creating a shockwave that devastated the landscape below.
2008 CT1 could possibly return to Earth’s vicinity in 2041, although its orbit has not yet been well defined, so that prediction could change. It is also a possible Mercury impactor, since that that planet is very near the asteroid’s currently calculated perihelion.
LINEAR uses a Ground-based Electro-Optical Deep Space Surveillance (GEODSS) telescope, and has detected over 3,000,000 asteroids since 1998, which is about 70% of the known near-Earth asteroids.
An asteroid between 150-160 meters in diameter will pass within 540,000 kilometers (334,000 miles) of Earth on January 29 at 08:33 UT (3:33 EST). Hopefully this news won’t cause any alarmist cries of doom, as the asteroid has no chance of hitting Earth. But there is one reason to get excited about this close approach by an asteroid: it will be close enough to likely be visible to amateur astronomers.
Asteroid 2007 TU24 was discovered by the Catalina Sky Survey on October 11, 2007 and will approach the Earth to within 1.4 lunar distances. During its closest approach, it will reach an approximate apparent magnitude 10.3 on Jan. 29-30 before quickly becoming fainter as it moves further from Earth. So, for a brief time the asteroid will be observable in dark and clear skies with amateur telescopes of 3 inch apertures or larger.
According to NASA’s Near Earth Object Program, since the estimated number of near-Earth asteroids of this size is about 7,000 discovered and estimated undiscovered objects, an object the size of 2007 TU 24 would be expected to pass this close to Earth, on average, about every 5 years or so. They also say the average interval between actual impacts of Earth for an object of this size would be about 37,000 years. But rest assured, for the January 29th encounter, near Earth asteroid 2007 TU24 has no chance of hitting, or affecting, Earth.
2007 TU24 will be the closest currently known approach by an asteroid of this size or larger until 2027. Plans have been made for the Goldstone planetary radar to observe this object Jan 23-24 and for the Arecibo radar to observe it Jan 27-28, as well as Feb 1-4. The NEO office says they should be able to image the object with high resolution radar, and if so, 3-D shape reconstruction images should be possible. Way cool.
The illustration below is courtesy of amateur astronomer Dr. Dale Ireland from Silverdale, WA. The illustration shows the asteroid’s track on the sky for 3 days near the time of the close Earth approach as seen from the city of Philadelphia. Since the object’s parallax will be a significant fraction of a degree, observers are encouraged to use the NEO office’s on-line Horizons ephemeris generation service for their specific locations.
Now, we’re aware that there are some alarmists out there trying to freak people out about this asteroid visit. They’re posing the usual conspiracy theories about the astronomy community’s cover up. Don’t worry, there’s absolutely nothing to fear except a little cold weather as you stand outside, hoping to see the asteroid pass by with your telescope. If you want a more detailed debunking of this myth, check out Bad Astronomy’s excellent coverage.
Here’s another wonderful example of how amateur astronomers can make important discoveries. Three high school students from Wisconsin discovered an asteroid while doing an astronomical observation project for a class in school. Connor Leipold, Tim Patika, and Kyle Simpson of The Prairie School near Racine were notified this week by the Minor Planet Center in Cambridge, Massachusetts that the object they discovered has been verified as an asteroid.
The students will have the opportunity to name the asteroid, temporarily designated as 2008 AZ28. They spotted the asteroid through telescopes located in New Mexico that operate remotely via the internet. The technology was provided through a project sponsored by Calvin College in Grand Rapids, Michigan.
As Fraser and Pamela commented on their Astronomy Cast episode about amateur astronomy, “Astronomy is one of the few sciences where amateurs make can meaningful contributions and discoveries.” And here’s proof. So the rest of you, go out there and start looking!
Sorry to disappoint those of you hoping for some Martian fireworks the end of this month. NASA’s Near Earth Object (NEO) Program office has effectively ruled out the possibility of Asteroid 2007 WD5 impacting Mars. The probability of such an event has dropped dramatically, to approximately 0.01% or 1 in 10,000 odds of an impact. Observers also say the asteroid has no possibility of impact with either Mars or Earth anytime in the next century.
Recent tracking measurements of the asteroid from several Earth-based observatories have provided a significant reduction in the uncertainties of the asteroid’s position during its close approach to Mars on Jan. 30, 2008. The best estimates now have 2007 WD5 passing about 26,000 km (16,155 miles) from the planet’s center at approximately 12:00 UTC (4:00 am PST) on Jan. 30th. The NEO office at the Jet Propulsion Laboratory has 99.7% confidence that the pass should be no closer than 4000 km (2,485 miles) from Mars’ surface.
The 50 meter (164 feet) wide asteroid was discovered in late November of 2007 by astronomers at the University of Arizona as part of the Catalina Sky Survey. Other telescopes used to track the asteroid are the Kitt Peak telescope in Arizona, the Sloan Digital Sky Survey telescope in New Mexico, New Mexico Tech’s Magdalena Ridge Observatory, the Multi-Mirror Telescope in Arizona, the Mauna Kea telescope in Hawaii and the Calar Alto Observatory in Spain.
An impact on Mars by 2007 WD5 could have created a .8 km (1/2 mile) wide crater on the planet’s surface. Many scientists were excited by the prospect of such an event, one that could possibly be tracked by the many spacecraft orbiting and on the surface of the red planet.
NASA’s Spaceguard Survey continually searches for Near-Earth Asteroids such as 2007 WD5, and their goal is to discover 90% of those larger than 1 km in size. JPL’s NEO office says that goal should be met within the next few years. Each discovered asteroid is continually monitored for the possibility of impact on Earth.
