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FAQ - Asteroids & Comets

1. Is Ceres an asteroid or a dwarf planet? I have seen it classified as both.

On January 1, 1801, Giuseppe Piazzi discovered the first asteroid, 1 Ceres. In 2006, when Pluto was reclassified as a dwarf planet, Ceres, because of the new definition, was also classified as a dwarf planet. The asteroid designation of Ceres was not addressed and so it can be assumed that Ceres has dual classification: an asteroid and a dwarf planet. It should be noted that all of the dwarf planets have numbers attached to their names, as do all of the asteroids and Trans-Neptunian Objects. Thus, all dwarf planets technically have dual classification. When Pluto was reclassified, it became 134340 Pluto to signify the sequence of its numbering along with the asteroids and Trans-Neptunian Objects.

2. Are asteroids and meteoroids made of similar things?

Meteoroids are bits of solid material of different sizes orbiting the Sun. Some of the bits come from comets, some of the bits come from asteroids, and a very few from the Moon and planets. All those bits originally come from the same “pot”—the solar nebula—and so they are mostly made of the same stuff. However, some asteroids (and planets!) became hot enough to melt and differentiate, or separate, into distinct layers of rock and pure iron/nickel. Bits of material blasted off asteroids with those differentiated layers will have those compositions—rock or metal—and so be different from most meteoroids.

3. Why or how would studying these asteroids help our understanding of the terrestrial planets?

The following is from and is a good example of why it is important to study asteroids:

1) they tell us about the origins of our Solar System, 2) they help us understand more about the origin of life, 3) we may want to mine near-earth asteroids for metals, 4) they may someday threaten to collide with the Earth, 5) astronauts may go visit one, according to Obama's new plan for NASA.

4. Why is it believed that comets are derived from the Oort Cloud? What is in the Oort Cloud that may give birth to comets (ice?)?

There are two types of comets: 1) short period comets with orbits that are primarily prograde-- they orbit in the same direction around the Sun as do the planets; 2) Long period comets with orbits that can be either prograde or retrograde. This then leads to two models for cometary sources:

1) Short period comets come from a source at the outer edges of the known Solar System (a disk of material called the Kuiper Belt) and are sometimes perturbed (usually by interactions with each other) into orbits that bring them into the inner Solar System. This disk was predicted to be similar to the asteroid belt, but made up of icy bodies (colder out there) that formed beyond Neptune and have been there since the formation of the Solar System. We now have confirmed this and know of more than 1,000 objects and it is estimated that there are perhaps tens of thousands of objects larger than 100 km with a total mass at least 100 times as massive as the asteroid belt.

2) Long period comets, because of their orbits, are thought to come from a cloud of material (too far out to be seen at present though similar "clouds" have been seen around other stars) that is made up of billions of objects with a total mass several times greater than the Earth’s mass. It is theorized that, as the Solar System was forming, icy objects that resided (again similar to the asteroid belt) between Jupiter and Saturn were perturbed by the newly-formed Jupiter and Saturn and tossed all over the place, hitting the planets and their moons or being tossed completely out of the Solar System into interstellar space. However, a lot of this material did not get tossed far enough to escape the gravity of the Sun and got trapped in very distant orbits around the Sun (up to 50,000 times the Earth’s distance), creating this vast “cloud” of icy bodies. Every once-in-a-while, interactions among these objects (or the effects of passing stars) perturb some of these into the inner part of the Solar System to become comets. Space is so vast, that, even though there are billions of these objects, collisions are rare. So, to answer the last part of this question, all of these objects formed in the same way as the asteroids—made up of the same “building blocks” as the planets, but not large enough to become planet-size.

5. Why did the asteroid belt between Mars and Jupiter not form into a planet?

In a single word, Jupiter. Jupiter probably grew very large very quickly, probably in about 3 million years, early in the history of the Solar System. Much of the material that went to form Jupiter would have come from the region of the present asteroid belt and the remaining material was moved around enough by the gravity of Jupiter to prevent it from forming a planet.

6. How far out have we explored to know about the size of Near Earth Objects?

As of today, there are 6,950 known NEOs. Of these, 807 are larger than 1 kilometer in diameter. The discovery rate for the large ones has decreased significantly in recent years, implying that we are getting close to finding all of them! This is not true for the smaller ones. There is a mandate by NASA is find 90% of all NEOs larger than 140 meters. A recent report states that this will probably not be met (you must find them but we will not give you the money to support this effort). Larger ones can be seen farther away (out to the orbit of Mars), but smaller ones have to come much closer in order for them to be bright enough to be seen with even large telescopes. For a typical small asteroid, this may be only once in 5 to 10 years and one of the surveys has to be looking in the right place at the right time in order to see it.

7. How can we study so many Near-Earth-Objects as they have so far?

There are several levels at which one can study NEOs. Initially, an NEO is discovered by a telescope on the ground or in orbit around the Earth. Email alerts go out for observers, mostly amateurs, to observe these newly-discovered objects. These observations help us to determine an accurate orbit for the NEO. If the object is large enough or interesting enough, it will be observed with other instruments on the ground and it may be possible to determine its size, shape, rate of rotation, and composition. While there are nearly a thousand know NEOs, probably less than 100 or so have been observed by a variety of techniques.

8. How are the regular meteor showers we see related to the NEOs?

Generally, they are not! Most meteor showers are related to comets. The tails of comets are made up of dust and gas. This dust spreads out over time and orbits the Sun just as the comet does (but spread out in a sort of doughnut shape all the way around the orbit of the comet. When this dust intersects the orbit of the Earth (and the Earth is there), we get a meteor shower. This is why there has never been a meteorite fall from a meteor shower (the particles are too small).

However, in 1983, an NEO, 3200 Phaethon, was discovered by IRAS (Infrared Astronomical Satellite). Its orbit takes it with 0.14 AU (Astronomical Units, the distance of the Earth from the Sun) from the Sun, closer than any other known asteroid. It greatest distance from the Sun is 2.4 AU, well beyond the orbit of Mars. It orbit is comet-like (very elliptical) and its composition is unusual. For this reason, it is thought to be an extinct comet (it has lost most of its ice). To add to this, it was soon realized that its orbit is similar to the orbit of the Geminid meteors—making it the only asteroid (since it does not have a coma like a comet) the is associated with a meteor shower.

9. Do we have the technology to change the path of an asteroid?

At a basic level, the answer is yes. There are a variety of ideas for how to move an asteroid or comet (you do not want to blow it up). First, you need to be able to get to an asteroid and we have the technology to do that now. Next, what you want to do is nudge it ever so slightly. You can pull on it with the mass of the spacecraft (no, we cannot do it with a tractor beam as in Star Trek), so would need to send something as large as possible to the asteroid. The other choice is to push it. In this case, what you want to do is vaporize a part of the asteroid so that the material coming off would push the asteroid the other way (like a rocket). Ideas for doing this include nuclear bombs (we have these), lasers (we have these, but maybe not powerful enough), or you could even paint a part of the asteroid in order to change how it absorbs light! So, right now we cannot just go out and stop The Big One. However, we have enough ideas for how to do it, so that, hopefully, with enough warning, put the resources into the technology so that we can refine these ideas and build something big enough that will be capable of diverting an asteroid.

10. What is the difference between asteroids, meteors, meteorites, and comets?

Let us start with the easier one, asteroid vs. comet. The observational answer is that when you observe an object for the first time, if it is star-like, it is called an asteroid (which means star-like) and given an asteroid designation. If it is fuzzy (it has a coma), it is called a comet and given a comet designation. As I mentioned above, Phaethon is an asteroid (not fuzzy), but has some of the characteristics that we come to equate with comets: its elongated orbit, the fact that it appears that it once had a tail (the Geminids), and possibly its composition. This is why, even though they are given asteroid designations, NEOs are, to most people who study them, “objects,” not asteroids—some of them may be extinct comets! The same is true for the Trans-Neptunian Objects—most of them are icy, and they may be the source of many of our comets, but from the point of view of how they are designated and named, they are star-like and thus asteroids. To add to this, some asteroids have been “discovered” that, when we look at previous observations, were at one time comet-like, but now show no cometary activity. Also, several asteroids have been known, once they came closer to the Sun, to start showing cometary activity. The implication is that there is probably a range of objects out there that run the range from “traditional” asteroids to “traditional” comets and what they really are depends on where they were formed, where they are now, and what had happened to them since the formation of the Solar System.

Asteroids vs. meteoroids. Both of these are objects that orbit the Sun. There are definitions of what is a big meteoroid and what is a small asteroid. This is usually given in the range of a few tens of meters in diameter (the Royal Astronomical Society is proposing 10 meters on the large end, down to 100 microns on the small end). However, thanks to the ever-improving techniques for detecting NEOs, we are seeing ever smaller and smaller objects. In reality, these are large meteoroids (down to a few meters in diameter), but once they are actually observed and an orbit determined for them, then they get a designation and maybe even numbered. At that point, they are technically asteroids.

Now, a meteoroid/asteroid enters the Earth’s atmosphere. The object itself is still a meteoroid or asteroid (or comet). The glow we see is what we call a meteor. If it is bright, we call it a fireball. If it is really bright, brighter than a full moon, it is called a bolide. People like Betty who study what happens when things hit the Earth or other solid surfaces will sometimes use the term bolide to define something that makes big holes in the ground. They call them bolides so they do not need to worry about whether it was an asteroid or a comet. Eventually, the meteoroid slows down (asteroids probably do not slow down very much and vaporize on impact) due to the effects of the atmosphere. At that point, it is assumed that they are now “part of the Earth” and will leave “rocks” that we can pick up—these are then meteorites.

11. Do asteroids and meteoroids orbit on the same plane as planets or do they have their own plane(s)?

This is where the terminology for asteroids and comets gets fuzzy (pun intended). When a new object is discovered, it is given an asteroid designation if it is star-like or comet designation if it shows a coma. There have been a number of cases where asteroids have “turned on” and become comet-like (got closer to the Sun) or when comets have ceased to show cometary activity. It is believed that as much as 10% of the Near-Earth Objects are actually extinct comet nuclei. More than 40 asteroids have “unusual” orbits: highly elliptical, highly inclined, and/or retrograde (orbit the Sun opposite the planets and most of the asteroids). Many of these have orbits similar to that of Comet Halley. This means that these are really dead comets that originated far from the Sun. Forty out of several hundred thousand is not many. Other than these, most asteroids have orbital inclinations less than about 30 degrees. Below is a plot of these orbits.

12. Which asteroids are being considered for possible landings?

Probably not B612! The Japanese Space Agency, JAXA, recently returned a spacecraft from the 500-meter asteroid 25143 Itokawa. The Hayabusa spacecraft touched down on the surface of Itokawa and may have returned a dust sample from Itokawa. There are a number of planned robotic sample-return missions to asteroids. JAXA and ESA (European Space Agency) are both planning missions to the asteroid 162173 1999 JU3. The University of Arizona has proposed to NASA to send a sample-return mission to 101955 1999 RQ36. This is an asteroid that has a very small chance (about 0.07%) of hitting the Earth in 160 years! Finally, NASA is looking at the possibility of sending astronauts to one of two asteroids: 2009 OS5 (launch date in 2020) which has a diameter of 60 meters or 1999 AO10 (launch date in 2025) which is 100 meters in diameter. The reason for the different asteroids is that opportunities to reach any particular Near Earth Asteroid do not happen very often (decades apart).

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