2008 BT18 Passing Earth
Published on Jul 13, 2008 at 1:14 pm.
3 Comments.
Filed under asteroids.
Tomorrow, July 14, asteroid 2008 BT18 will pass Earth at a distance of only 0.0151 AU (less than six times the distance between the Earth and the Moon). Asteroids pass Earth all the time. It is actually pretty sobering to realize just how many of those things are going by. When I first began teaching astronomy, asteroids were sort of a footnote to the class. Once you had covered all the important things, like planets, you talked about the miscellaneous other things. Many books even titled the chapter on asteroids “Miscellaneous Other Solar System Objects” or “Solar System Debris.” The implication was that these bodies were not important. But, even back then, we knew that asteroids occasionally run into Earth. Craters exist all over the planet. Granted, they are often hard to find because they are covered by erosion and tectonic processes, but they are here. Astronomers knew of a few dozen asteroids whose orbits come near Earth. But, then technology became available to permit extensive surveys looking for these bodies. We now know of nearly a thousand such bodies that come close enough to be categorized as potentially hazardous, and more are being discovered all of the time. We also now realize that asteroids are an important part of the Solar System. Many are remnants of the objects that were coming together to form planets as the Solar System was forming. There are likely millions of the things out there. And, impacts are far more frequent than we had first suspected. So, when I teach my planetary astronomy class, I now move the discussion of asteroids up to before I cover the planets in detail.
The asteroid 2008 BT18 was discovered January 31, 2008, but the LINEAR program. 2008 BT18 is one of those potentially hazardous objects. Tomorrow it passes Earth, but at a very safe distance. In fact, for as far as we can reliably compute its orbit, it will continue to miss Earth. In fact, it may never run into Earth. But, it comes close enough that it needs to be watched. Above is an orbital diagram, courtesy JPL, showing the current position of the body. As with many Earth crossing asteroids, its orbit is not just near Earth. In fact, the semimajor axis of its orbit is 2.22 AU, out in the asteroid belt. But, it has a very elliptical orbit, ranging from about 0.89 AU out to almost 3.55 AU. Interestingly, the orbit of 2008 BT18 has a semimajor axis that is very close to one of the asteroid belt’s Kirkwood gaps. That may have something to do with its large eccentricity.
If that were all that there was to this asteroid, it would still be interesting to write about. However, there is more! It is close enough to Earth already that the giant radar at the Arecibo observatory is able to monitor the object. What researchers have found is that 2008 BT18 is not one asteroid, but two! A story at Spaceweather.com labels 2008 BT18 as a binary asteroid. However, the Arecibo radar image, shown here, seems to show the secondary as just a dot. Wouldn’t that mean that it is more like a moon than a binary asteroid? Well, that is hard to say just yet. Getting the size right on radar images is tough. But reports are that the primary (larger object) is about 600 meters across, and the secondary (smaller object) is about 200 meters acrossSo, we’ll have to wait a bit to see if 2008 BT18 is really a binary asteroid, or an asteroid with a moon. But, what is the difference? In either case, these bodies, as they orbit the Sun, will orbit each other around the center of mass point between them. If that center of mass point is located within the larger body, then the smaller one is a moon. If the center of mass point is located outside of the primary body, then they form a binary asteroid. The sticking point, though, is that if the orbit of the secondary is elliptical enough, then sometimes the center of mass point will be inside the primary, and sometimes outside of it. Then, what do we call these things? Also, as you can see from the radar image, the primary is not spherical. That is pretty typical of asteroids. Only the largest ones would be expected to be spherical. But, that might mean that the center of mass is sometimes inside the primary and sometimes outside of it depending upon the primary’s orientation! Now, I don’t know anything about the orbit of the secondary, so I am just listing the possibilities. But, this does illustrate how these things can get complicated.
Binary asteroids, or asteroids with moons, are not all that uncommon. In fact, a lot of asteroids seem to be double lobed, perhaps being a binary asteroid in which the two bodies have drifted together. We also have known that asteroids can have moons since August of 1993, when the Galileo spacecraft flew past the asteroid 243 Ida, discovering a moon as seen in the image below. That moon eventually was named Dactyl (243 Ida I).
We are still learning about asteroids. So, you can see why I now cover them early in the semester rather than at the end as a footnote to the Solar System. And, I seem to be winning converts to my way of thinking. After explaining what I do to other astronomy faculty, several have decided that my approach is a good idea and they are restructuring their courses to follow the same approach.
-Astroprof
Images courtesy JPL, Arecibo
Joshua Brotman on July 13, 2008 at 9:21 pm: 1
How can .0151 AUs be less than 1/6 the distance from earth to the moon?
1 AU is about 150 000 000km
and so 0.151 * 150m = about 2 000 000km
however, the distance to the moon is only 384 000km. Which would make it closer to six times /further/ than the distance from Earth to the Moon!
I think you just made a typo for LD
Joshua Brotman on July 13, 2008 at 9:23 pm: 2
Oh, wait. You’re right…
Ron Drummond on July 15, 2008 at 8:53 am: 3
My cousin John Drummond in New Zealand is an amateur astronomer at Possum Observatory, and he filmed the flyby:
http://www.possumobservatory.co.nz/asteroid_2008bt18_video-15july2008-41cm.htm