2007 WD5
Published on Jan 27, 2008 at 4:37 pm.
8 Comments.
Filed under asteroids.
Yesterday, I wrote about the asteroid 2007 TU24 that is about to pass close to Earth. Interestingly, there is another planetary near miss going on. Another asteroid, also discovered by the Catalina Sky Survey late last year, is about to pass very near a planet. But, this time, the planet getting the near miss is Mars. The asteroid will pass very near Mars on January 30, 2008.
This asteroid created quite a stir in the blogosphere and in astronomy news because for a while it was widely reported that it might hit Mars. However, with refined measurements of its position, the orbit has been computed much more accurately. It now seems almost certain to miss Mars.
Again, as with 2007 TU24, there has been a lot of misinformation about 2007 WD5. At no time was Mars every in any danger whatsoever of being blown apart by the impact. The asteroid itself is fairly small. It is only about 160 feet across. Now, if you happen to be near where it hit, you’d be in trouble. But, the impact and resulting explosion would not harm things a hundred or more miles away, though you’d still notice it. 2007 WD5 is probably about the same size, or only slightly larger, than the impactor that created the Tunguska blast in 1908. At that time, a body of comparable size hit Earth, creating a massive blast equivalent to about two dozen megatons of TNT, wiping out a forest near the Tunguska river in Siberia. While that is a big explosion, it is just a mosquito bite to a planet the size of Mars. Scientists at NASA’s Jet Propulsion Laboratory (JPL), rather than being horrified about the possible fate of our two rovers Spirit and Opportunity, were rather excited to have them there to witness any aftermath of the explosion. But, now it looks like the asteroid will pass harmlessly past Mars.
Even at the height of all the speculation about the asteroid hitting Mars last December, I kept quiet about it. I figured that I had a month to write about it. And, besides, a lot of other people were writing about it. Even at the highest probability of impact, there was only about a 4% chance of the asteroid hitting Mars. I decided to wait and see what later refinements of the orbit yielded. Now, it seems that the asteroid is likely to miss. So, why didn’t we know that earlier? What was up with all of the talk of an impact? This sort of thing has happened before, where astronomers find an asteroid and there is a lot of talk about it possibly hitting Earth, and then they say, “Oh, never mind.” How does this happen? Can’t they get the orbit right?
Well, it is not that simple. With the distances involved in interplanetary space, even a tiny difference in any parameters of an orbit (speed, position, inclination, eccentricity, etc) can mean that the path of an orbit can be shifted by tens of thousands of miles, or even much, much more. This asteroid was only recently discovered (in November). It is one of a group of asteroids known as Apollo asteroids, whose orbits cross that of Earth. But, it the few observations of it were only good enough to show that it was passing near Mars at the end of January. The cone of error was very large. Consider, for example, this schematic:
This is not exactly what the error situation was, but it is illustrative of the concept. Imagine that Mars is the small red circle. The asteroid is on an orbit that is calculated to pass very near Mars. The orbit may, for example, pass near the center of the large light blue ellipse. However, there are small uncertainties in the orbit of the asteroid. So, knowing the level of uncertainty, one can estimate about how far from the calculated orbit the asteroid might actually pass. So, depending on the exact values of the orbital parameters, the asteroid might pass Mars anywhere in the large light blue ellipse. Well, that ellipse overlaps Mars, so that means that an impact is possible. Not, possible does not mean for certain. There is far more of the ellipse that misses Mars. If Mars occupies only 2% of the ellipse, they you would say that there is about a 2% possibility of a collision. If Mars occupies 5% of the ellipse, then there is about a 5% chance of collision. (Note: it is actually a little more complicated than that, but you get the rough idea.)
Now, 2007 WD5 has an orbit that nominally crosses Earth’s orbit, making it a potentially hazardous asteroid. However, we can compute its likely path for some time to come, and we see that it passes nowhere near enough to Earth to pose a danger in the foreseeable future. So, normally that would mean that the asteroid would be monitored, but it would likely only get enough observations to make sure that we knew where it was so that we could go find it again later to refine our understanding of its orbit. But, with the possibility of the asteroid hitting Mars, it got a LOT of attention. So, more observations narrow the errors and uncertainties in the measurements of its orbit. Eventually, a refined orbital path is determined. Unless the original observations and calculations were way off, then the refined pathway will be in the error cone of the original calculations. The next image shows a schematic of that that might look like.
Again, this is simply a schematic to illustrate the point, it is NOT an actual representation of the error cone of this particular asteroid. The additional observations yield a more precise measurement of the orbit, but still not exact. So, there is still some error possible. The uncertainly in the orbit, though, is far less, so the area of uncertainty is much smaller, as illustrated by the small dark oval. Here, we see that, for illustrative purposes, the area of uncertainty barely touches Mars, so the chance of a collision would then be considered far lower. Additional observations refine the orbit even more, and the uncertainly gets correspondingly less. Eventually, what may happen is that the final area of uncertainly misses Mars altogether, so we can say the the planet will be safe from an impact. This is what has happened with the asteroids that initially were claimed to pose a possible threat to Earth, too. Later observations narrowed the uncertainty in the orbit to the point that it became clear that the asteroid would indeed miss Earth. An important point, though, is that at no time did the calculations indicate an actual collision. The range of uncertainty for the orbit, though, overlapped the planet.
A great many writers were going on about the upcoming impact as if it were a near certainty. But, I wanted to wait until better data were in. There was never a greater chance than 4% of an impact. Put it another way, there was a 96% chance that nothing would happen and that the asteroid would pass harmlessly past Mars, albeit very close to the planet. A few bloggers, such as Phil, did in fact stress the low probability of an impact, even at its most probable. Since he had already posted about that, and a lot of my readers also read his site, then I decided to hold off for a while. But, now that the date of passage is near, I figured it would be an interesting topic to write about. The latest estimate that I’ve heard is that the chance of collision has now dropped to less than a 1 in 10,000 chance. In other words, there is better than a 99.999% chance that the asteroid will miss Mars. Still, it is interesting, and very instructive, how the story has developed. Remember this the next time that you hear about an asteroid that has a chance of hitting Earth.
-Astroprof
Jeffrey Simpson on January 27, 2008 at 5:10 pm: 1
I remember a couple of years ago when an asteroid hit 1 or 2 on the Torino scale (perhaps 99942 Apophis?) that as more observations came in, the probability increased and then suddenly dropped to zero. The explanation was that you start off with the very large area it could possibly pass. With more observations, this area gets smaller, but since the cross-sectional area of the Earth stays the same, the probability of hitting Earth increases. This goes on until the uncertainty shrinks to the point that the Earth is no longer inside the uncertainty area.
Ed Minchau on January 27, 2008 at 6:11 pm: 2
“…the chance of collision has now dropped to less than a 1 in 10,000 chance. In other words, there is better than a 99.999% chance that the asteroid will miss Mars.”
There are one too many nines there. 99.999% would be a one in one hundred thousand chance.
Astroprof on January 27, 2008 at 6:12 pm: 3
Yes, that is basically what happened in this case, too. The area of uncertainly dropped, but Mars was still in it, so the probability seemed to go up. Then, the area of uncertainty narrowed so much that it missed Mars. And, yes, Apophis reached the highest yet on the Torino scale until more observations came in and it became clear that it would miss us (at least in the near term).
Astroprof on January 27, 2008 at 6:13 pm: 4
Oops. Thanks for catching that, Ed. It should be 99.99% chance of missing.
Bruce on January 28, 2008 at 5:30 pm: 5
Nice explanation on this subject! I was inspired to write a post recommending this one and adding some follow-up comments:
http://flyingsinger.blogspot.com/2008/01/astroprof-on-asteroid-uncertainty.html
Asteroid TU24 and the NWO « Children of the Lens, Seekers of Tomorrow on January 29, 2008 at 8:34 am: 6
[…] Much tinfoil and conspiracy theory has been spun concerning these events, but in fact, these events might be common (link). We’re just more aware of them now because we have good evidence that an asteroid wiped out the dinosaurs (link) and we’re not too keen about the same thing happening to us. Except for the politicians, who obviously think that ignoring the problem ( or are they? ) will make it go away. Probably because they already have safe havens underground to save theirs’ and their families hides in case of an Extinction Level Event (link). If it weren’t for college astronomers, we would know next to nothing about this (link). […]
Pyro on January 30, 2008 at 5:56 pm: 7
Ok i heard some rumors that WD5 have some chances to hit us(earth) on 13th of april. Is that true ?
Astroprof on January 30, 2008 at 6:08 pm: 8
Pyro,
2007 WD5 does not pose any danger at all to Earth in the foreseeable future. As for April 13, it will be nowhere near Earth.