On Wednesday, September 29, Earth will dodge a cannonball: the Near-Earth Asteroid known as 4179 Toutatis will buzz by at a distance only four times the distance from the Earth to the Moon -- about one and a half million kilometers, or about a million miles. This is very close in astronomical terms. In fact, according to Jet Propulsion Laboratory radio scientist Steven Ostro, "The September 29, 2004, approach is the closest in this century of any known asteroid at least as large as Toutatis." But, as the wisdom goes, "close" only counts in horseshoes and hand grenades; Toutatis' flyby will have no effect whatsoever on us.
Although Toutatis won't hit us, that doesn't mean the close approach is uninteresting. It is rare to have an opportunity to study a small body up close without sending a spacecraft to it. Because of an orbit that lies nearly in the plane of the Earth's orbit, Toutatis has passed quite close to Earth on three previous occasions since its discovery, so it is already a very well-studied object.
Scientists are fortunate in studying Toutatis because it is in a very unusual rotational state. Most familiar objects in the solar system are in "principal-axis rotation," in which they spin like a top. But Toutatis is in a rotational state called "non-principal-axis," in which it has two simultaneous rotational motions with different repeat periods. The end result is a tumbling motion like that of a flubbed pass in American football. The tumbling motion means that, if you watch it for long enough, Toutatis presents every point on its surface to view from the Earth.
This asteroidal exhibitionism allowed Ostro and his coworkers to use the powerful radio telescopes at Goldstone in California and Arecibo in Puerto Rico to develop an extremely good physical model for Toutatis' shape. "I would say we have more detailed physical information about Toutatis than any other Near Earth Asteroid except for Eros," Ostro claims. Toutatis is a very irregularly shaped object measuring roughly 4.6 by 2.3 by 1.9 kilometers (2.9 by 1.4 by 1.2 miles). Unusual shapes are normal for such small bodies, which have very little self-gravity to tug themselves into spherical shapes as larger bodies do. "This is one of the things that makes the Near Earth Asteroid population so interesting," Ostro says. "It's a zoo. We see objects that look like perfectly featureless spheres as well as those with highly complex topography."
Toutatis' complex topography may indicate that, rather than being one solid object, it is in fact two objects closely bound together. A thin topographic "neck" separates two lobes, one of which is substantially smaller than the other. "I think there's maybe a 30% chance that it was once two separate objects," Ostro says. Based on his radar observations, "the smaller lobe might be a little rockier than the larger lobe. Also, its unusual spin state had to come from somewhere." The spin state could have been caused by the gravitational tug of the Earth on its irregular shape in a previous, even closer approach, or "the small lobe collided gently with the larger lobe in the catastrophic destruction of a much larger parent object. But nobody knows and there's no way to know. There's no way to test all these hypotheses about what the interior is really like and where the spin state came from. It's a mystery."
In fact, the individual histories of asteroids are unfathomably complex. Ostro explains why. "With the collisional history of Mars, you're dealing with a history of billions of years. But after the Late Heavy Bombardment, all the impacts did was to make holes in the ground. With asteroids, every time you smack one of these objects you can change it quite a bit. You can do everything from making an inconspicuous hole to disrupting the object completely. That basically changes it into a new kind of beast, maybe giving it satellites, utterly destroying any aspect of it that existed before the collision. So you have a huge spectrum of possibilities of collisions over the age of the solar system."
One open question about asteroids is how the different types of fragments of asteroids that have landed on the Earth -- meteorites -- relate to the different types of asteroids observed out there in space. Toutatis' spectral properties identify it as an S-class asteroid, a type that is common in the inner solar system. According to Ostro, that means that Toutatis could be either an ordinary chondrite -- the most common type of meteorite that fall on the Earth -- or a stony-iron meteorite. Since no one has seen an S-class asteroid in space that has subsequently plunged to Earth and been identified as a stony-iron meteorite, the composition of Toutatis is uncertain. But Ostro and his collaborators are poised to take advantage of Toutatis' close approach to try to tackle this problem.
The main characteristic that separates one type of meteorite from another is composition and therefore density. Chondrites are made primarily of rock and so have a lower density than stony-iron meteorites, which contain some metal. The density of an object depends upon its mass and its volume. Because of the radar observations of Toutatis conducted in 1992 and 1996, the asteroid's shape, and therefore its volume, is extremely well understood. But "it's hard to measure the mass of an asteroid," Ostro says. "You usually have to send a spacecraft there or hope that the object has a satellite." However, the close approach of Toutatis will permit a nifty experiment that will allow Ostro to determine the mass without having to go there.
The experiment exploits a subtle phenomenon known as the Yarkovsky Effect. The Yarkovsky effect is a tiny acceleration on a rotating object that is illuminated by the Sun. The object absorbs some of the solar energy as heat, which is re-radiated away over time. But the rotation of the body means that the energy is not radiated away in the same direction from which it was absorbed. This re-radiation produces a tiny force on the object.
In October, when Toutatis becomes visible in the northern sky, Ostro will use the Arecibo radio telescope in Puerto Rico to measure very accurately the range from Arecibo to Toutatis. He will be armed with a highly precise prediction of Toutatis' location based upon its gravitational interactions with other bodies in the solar system and its observed position in 1992. The tiny force produced by the Yarkovsky effect will make Toutatis' actual course deviate slightly from what Ostro has predicted. If he can predict the strength of the Yarkovsky force, and measure the resulting deviation accurately, then he can use Newton's Second Law (force equals mass times acceleration) to determine Toutatis' mass and therefore its density.
"We did this experiment on [asteroid 6489] Golevka last year and it worked," Ostro says. They determined that the Yarkovsky effect had deflected little Golevka by about 16 kilometers (10 miles) from its predicted path in 12 years, which allowed them to fix Golevka's density. The density was low -- much lower than that of most asteroids -- meaning that Golevka is likely a fractured object containing a significant amount of open pore space. For Toutatis, Ostro hopes to find a higher density, because if the density he finds is high enough, it would be possible to exclude chondritic compositions and say for sure that Toutatis is a stony-iron object.
Even with the mass known, however, there will still be much that is unknown about the interior, and therefore the history, of Toutatis. In particular, Ostro believes that "there's probably no way of knowing" whether Toutatis is one funny-shaped asteroid or two asteroids touching "until we send a human crew." There are no current plans for that, but "because it's in the plane of the Earth's orbit it's a good target. Who knows, maybe some time in this century, people will go there." Humans will certainly be keeping a close eye on Toutatis. Its orbit is well enough known that it's safe to say it won't hit the Earth anytime soon. But "it's in the ecliptic, so it makes a lot of close approaches. Chances are excellent that it will collide with the Earth someday."
Unless, of course, we are prepared to prevent such an impact. Interestingly, one method of impact prevention would employ the Yarkovsky effect. If we had several decades to centuries of warning, it could be possible to send a mission to an asteroid to dust one side with chalk or soot, amplifying the Yarkovsky effect in such a way that the object would be just slightly deflected onto a different orbital path that would no longer threaten the Earth. We're not yet ready to perform such interventions, of course. We have to finish finding all of the potentially hazardous asteroids, mapping their orbits to identify whether any of them represent a short-term threat, and studying their physical properties to learn what sort of deflection methods are most likely to succeed. In the meantime, we will continue to enjoy Toutatis' close approaches for what they are: an opportunity to study a funny little body with a fascinating history.
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