25143 Itokawa
Published on Oct 13, 2006 at 1:03 am.
3 Comments.
Filed under asteroids.
I’ve been updating some of the materials for my planetary astronomy class. That is one thing that I like about this subject, it never gets boring. There is always something new, even at the freshman level. Among the things that I have been working on is my presentation of minor planets (asteroids would be the layman’s term). Back when I first began teaching this class, comets, asteroids, and meteorites were at the very end of the semester — something that we got to if we had time. They were miscellaneous topics. They were small things of little account. Now, I think of them as important relics of the formation of the Solar System, so I move discussion of them far forward in the semester, to near the beginning (even though they are still at the end of the planetary section of the textbook).
One of the new things that I can talk about is asteroid 25143 Itokawa. This small asteroid was discovered in 1998 by LINEAR (a project to detect and catalog near Earth asteroids). The asteroid first was called 1998 SF36, but was renamed Itokawa after it was selected for the Japanese Hayabusa spacecraft’s asteroid rendezvous. 25143 Itokawa is classified and Apollo asteroid (one whose orbit crosses inside Earth’s orbit), with a perihelion just inside Earth’s orbit, and an aphelion just outside Mars’ orbit. Studies of its orbit show that it most likely originated in the main asteroid belt, but near a resonance with Jupiter, so it got kicked out of its orbit. Now it flies around the inner Solar System. Gravitational interactions with its orbit make the orbit chaotic, changing parameters in an unpredictable manner. It’s orbit can only be computed a century or two into the future, but it appears that it will most likely run into one of the inner planets (or the Sun) within the next few tens of millions of years. Earth is one of those inner planets in its path.
Long before the Hayabusa spacecraft visited Itokawa, it was already known to be a strange asteroid, composed of two lobes at an odd angle to one another. Hayabusa provided the first detailed photos and measurements of it. The asteroid measures about 535 by 294 by 209 meters. It consists of the two lobes, but is also apparently composed of a large number of fragments and boulders, with over 500 boulders being 5 meters or more across. With a density of 1.9 g/cm3, Iotokawa has a porosity of about 40% (about that of course loosely packed sand). A smooth patch near the junction of the two lobes appears to be mostly gravel.
Many asteroids are described as being “rubble piles,” but 25143 Itokawa seems to be even more of a pile of rubble than most. Data suggest that perhaps Itokawa may once been a more solid body that was blown apart by a collision in the past. The pieces then coalesced into two loose bodies that then merged. This model also is consistent with the observed rotational axis of the asteroid. Itokawa rotates about a point perpendicular to its long axis, 
as you’d expect if it were formed by two bodies orbiting one another and coming together. The rotational period of 12.13 hours is slow enough to keep the pile of material from coming apart. This is important, because the escape velocity is only about 10 cm/s. The sparsity of craters suggests either that the surface is very young, or that impacts rattle the asteroid enough for the loose material to move around to fill up craters. The smooth region between the lobes is likely smaller material that has settled in that position between where the two parts join.
The 
Hayabusa spacecraft was designed to approach the asteroid and take samples of its surface. You can see in these photos the shadow of the spacecraft on the surface of the asteroid and an artists impression of the spacecraft at the surface. Unfortunately, there were several problems with the maneuver, and it appears to be unlikely that any samples were retrieved in any quantity. 
Even without a sample return, though, the Hayabusa mission was definitely a success, and a triumph for the fledgling Japanese space agencey. The data returned have shown this to be an asteroid unlike the others that have been studied up close by NASA probes.
One reason that 25143 Itokawa is of interest is the real (though very small) danger that it poses to Earth. Of greater interest, though, are other asteroids that are more likely to impact Earth. What would we be able to do if we were to find an asteroid on collision course with Earth? In science fiction it is easy: we just blow it up or push it out of the way. However, this thing is so poorly put together than any attempt to blow it up would likely succeed, only to produce a giant swarm of smaller bodies which would strike all over one hemisphere of Earth. Any attempt to deflect it may have a similar effect, given its fragile nature. So, we need more study.
We have in years past tended to think of asteroids as being pretty much alike. However, of the asteroids visited by spacecraft, Itokawa, Eros, Mahilde, Gaspra, and Ida, no two have been just alike. These bodies may be far more varied and complicated than we would like to think. Again, we need a lot more study.
-Astroprof
(Images courtesy of JAXA)
A Ler…-- Rastos de Luz on October 13, 2006 at 5:20 pm: 1
[…] “25143 Itokawa“, no Astroprof’s Page. Um artigo sobre o asteróide visitado pela sonda japonesa Hayabusa; […]
Astroprof’s Page » 433 Eros on February 14, 2008 at 9:10 pm: 2
[…] As NEAR studied Eros, it became apparent that the asteroid seemed to have striations or layers along its length. One possible explanation for this is that Eros may have once been part of a larger planetesimal that was shattered by a collision long ago. Again, that may be consistent with it being a mostly solid body. Part of Eros’ odd shape comes from a very large impact crater on one side. The interior of this crater is much less cratered than other parts of Eros’ surface, indicating that the impact may have been of fairly recent origin. When NEAR got close to the surface of Eros, images showed a lot of large rocks laying around. These rocks appear to be laying on the surface of Eros, rather than the asteroid being simply a pile of rocks, like the asteroid 25143 Itokawa that was visited by Japan’s Hayabusa spacecraft. These rocks laying all over Eros may be material thrown out by the large impact that created the large saddle shaped impact crater. Furthermore, craters near to and on the opposite side of Eros from this crater are fewer and rounder than other craters on Eros. Other asteroids, such as 253 Mathilda, have many such smooth craters. Those asteroids are believed to have smooth craters because those bodies are more porous and not solid like Eros. But, Eros does have some sharper defined craters, showing that such craters can form there. It has been speculated that the smooth craters near and opposite the larger impact feature may have been shocked and shaken by seismic waves from the large impact. That may have broken up the craters and caused them to soften in appearance. […]
Astroprof’s Page » Tunguska, one century later on June 30, 2008 at 4:12 pm: 3
[…] But, how can a huge chunk of rock not make it through the atmosphere? Well, remember the asteroid Itokawa. That is an example of a rocky asteroid that is not a solid chunk of rock. It is at best a […]