Asteroid (25143) Itokawa is a peanut-shaped near Earth asteroid that doesn’t actually present a threat to Earth, but has given us an amazing opportunity to study such asteroids up close. Although a lot smaller than their cousins Vesta and Ceres which are being visited by the Dawn spacecraft, such near Earth asteroids are important for studying the possibilities for visiting such an asteroid, future asteroid mining, and, oh yeah, learning how to deflect one if it ever does come our way.
Itokawa was visited with the Habayusha spacecraft in 2005. Its odd shape seems to be covered in boulders and debris, making it a poster-child for the “rubble-pile” model of asteroids. Though rocky, its overall density was only about 2 grams per cubic centimeter, twice the density of water but less than that of most types of rocks. If an asteroid like Itokawa came hurtling towards us, it wouldn’t be one solid rock that needed to be deflected, but a whole pile of them. Suddenly, Sending Bruce Willis with a drill team and a nuke doesn’t seem like such a good idea.
There is more that can be learned and, in fact, needs to be learned before we can be really confident about visiting, mining, or stopping these asteroids. Luckily, astronomer Stephen Lowry of the University of Kent in the UK and his collaborators have been monitoring this asteroid from 2001 to 2013, before and after the Habayusha closes approach. By taking many observations of the asteroid over that time period with the 3.58-meter New Technology Telescope in Chile, they produced a light-curve of the asteroid. Since the asteroid is oddly shaped, that tells you how much of its surface is reflecting sunlight back at us. With that information and a highly detailed shape model from the Habayusha data, they could track the rotation of Itokawa with extreme accuracy. The rotation period was changing by a small factor, 0.045 seconds per year.
Read that again… they measured a rotation change of 45 milliseconds per year. Wow. This change can be explained by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. That mouthful means that as the asteroid absorbs heat from the Sun, it re-emits it in a certain way depending on the asteroid’s shape and composition. In order for the rotation change that was measured to be true, the center of mass of the asteroid had to be 21 meters away from where astronomers thought it was. THAT means that one side of the asteroid is quite a bit denser than the other. A small chunk of it is 2.85 grams per cubic centimeter, and the other side is 1.75 grams per cubic centimeter. Altogether, it makes the whole asteroid the 1.9 grams per cubic centimeter that was measured by the spacecraft. Below is a schematic of the different parts of the asteroid.
So with this clever and dedicated observing technique, we have our first look at the interior and composition of a near-Earth asteroid. Pretty cool, huh?
Remember, you can help us map the surface of one of the largest asteroids in the solar system, 4 Vesta, with data from the Dawn spacecraft in Asteroid Mappers. There are LOTS of boulders and craters still to be measured and marked there. Meanwhile, we anxiously await Dawn’s arrival at Ceres, and you can follow its progress on the Dawn website.