NASA takes aim at bizarre little 'rubble pile' asteroid for future capture mission

Credit: NASA/JPL-Caltech/Northern Arizona University/SAO

Friday, June 20, 2014, 5:25 PM - It may not look like much in this image, taken by the Spitzer Space Telescope back in February, but Asteroid 2011 MD is a prime candidate for the asteroid capture mission NASA is planning for 2019, and it's proving to be quite a strange object - resembling a pile of debris flying through space rather than one solid rock.

Perhaps we're just so used to seeing amazing optical images from the Hubble Space Telescope in orbit or the any number of telescopes and observatories here on the ground, but the picture above probably looks more like something from biology than astronomy. This is because NASA's Spitzer Space Telescope is designed to look at a the universe in a completely different part of the light spectrum - the 'infrared' part, that's very useful for seeing through the dust that permeates our galaxy and for spotting small asteroids that whiz around near our home planet. Essentially, the image above is showing the heat given off by the objects in the view. There are several distant and dim stars in the view, but the orange splotch in the middle is what we're interested in. 

In visible light, it would be much harder to figure out what that is. It could be a tiny, bright snowball, or a massive dark rock, or something in between.

Credit: NASA/JPL-Caltech

So, that orange splotch is the heat given off by a 20-metre-wide asteroid named 2011 MD, that was discovered in the latter half of June 2011, roughly a week before it slipped by Earth at a distance of only around 12,000 kilometres. That's a little over 3 per cent the distance to the Moon, and less than half the distance to the ring of geostationary communications satellites that orbit the planet. Having plotted out the asteroid's orbit around the Sun, astronomers found that it very closely matches the orbit of Earth. It's just a little bit wider, and little bit more 'off-centre', giving the asteroid a year that's just shy of 400 days long. We don't see it very often. It came really close in 2011, but the next time our orbits sync up looks to be in the summer of 2023. The nice thing about this arrangement is that, while the asteroid is moving very quickly around the Sun, at that point in time, it will be traveling at roughly the same speed as the Earth. So, it would be fairly easy for us to send a spacecraft out there, nab it, and drag it back for study.

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One of the things complicating this is the fact that this asteroid appears to be more than just one asteroid - basically a rubble pile flying through space, kept together by the mutual gravitational pull between the chunks of rubble. The astronomers figured this out by combining visible-light images from back in 2011 with these new Spitzer observations. This allowed them to estimate the mass and density, and rather than coming up with a density close to rock, it turned out to be more like one-third that - closer to water. Since you're not going to have a great glob of water floating around in space (only Earth's atmosphere allows it to have consistent liquid water on its surface), this has to mean that only around one-third of the visible volume the asteroid fills is actually rock. The rest is empty space!

"With Spitzer, we have been able to get some of the first measurements of the sizes and compositions of tiny asteroids," said David Trilling, the astronomer from Northern Arizona University that led the team that checked out 2011 MD, according to a NASA statement. "So far, we've looked at two asteroids and found both of them to be really weird - not at all like the one solid rock that we expected. We're scratching our heads."

Credit: NASA/JPL-Caltech

These rubble piles are thought to form when two asteroid smack into each other, but at just the right angle and impact so that the resulting debris manages to clump back together over time, to continue on a mutual journey around the Sun. According to the astronomers, 2011 MD could be a larger rock surrounded by a cloud of dust and smaller debris, or it could resemble a collection of boulders. Astronomers have known about this kind of asteroid for some time, and they've discovered other examples of it, and each new discovery is giving them more insight into their nature.