![]() ![]() ![]() This made selecting a site for sample collection more difficult. One of the consequences of the activity – explained by changes in temperature fracturing larger boulders and breaking up the rocks – is that the surface of Bennu is completely covered in rubble, much more than had been expected. The particles were less than a centimeter across, and most of them landed back on the asteroid – generally closer to the equator than the poles, which changed its shape over time. Because it is so small it was assumed that Bennu would be quiescent – it wouldn’t, for instance, be behaving like a comet and ejecting bursts of gas and rocks.īut because nothing in the solar system is simple, when OSIRIS-Rex got close to Bennu, it found that the asteroid was throwing small amounts of material from its surface. It looks a bit like an old-fashioned spinning top, or a rough diamond, pointed at the top and bottom and fatter in the middle. But it is oddly shaped and active – two things that were not known. The relief was because Bennu is small – only about 500 meters across – a fact that was known when it was selected as a target. And I still remember the emotion in the control room when Philae landed on comet 67P/Churyumov-Gerasimenko in 2014. ![]() That record is held by the NEAR spacecraft that made a controlled crash-landing on Asteroid 433 Eros in 2001. Why Bennu? And why the relief? After all, this is not the first asteroid that a spacecraft has visited – and it is not the first small body that has been landed on. It was completion of the touch-and-go manoeuvre that prompted the clapping and cheering in mission control. Or, rather, not to land, but to make a very rapid “touch-and-go” visit to the surface – where it would collect fragments of material to return to Earth. The spacecraft spent two years circling Bennu, making detailed maps of its surface, learning as much as possible about the asteroid before the next phase of the mission: looking for somewhere safe to land. The purpose of the mission was to characterize the asteroid, then bring some of it back for study on Earth. OSIRIS-REx (for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer)) was launched in September 2016, arriving at its target asteroid 101955 Bennu in December 2018. ![]()
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