When we think of observing asteroids, the first things that come to mind, for me at least, are telescopic observations of small moving dots in the sky and space missions taking the iconic images we have seen of objects like Bennu, Itokowa, and Ida with its little moon Dactyl. It’s easy to forget that powerful radar facilities are able to obtain cool 3D shape models of passing space rocks.
Since 1968, NASA’s Deep Space Network has used the Goldstone facility to both communicate with distant spacecraft and to radar image nearby asteroids. We are lucky that small asteroids don’t get super close to Earth all that frequently, and large asteroids very rarely get anywhere near us. This means that it can be days between targets, and it has taken 53 years for Goldstone to rack up its thousandth rock.
That thousandth rock, currently designated 2021 PJ1, was observed on August 14. It was just 65-100 feet wide and passed at a distance of over one million miles, or more than four lunar distances away. Such a small rock at such a large distance didn’t allow for particularly interesting radar returns to be acquired, so we are actually showing you the August 22 radar returns of asteroid 2016 AJ193, which was twice far away but also a whopping forty times larger. This sequence shows how radar can be used to see both the shape of asteroids and to also capture their spin rates.
NASA JPL press release