FRED ANSWERS YOUR QUESTIONS

Why does asteroid 2014 JO25, which recently flew by Earth, look like two that have been welded together? This isn’t the first like this. Is it electrostatically welded?

John Maksuta, New York

These ‘rubber-duck’ asteroids aren’t welded as such but, like comet 67P/Churyumov-Gerasimenko that was visited by the European Space Agency’s Rosetta spacecraft in 2014–16, consist of two chunks that have come together through their mutual gravities. They have attracted a dusty collar between the two halves, giving the appearance of being a single object, but remain essentially separate bodies. Gravimetric analysis of 67/P by Rosetta proved this to be the case.