On July 16, the Dawn spacecraft went into orbit around the asteroid Vesta, the first spacecraft ever to go into orbit around an asteroid. Dawn has returned pictures like NASA – All Eyes on Vesta. It has a typical asteroid appearance, cratered and lumpy, though it is more spherical than the smaller asteroids. It also has some odd grooves.
Dawn is powered by ion engines, which make very fast exhaust (30 km/s), and which are powered by the spacecraft’s solar panels. This enables it to carry much less fuel than what it would otherwise need. By comparison, its booster rocket, the Delta II, has much slower exhaust, at 3 km/s. By simple Newtonian mechanics, that means needing over 13 times less fuel. But the engines have VERY low thrust, accelerating the spacecraft from 0 to 60 mph in 4 days. By comparison, a fully-loaded Delta II can go 0 to 60 mph in 2 seconds.
Not surprisingly, Dawn has taken a long time to arrive; it was launched nearly 4 years ago. Even so, it flew by Mars 1 1/2 years after launch to get a gravity assist. It will stay at Vesta for a year, then depart for the asteroid Ceres, arriving there 2 1/2 years later.
In other spacecraft news, the MESSENGER spacecraft went into orbit around Mercury last March, and it has been returning lots of pictures and other data ever since. It was launched 7 years ago, and it flew by the Earth once, Venus twice, and Mercury three times to avoid consuming a lot of fuel.