I admit to being a bit sad that the last space shuttle mission is in progress (the Atlantis is due to return to Earth on July 20). I remember how impressive the first mission was, 30 years ago. We watched its launch at the planetarium at OMSI (the science museum in Portland), and we anticipated a new era of inexpensive space travel; the shuttle was supposed to be launched so frequently that its missions would cease being newsworthy. Of course, that didn't happen; there were highlights such as Hubble telescope repairs, but there were also the terrible disasters that destroyed two shuttles and took 14 lives. And the shuttle was vastly more expensive per mission than originally promised (it would have been cheaper to build and launch a new Hubble telescope than to repair it in orbit).
But while everyone is following the Atlantis, and discussing the fate of the American manned space program, there is another remarkable mission in progress that has reached its first objective: the Dawn spacecraft is this evening entering into orbit around the asteroid Vesta. This is the second-largest asteroid, a more-or-less round proto-planet 350 kilometers in diameter. Dawn will remain in orbit around Vesta until July of next year, when it will depart for its next destination: the largest asteroid, Ceres (975 kilometers in diameter). The spacecraft was given the name Dawn because its primary scientific goal is to study the origins of the solar system preserved in the asteroids.
Dawn will enter into orbit around Vesta tonight, but the first imagery and scientific results are due back in the first week of August. NASA will celebrate a "Vesta Fiesta" August 5-7.
One interesting thing about Dawn is its use of solar-powered ion engines for propulsion. These don't provide quite as much thrust as the TIE (twin ion engine) fighters in Star Wars: only 91 mN (millinewtons). One newton is 0.22 pounds of thrust, so the ion engines on Dawn provide about 0.0018 pounds of thrust, about the weight of a piece of notebook paper according to the NASA Dawn website (look for the page about the spacecraft itself). This is very small, but the engines can run for years at a time, and the velocity builds inexorably. If Dawn had relied on chemical rockets, it would have needed to be far bigger and much more expensive.
Now the one thing that Dawn will not do is land on Vesta, much less bring samples of Vesta back to Earth. But it turns out we already have samples of Vesta: certain kinds of meteorites are known to have come from Vesta. Something big hit Vesta long ago, and produced meteoroids that eventually found their way to Earth.