Although they’re both enormous asteroids, protoplanets really, and lie within the asteroid belt between Mars and Jupiter, Vesta and Ceres couldn’t be more different.
Vesta formed closer to the Sun, and probably shares many features of the inner planets. Scientists believe it formed in a hot, dry environment and will probably have layers of volcanic flows and a solid metallic core. But even the best photos from Hubble show a blurry gray world, bringing more questions than answers. It’s the brightest asteroid in the Solar System, measuring 530 km (329 miles) across. You can even see it with the unaided eye; in fact, it’s the only main belt asteroid you can see. Traveling to Vesta could be a little dangerous. “We know very little about Vesta’s internal structure,” explained Chief Engineer Dr. Marc Rayman, “it
has an unpredictable and possibly very irregular gravity field.”
Just a little further out – across an invisible line that separates the inner rocky planets from the outer planets – is Ceres; the largest asteroid in the Solar System, measuring 957 km (595 miles) across. Unlike Vesta, Ceres is believed to have formed in a cool, wet environment, and in the presence of water. This water is probably still there, in the form of ice caps, a thin water vapour atmosphere, or even as a liquid underneath the surface.
While most of the objects in the asteroid belt are pulverized chunks of rock, accumulations of material from different bodies, Vesta and Ceres remain largely unchanged from when they first formed 4.6 billion years ago. Revelations about the early history of the Solar System could be written on their surfaces.
The $370 million US spacecraft is scheduled for liftoff in June, 2006. After 4 or 5 years of travel time (depending on whether or not it’ll be making a flyby of Mars first) Dawn will arrive at Vesta in 2010 or 2011, studying it for almost a year before flying off to rendezvous with Ceres three years later. It has a suite of scientific instruments on board to study the two asteroids in great detail: their mass, volume, spin rate, chemical and elemental composition, and gravity. Oh, and it’ll be taking pretty pictures too.
Dawn will be the first spacecraft ever to orbit two separate objects in the solar system (and no, orbiting the Earth doesn’t count here). A feat that wouldn’t even be possible without its ion engine. A very similar engine helped Deep Space 1 set speed and duration records, and served as a model for Dawn’s development. It uses solar electricity to ionize xenon atoms and then hurl them out the back of the spacecraft. The thrust is tiny but fuel efficient, and the engine can keep running for months or even years providing a tremendous velocity.
And an ion engine gives controllers flexibility. “It gives us a very long launch window. We’re launching in June 2006 because that’s when the spacecraft will be ready. But we could still make it in November or even after that,” said Dr. Rayman. So far, though, the project is right on schedule. The completed spacecraft shipped this week from NASA’s Jet Propulsion Laboratory to Orbital Sciences for the next stage of assembly and testing.
If you’re interested in finding out more about this mission, stay tuned. Dr. Rayman is planning on keeping the world well informed, through the Internet. He learned how important this can be while working on Deep Space 1, taking the unusual step – at the time – of maintaining a web log to describe his experiences working with the spacecraft. “I was in the airport when I realized that we needed to get the word out. I dictated my first entry over the phone,” recalled Dr. Rayman. Rayman continued maintaining his popular DS1 blog, giving armchair mission controllers a unique insight into the day-to-day challenges and decisions that go into managing a spacecraft half a solar system away.
Expect more of the same with Dawn. “These missions belong to more than just NASA, or the United States. They’re humanity’s emissaries to the cosmos, and we want everyone to come along for the ride,” explained Rayman. But this time, he’ll get started earlier, bringing an Internet audience into the development stages as well as post-launch.
Written by Fraser Cain