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Dawn Begins Year-Long Vesta Fiesta
Columnist: Dr. James Green

Dawn Begins Year-Long Vesta Fiesta
18 July 2011

Beyond the orbit of Mars is a solar system region known as the Main Asteroid Belt, where asteroids of all sizes and shapes twirl and spin around our sun. Scientists believe these bodies are planetary building blocks. But some of these "blocks" are so large and mysterious that they are worlds in their own right.

One of these massive worlds has just received a visitor.

On July 15th, our Dawn spacecraft defined "Arriva!" in a new way when it went into orbit around Vesta - an enormous asteroid with an average diameter of about 330 miles. After a year there, Dawn will pull off an unprecedented feat - break away from its orbit around one solar system body and travel to a second one. The probe's next target is the icy dwarf planet Ceres, another totally unexplored world in the outer part of the asteroid belt.

Each of these two very different places has its own tale to tell about the solar system's early formation and evolution. Vesta's surface has been bombarded by meteorites for 4.5 billion years, so it may just be the oldest surface we'll study. Dawn's cameras will discover this ancient relic's story by studying lava flows and huge craters on that surface, including a giant crater the Hubble Space Telescope revealed in 1997. Ceres, about 590 miles wide, is the largest body in the asteroid belt and is actually classified as a dwarf planet. It seems to be like the icy moons we find in the outer solar system, while Vesta is more like the rocky, inner planets.

Why are they so different? Dawn will complete a detailed survey of both asteroids to answer that question and more.

The ion-powered Dawn has been traveling for nearly 4 years and is close to the end of its second loop around the sun. During its voyage, Dawn has been gradually climbing away from the sun and slowing down. The spacecraft had to be traveling at the same rate as its target - Vesta - for the asteroid's gravity to take hold of it and let it enter orbit there.

Getting Dawn into orbit is an amazing achievement. Instead of "fire the thrusters full blast," we just sort of slid into orbit, letting gravity grab the spacecraft with a light tug. This gives us great confidence that the big challenge down the road of getting into orbit around Ceres can be accomplished just as easily.

In early August, Dawn will begin its first science orbit at 1700 miles altitude.
For 3 weeks it will cease thrusting and will instead take pictures and science measurements and return them to Earth. At this altitude, it takes about 68 hours (a little under 3 days) to orbit once. For each orbit, Dawn will spend half of that time on the day side, pointing its camera toward the surface, taking pictures and spectra. Then for the half orbit on the night side, the spacecraft will point its antenna back to Earth to send the images and data home.

Next Dawn will thrust for a month to spiral down to around 425 miles altitude for the second science orbit. Here the orbital period is 12 hours. In this phase the probe will do some high-altitude mapping, study Vesta's topography via stereo images, and gather high-resolution data to map surface rock types.

Then for about 6 weeks the spacecraft will thrust and spiral still lower, to just 110 miles or so altitude. That's closer than the space station is to Earth. We'll have an even more fabulous view! Here Dawn will orbit once every 4 hours, taking measurements for its third science orbit. These observations do not depend on illumination by the sun like the camera and spectrometer observations do. Instead, in this phase, Dawn will focus on measuring Vesta's gravity with the greatest possible accuracy. It will do this by using its radio signal to measure the motion of the spacecraft in relation to Earth. Also in this science orbit Dawn's gamma ray and neutron detector will make measurements that reveal the elements present in Vesta's surface.

For now, we are viewing an incredible new world up close as we zero in on its southern hemisphere, looking at the huge central peak and wondering how it got there. It is believed that Vesta was nearly spherical at one time, and then a collision occurred in its southern hemisphere, blowing off an enormous amount of material where a central peak now remains. Is that peak part of the core or mantle exposed? Was it formed as a result of the impact, or did it arise from volcanic action? The Dawn team hopes to answer these questions. I can't wait!

We've come a long way since the first telescope sighting of an asteroid. In 1801, Giuseppe Piazzi spotted something shining like a tiny, faint star. That was Ceres. A few years later, in 1807, German astronomer and physician Wilhelm Olbers spotted Vesta in the night sky. Now we are traveling across the solar system for a closer look at these mysterious worlds. Come to the party. Arriva!

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About: Dr. James Green
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Dr. Green is the Director of Planetary Science at NASA Headquarters. He views his job as the keeper of NASA's planetary program and the top advocate for that program to flourish and grow.
Read More by Dr. James Green
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Last Updated: 18 Jul 2011