NASA’s Dawn spacecraft entered orbit around the asteroid Ceres on Friday morning at an altitude of just under 38,000 miles. Currently, the spacecraft is above the night side of Ceres, limiting the amount of information that can be gathered.
When conventional spacecraft enter orbit, they fire their engines for less than an hour to slow down and be captured by the gravity of their target. But Dawn is powered by an ion engine, which generates a small amount of thrust but can fire continuously for weeks, and there was no big change in Dawn’s course on Friday.
“Ion propulsion, with its continuous thrust, produces trajectories that don’t fit with intuition,” said Marc D. Rayman, Dawn’s chief engineer. “But any point along that trajectory after orbit capture, if we did stop thrusting, it probably would look to you more the way you think of an orbit.”
By late April, it will move into a 8,400-mile circular orbit where it can start examining Ceres in detail. Ceres, at almost 600 miles wide, is the largest of the asteroids and is now counted as a dwarf planet; it follows an orbit between Mars and Jupiter. Already, scientists are intrigued by bright spots on its surface, which may be reflective patches of ice or salt.
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