With two months of observations at an altitude of 915 miles above Ceres under its belt, NASA’s Dawn spacecraft fired up its ion engine to change its orbit one last time.

On Friday, Dawn began maneuvering into its fourth and final science orbit above the dwarf planet of Ceres. It will take seven weeks for the spacecraft to descend nearly 700 miles to a new orbit just 235 miles above Ceres’ surface. By mid-December, Dawn will be capturing images at a resolution of 120 feet per pixel.

Dawn’s gamma ray and neutron detector (GRaND) will get a workout at this new orbit altitude. The GRaND instrument measures the energy from gamma rays and neutrons to figure out what the surface of Ceres is made up of. Ceres is thought to be rich in water. Data from the GRaND instrument should be able to tell the Dawn team if their theory is right. Scientists are hoping this instrument will shed more light on the curious bright spots sitting in the Occator crater.

Ceres bright spots mosaic

Mosaic taken from an altitude of 915 miles at a resolution of 450 feet per pixel.

Not ice

Dawn’s science team knows the bright spots aren’t ice. The albedo, or reflectivity, of the spots rules out ice. So, what are they? Scientists are leaning more towards salt. Here’s Dawn’s principal investigator Chris Russell talking about the bright spots earlier this month.

“We believe this is a huge salt deposit,” said Russell (fast forward to 15:30). “We know it’s not ice, and we’re pretty sure it’s salt, but we don’t know exactly what salt at the present time.”

Where did the salt come from? Now this is where it gets interesting. Ceres may be active. “Some comet or asteroid did not come in carrying salt to this object,” said Russell. “This is derived from the interior somehow.”

Dawn gets one more shot to figure out exactly what these bright spots are.

Dawn’s future

Fuel. It’s the one resource that determines how long missions like Dawn last. Without hydrazine fuel, Dawn can’t point its instruments at the surface. It can’t point its antenna at Earth. And it can’t point its solar arrays at the Sun.

Once Dawn runs out of hydrazine fuel, the science gathering is done. That doesn’t mean the discoveries are finished, though. Scientists will pour over data from Dawn and publish studies for years to come.

So, will Dawn just crash into Ceres? Not for a while. Because Ceres is believed to have water ice, NASA’s Planetary Protection Office required the Dawn mission team to set an orbit that avoids Ceres’s surface for at least 20 years. The Dawn team went even further and chose an orbit that won’t impact Ceres for at least 50 years.