NASA’s Dawn mission to Vesta and Ceres, the two largest bodies in the main asteroid belt, is nearing the end of its mission as a key fuel runs low. Hydrazine is used to control the spacecraft’s orientation in space. And Dawn only had as much as it carried from Earth. When Dawn runs out of hydrazine, it will no longer be able to point its antennas toward Earth for communication. Once communication is lost, mission managers will continue to attempt communicating for a limited period – a couple of days – to verify that there is not an unexpected problem. If there is no response, they will officially end Dawn’s mission.

Deep Space Network

Science ‘til the End

Dawn will continue to gather science data about Ceres and return it to Earth, right up until the spacecraft can no longer do so. The spacecraft has been and will be collecting high-resolution gamma ray and neutron data, images, infrared spectra and gravity data from an altitude as low as 22 miles (35 kilometers). These observations initially will be focused on a region that contains Occator Crater and a diversity of geological terrains. The main goal is to understand the evolution of the dwarf planet and the origin of the bright regions in the famous crater.

When Will Dawn Run Out of Fuel?

Mission managers can’t predict exactly when Dawn will run out of fuel. There is no real-time gauge for hydrazine levels. Engineers are regularly adjusting their calculations of the fuel available, and the current best estimate is that Dawn’s mission will end between mid-September and mid-October. However, there are uncertainties, meaning the mission could end in August or perhaps even late October.

The estimates are based on mathematical modeling that predicts how much hydrazine is used each time Dawn turns, and as it holds its orientation stable. Engineers do have physical measurements of the temperature and pressure in the fuel tank, but those are less than perfect, partly due to the wear and tear on the instruments over time. There is another source of uncertainty -- tubing, valves and filters connecting the fuel tank with each of the 12 thrusters may hold onto some hydrazine after the pressure in the lines is too low for the thrusters to operate. Engineers have a good estimate as to how much will be trapped in the system, but it’s not perfect.

Dawn animation

How Will We Know It’s Out of Hydrazine?

Dawn will give mission controllers no heads-up that it’s getting low on hydrazine. Performance will go along the same as it has been, then all of a sudden, Dawn will have expended its last puff of hydrazine.

What will happen is that Dawn will no longer be able to control its orientation with its reaction control thrusters. It will start rotating very slowly. Dawn’s solar arrays will no longer point to the Sun; its camera and other sensors won’t point at Ceres. Its main and low-gain antennas will not point toward Earth.

Calling It

Within a few days of Dawn losing its ability to point, it will become clear to Dawn mission managers that the chances of recovering communications with Dawn are very low. They will attempt contact through different Deep Space Network stations. The Dawn project manager at JPL will determine when those communications attempts will cease and will alert NASA managers. NASA will share the news, publicly signaling Dawn’s official end of mission.

At this point, Dawn will simply remain a new “moon” of Ceres in orbit around the object it has been studying since 2015.

Because Ceres has conditions of interest to scientists who study chemistry that leads to the development of life, NASA follows strict planetary protection protocols for the disposal of the Dawn spacecraft at the end of the mission. Engineers put the spacecraft in an orbit that guarantees it will not crash for at least 20 years – and likely for decades longer.