Back in May 2015, NASA's Dawn spacecraft became the first man-made object to enter orbit around a dwarf planet. It has circled Ceres ever since, with NASA extending its mission not once but now twice. The latest lifeline involves sending the unmanned probe closer to the surface of Ceres than ever before, as it shifts its focus to measuring gamma rays and neutrons.
Dawn arrived at Ceres after a seven and a half year journey, making a stop to orbit the giant asteroid Vesta and then continuing on with the goal of investigating the dwarf planet. That primary mission drew to a close midway through last year, meaning the probe has already far outstripped its initial scientific goals.
With supplies of hydrazine fuels waning quickly, NASA last year moved to extend Dawn's mission by maneuvering it into higher orbit, where it required less juice to counter Ceres' gravitational pull. The agency has now authorized another extension, where if all goes to plan, Dawn will be lowered to within 120 mi (200 km) of the dwarf planet's surface.
At that altitude, Dawn's priority will be to collect data with its gamma ray and neutron spectrometer, used to measure the number and energy of gamma rays and neutrons. This data will help scientists understand more about the composition of Ceres' uppermost layer and how much ice it holds.
It so happens that this extension will also station Dawn in Ceres' orbit as the dwarf planet goes through its closest approach to the Sun, something known as perihelion. Taking place in April next year, this may cause ice on the surface to turn into water vapor, which scientists suspect may contribute to a weak transient atmosphere. Dawn's observations during this time will enable them to further explore this theory.
Unlike Cassini, Dawn won't be deliberately destroyed – a conscious decision made by the mission team to protect the dwarf planet from Earthly contamination. It will remain in stable orbit long after it loses communication with Earth, which the team expects to occur some time in the second half of 2018.