NASA’s MESSENGER orbiter has been flying on fumes and improvisation for the last month, but it will finally crash into Mercury on April 30—bringing to an end its 11-year science mission, which included four years orbiting the innermost planet.
MESSENGER was in fact the first spacecraft to orbit Mercury, which turned out to be a challenge. This close to the sun, the intense solar gravity has to be taken into account: During the last phase of the mission, the geometry of MESSENGER’s orbit has been such that the sun is actually pulling the spacecraft toward the planet.
Sunlight also exerts physical force at this close range; solar radiation pressure nudges the spacecraft enough to change its speed by one or two centimeters per second, every day. Mission Systems Engineer Dan O’Shaughnessy describes it as a small but persistent force, but it adds up over time to a large effect, which can be difficult to calculate precisely for the complex shapes of the spacecraft’s surface. Radiation reflecting back from Mercury is another force to reckon with.
As a result, maintaining an orbit around Mercury requires frequent boosts, known as orbital correction maneuvers (OCMs). Until March, MESSENGER had relied on its liquid propellant for those maneuvers, but the team knew the propellant was running out. Late last year they came up with an idea: MESSENGER’s propellant tanks use helium gas for pressurization, and switching to helium for thrusting after the propellant ran out might keep the spacecraft flying for another month.
The first helium gas boost happened on March 18, just before Mercury’s orbit carried it around to the far side of the sun, where it’s mostly out of contact with Earth. The maneuver provided some thrust but not quite enough altitude, and the team worried that the spacecraft’s orbit might decay while it was still out of contact. They scheduled another correction, using a larger set of thrusters, and MESSENGER survived—the first time helium has been used for propulsion. The experiment bought the spacecraft an extra month of time in orbit.
Knowing the end is near, the MESSENGER team took some big risks to gather high-resolution images and other data in the final month, when the low part of the orbit dipped just a few miles above the planet’s surface. “Now we can operate really close to the surface, because we've almost got nothing to lose,” said O’Shaughnessy.
Mission scientists wanted to get a closer look, using the spacecraft’s neutron spectrometer, at water ice and possible organic compounds in shadowed craters in the planet’s polar regions. MESSENGER had confirmed the presence of water ice in higher latitudes, where crater floors sit in permanent shadow, and it had also identified some darker material that may be deposits of carbon-based organic compounds. Scientists also wanted to get MESSENGER’s on-board magnetometer closer to the planet’s surface to look for signs of magnetism in Mercury’s crust. If it exists, crustal magnetism could be a record, trapped in cooled volcanic rock, of the history of Mercury’s magnetic field.
For engineering and operations teams on Earth, MESSENGER’s demise won’t be a crash, but a quiet fade into silence. Its trajectory will intersect the planet’s surface when it’s on the far side of Mercury as seen from Earth. Mission control at the Johns Hopkins Applied Physics Laboratory outside Baltimore will hear the spacecraft’s radio signal fade as it goes around the planet, and it won’t re-emerge on the other side. MESSENGER will die alone.
“We'll just be looking for a signal, and the absence of signal will sort of confirm our demise,” said O’Shaughnessy.
In fact, MESSENGER will already have hit Mercury by the time of that non-contact, because it takes about 11 minutes for signals to travel from Mercury orbit to Earth.