MESSENGER uses helium for last minute reprieve

Artist's concept of the MESSENGER orbiter (Image: NASA)

Now orbiting the planet Mercury after over ten years in space, NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is still functioning better than expected. Its mission will soon come to an end though – it's running out of fuel and is scheduled to crash into the planet in March. However, mission control have come up with a novel plan that will use the helium used to pressurize the unmanned probe's engine to give it another month of life.

According to the MESSENGER team, fuel is usually the last problem that a robotic exploration team worry about because there are so many other things that can go wrong long before it runs out. That being said, the fuel is the single most important consumable aboard an orbiter mission because it not only allows the spacecraft to maintain the correct attitude and keep its antennae pointed at Earth, it also lets it use the main engine to boost its orbit against atmospheric drag, which decays the orbit. The upshot is that when the propellant runs out, the spacecraft starts to tumble and spirals in to burn up in the atmosphere or, in MESSENGER's case, crash into the surface at hypersonic speed.

So normally when the fuel runs out, that's it, but NASA reasoned that MESSENGER's liquid-fuel rocket engine design meant there was still a bit of thrust left even after all the propellant was expended. The MESSENGER's engine is pressure fed, which means that it uses helium from a separate tank to push the fuel and oxidizer into the engine's combustion chamber. Since the helium needs to work against the force of the rocket's combustion, it's under considerable pressure, and when the fuel is gone, there will be some helium left in the pressure tank.

The idea is to use the helium as a cold propellant. In other word's where the rocket engine gets its thrust by burning fuel, the helium pushes the spacecraft by simple gas pressure like a toy balloon when the open neck is let go.

Unfortunately, this is the first time a pressurant has been as an improvised thruster and MESSENGER's engine is a bit more complicated than a balloon. According to MESSENGER Mission Systems Engineer Dan O’Shaughnessy, of the Johns Hopkins University Applied Physics Laboratory (APL), the pressure in the helium tank isn't much compared to a firing engine. In addition, the gas passes through a number of reduction valves and nozzles that have to be taken into account, and helium is the second lightest of gases, so it doesn't provide much in the way of thrust.

If these problems can be overcome, NASA estimates that it will give MESSENGER another month of active life before impact on Mercury. MESSENGER's current closest approach to Mercury is 25 km (15 mi). If a scheduled course correction using the helium is successful, this will rise to 80 km (50 mi). This will allow the orbiter to carry out additional low altitude observations, including collecting a new set of high-resolution images.

"During the additional period of operations, up to four weeks, MESSENGER will measure variations in Mercury’s internal magnetic field at shorter horizontal scales than ever before, scales comparable to the anticipated periapsis altitude between 7 km (4 mi) and 15 km (9 mi) above the planetary surface," says APL’s Haje Korth, the instrument scientist for the spacecraft's magnetometer. "Combining these observations with those obtained earlier in the mission at slightly higher altitudes will allow the depths of the sources of these variations to be determined. In addition, observations by MESSENGER’s Neutron Spectrometer at the lowest altitudes of the mission will allow water ice deposits to be spatially resolved within individual impact craters at high northern latitudes."

Built and operated by John Hopkins University for NASA, the MESSENGER spacecraft was launched from Cape Canaveral on August 3, 2004 as the first mission aimed to place an orbiter around the innermost planet Mercury. It wasn't until March 18, 2011 that it went into orbit around its target because matching orbits with Mercury required an elaborate series of flybys of Earth, twice of Venus, and three times of Mercury before it obtained the proper trajectory. After completing its year-long mission to study Mercury's surface and exosphere, the orbiter was then placed on two extended missions; the second of which is now scheduled to end in April.

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