"To my knowledge this is the first time that helium pressurant has been intentionally used as a cold-gas propellant through hydrazine thrusters," says Stewart Bushman. (Credit: "thank you balloons" via Shutterstock)

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Helium ‘fumes’ buy Messenger a bit more time

A NASA spacecraft circling Mercury will soon be running on fumes. This clever strategy helps squeeze a few more weeks of valuable science from a mission already nearly three years into overtime.

After more than 10 years traveling in space, nearly four of those orbiting Mercury, the Messenger probe is just about out of hydrazine fuel for its thrusters. It has been on track to drop from orbit and crash on the innermost planet at the end of March.

Messenger probe orbits Mercury
This artist’s impression shows the MESSENGER spacecraft in orbit at Mercury. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

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But engineers at the Johns Hopkins Applied Physics Laboratory have devised a way to generate thrust using the non-propellant helium gas that keeps the propulsion system pressurized. That trick will keep Messenger aloft for as long as another month and allow scientists to collect even more data about the planet closest to the Sun.

“The team continues to find inventive ways to keep Messenger going, all while providing an unprecedented vantage point for studying Mercury,” says APL’s Stewart Bushman, lead propulsion engineer for the mission. “To my knowledge this is the first time that helium pressurant has been intentionally used as a cold-gas propellant through hydrazine thrusters.”

Helium will be a less-efficient propellant, but it will work—and it’s just about all that’s left on a spacecraft that’s continued its mission far longer than originally expected.

“Propellant, though a consumable, is usually not the limiting life factor on a spacecraft, as generally something else goes wrong first,” Bushman says. “As such, we had to become creative with what we had available. Helium, with its low atomic weight, is preferred as a pressurant, because it’s light, but rarely as a cold-gas propellant, because its low mass doesn’t get you much bang for your buck.”

Getting closer

Messenger’s orbits are already decaying; by January 21, the low point in its swings around Mercury will be down to about 25 kilometers [just over 15 miles]. The helium-based maneuver scheduled for that day should raise altitude at closest approach to just over 80 kilometers [almost 50 miles].

That will give scientists extra time to explore Mercury at close range. This past summer, the team launched low-altitude observations, seeking the highest-resolution Mercury images ever.

Those images are enabling scientists to search for volcanic flow fronts, small-scale tectonic features, layering in crater walls, locations of impact melt, and new aspects of hollows.

The detailed views are expected to provide a new understanding of Mercury’s geological evolution.

“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,” says APL’s Haje Korth, instrument scientist for the 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.”

The spacecraft’s neutron spectrometer should be able to study water ice deposits within individual impact craters in the planet’s high northern latitudes, Korth says.

Messenger is an acronym for Mercury Surface, Space Environment, Geochemistry and Ranging. It is the first space mission to orbit the planet closest to the sun.

The spacecraft was launched on August 3, 2004, and entered orbit on March 18, 2011, to begin a yearlong study. The orbital phase has since been extended twice and should now last a total of more than four years.

Sean C. Solomon of Columbia University’s Lamont-Doherty Earth Observatory is principal investigator. APL built and operates the Messenger spacecraft for NASA.

Source: Johns Hopkins

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