[meteorite-list] Innovative use of Pressurant Extends MESSENGER's Mission, Enables Collection of New Data

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=269

MESSENGER Mission News
December 24, 2014

Innovative use of Pressurant Extends MESSENGER's Mission, Enables Collection 
of New Data

The MESSENGER spacecraft will soon run literally on fumes. After more 
than 10 years traveling in space, nearly four of those orbiting Mercury, 
the spacecraft has expended most of its propellant and was on course to 
impact the planet's surface at the end of March 2015. But engineers on 
the team have devised a way to use the pressurization gas in the spacecraft's 
propulsion system to propel MESSENGER for as long as another month, allowing 
scientists to collect even more data about the planet closest to the Sun.

"MESSENGER has used nearly all of the onboard liquid propellant. Typically, 
when this liquid propellant is completely exhausted, a spacecraft can 
no longer make adjustments to its trajectory. For MESSENGER, this would 
have meant that we would no longer have been able to delay the inevitable 
impact with Mercury's surface," explained MESSENGER Mission Systems Engineer 
Dan O'Shaughnessy, of the Johns Hopkins University Applied Physics Laboratory 
(APL), in Laurel, Md. "However, gaseous helium was used to pressurize 
MESSENGER's propellant tanks, and this gas can be exploited to continue 
to make small adjustments to the trajectory."

This gas is less efficient, he added, but as effective as the liquid propellant 
at modifying the spacecraft's trajectory.

"The team continues to find inventive ways to keep MESSENGER going, all 
while providing an unprecedented vantage point for studying Mercury," 
said 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. 
These engines are not optimized to use pressurized gas as a propellant 
source. They have flow restrictors and orifices for hydrazine that reduce 
the feed pressure, hampering performance compared with actual cold-gas 
engines, which are little more than valves with a nozzle."

"Propellant, though a consumable, is usually not the limiting life factor 
on a spacecraft, as generally something else goes wrong first," he continued. 
"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."

Adjusting MESSENGER's trajectory will allow scientists to spend extra 
time exploring Mercury from close range. This past summer, the team launched 
a low-altitude observation campaign to acquire the highest-resolution 
images ever obtained of Mercury, 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 -- detailed views 
that are providing 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, scales comparable to the anticipated 
periapsis altitude between 7 km and 15 km above the planetary surface," 
said APL's Haje Korth, the 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. 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."

MESSENGER's periapsis altitude is now approximately 101 kilometers and 
decreasing. The next orbit-correction maneuver on January 21, 2015, will 
raise the altitude at closest approach from approximately 25 kilometers 
to just over 80 kilometers.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) 
is a NASA-sponsored scientific investigation of the planet Mercury and 
the first space mission designed to orbit the planet closest to the Sun. 
The MESSENGER spacecraft was launched on August 3, 2004, and entered orbit 
about Mercury on March 17, 2011 (March 18, 2011 UTC), to begin a yearlong 
study of its target planet. MESSENGER's first extended mission began on 
March 18, 2012, and ended one year later. MESSENGER is now in a second 
extended mission, which is scheduled to conclude in March 2015. Dr. Sean 
C. Solomon, the Director of Columbia University's Lamont-Doherty Earth 
Observatory, leads the mission as Principal Investigator. The Johns Hopkins 
University Applied Physics Laboratory built and operates the MESSENGER 
spacecraft and manages this Discovery-class mission for NASA.