18 March 2017

Lunar GAS (1987)

During the STS-91 (2-12 June 1998) mission to the Russian Mir space station, the Space Shuttle Orbiter Discovery carried four pairs of GAS canisters along its Payload Bay walls. The red arrow points to one pair. Image credit: NASA
NASA's Get Away Special (GAS) Program (officially the Small Self-Contained Payloads Program) was conceived in 1976 as a way of providing researchers with low-cost opportunities to fly experiments in the Space Shuttle Orbiter's 15-foot-by-60-foot payload bay. The first operational GAS canister, with a suite of 10 experiments developed by students at Utah State University, Weber State University, and the University of California at Davis, reached low-Earth orbit (LEO) during mission STS-4 (27 June-4 July 1982) on board the Orbiter Columbia. By 17 March 2005, when NASA terminated the GAS Program in the aftermath of the 1 February 2003 Columbia disaster, nearly 170 GAS canisters had flown in low-Earth orbit (LEO).

If four engineers at the Jet Propulsion Laboratory (JPL) in Pasadena, California, had had their way, a GAS payload might have traveled far beyond LEO. In May 1987, the team proposed that an advanced-design small spacecraft be launched on board a Space Shuttle inside an Extended GAS canister and ejected into Earth orbit. The spacecraft, called Lunar GAS (LGAS), would then use electric-propulsion thrusters to spiral outward to the moon.

Close-up of two of the STS-91 GAS canisters in Discovery's Payload Bay. Image credit: NASA
LGAS anticipated the small, relatively cheap spacecraft of NASA's 1990s Discovery Program, the first mission of which, Near Earth Asteroid Rendezvous (NEAR), departed Earth in 1995. The Discovery Program, a significant break from the large-spacecraft paradigm that characterized much U.S. planetary mission development in the 1970s and 1980s, got its start in 1991-1992 as Defense Department space technology developed for President Ronald Reagan's Strategic Defense Initiative "missile shield" trickled into the civilian space sector. The Discovery Program would become an intermediate evolutionary step leading toward the present-day Cubesat revolution.

The LGAS mission would begin up to three months before planned Space Shuttle launch with the insertion of the 149-kilogram spacecraft into its Extended GAS canister. The spacecraft would at that point enter the routine GAS payload processing flow and no one would see it again until it left its canister in LEO.

The Shuttle Orbiter bearing the LGAS spacecraft would lift off from Kennedy Space Center in Florida and enter an orbit inclined 28.5° relative to Earth's equator. The astronauts would then open its payload bay doors, exposing the closed Extended GAS canister bearing LGAS to space.

NASA required that GAS experiments place minimal demands on Shuttle expendables and astronaut time. The JPL team insisted that, despite its complexity, the LGAS mission could meet this requirement. A few hours after launch, one astronaut would flip a single switch on the Shuttle flight deck to open the motorized Extended GAS canister lid, then would flip two more to release a latch and activate a spring ejection mechanism.

Simplified schematic of the LGAS spacecraft following deployment from its GAS canister. Image credit: JPL/NASA
The barrel-shaped LGAS spacecraft would leave the Extended GAS canister moving at one meter per second; then, as it moved away from the Shuttle Orbiter, it would automatically extend its twin accordion-fold solar-array wings and its science boom. The slender advanced-design rectangular solar arrays would each have a mass of about 15 kilograms. Their combined 7.25 square meters of collecting area would generate 1.467 kilowatts of electricity at mission start.

Two small chemical-propellant thrusters would turn the spacecraft to point its solar arrays and spin axis toward the Sun, then would spin its barrel-shaped body end over end at up to five revolutions per minute to create gyroscopic stability. After it had moved a safe distance away from the Shuttle, the LGAS spacecraft would switch on one of its twin electric thrusters. Mounted on opposite sides of the spacecraft body, these would take turns thrusting parallel to its spin axis. Fueled from a round tank containing 36 kilograms of compressed xenon gas, the thrusters would each be designed to withstand 3500 start/stop cycles and to operate for a total of 4500 hours (187.5 days).

LGAS spacecraft electric-propulsion thrust and coast arcs during escape from Earth orbit. Image: JPL/NASA
The LGAS spacecraft's orbit about the Earth would for mission operations purposes be divided into four 90° arcs, the JPL engineers explained. In the first arc, one thruster would point opposite the LGAS spacecraft's direction of motion so that when it operated it would accelerate the spacecraft. In the second arc, which would occur in Earth's shadow, both thrusters would point perpendicular to the spacecraft's direction of motion; this would mean that they could not contribute to accelerating the spacecraft, so they would not operate.

In the third arc, the second thruster would point opposite the LGAS spacecraft's direction of motion, so it would switch on to take its turn accelerating the spacecraft. In the fourth arc, which would see the spacecraft pass between the Earth and the Sun, the thrusters would again point perpendicular to its direction of motion, so would not operate.

Overcoming drag from Earth's atmosphere would require about one-third of the LGAS spacecraft's thrust early in the departure spiral, the team calculated, but drag would taper off quickly as the spacecraft raised its orbital altitude by up to 20 kilometers per day. Starting about three months after launch from the Shuttle, the LGAS spacecraft would spend between 100 and 150 days inside the Earth-girdling Van Allen Belts. High-energy particles in the Belts would gradually degrade the twin wing arrays, reducing their electricity output.

Image credit: JPL/NASA
About 600 days after launch, the LGAS spacecraft would reach a point about 130,000 kilometers away from the Earth. It would then turn off its thrusters and coast in a lazy 15-day "linking orbit" that would deposit it into a loosely bound 40,000-kilometer circular lunar polar orbit.

The xenon-fueled thrusters would then resume alternating operation with their 90° thrust arcs centered over the moon's polar regions; this time, however, the thrusters would point in the spacecraft’s direction of motion when they operated, gradually slowing the LGAS spacecraft so that it would spiral in toward the moon.

The spacecraft would achieve a 100-kilometer-high, two-hour lunar polar orbit about two years after it departed its Extended GAS canister. In its orbit over the moon's poles, the moon would rotate beneath it about once per month, enabling it to eventually overfly the entire lunar surface. Irregularities in the moon's gravity field would mean that the electric thrusters would need to adjust the spacecraft's orbit about every 60 days.

The LGAS spacecraft would have room for only one science instrument: a 15-kilogram gamma-ray spectrometer (GRS) for charting the composition of the moon's crust. The JPL engineers proposed that the unflown Apollo 18 GRS be mounted on the LGAS science boom. Lunar-orbital science operations would continue for about one year.


"Lunar Get Away Special (GAS) Spacecraft," AIAA-87-1051, K. T. Nock, G. Aston, R. P. Salazar, and P. M. Stella; paper presented at the 19th AIAA/DGLR/JSASS International Electric Propulsion Conference in Colorado Springs, Colorado, 11-13 May 1987

"Getaway Special," Wikipedia,
https://en.wikipedia.org/wiki/Getaway_Special (accessed 18 March 2017)

More Information

The Eighth Continent

On the Moons of Mighty Jupiter (1970)

Cometary Explorer (1973)

Catching Some Comet Dust: Giotto II (1985)


  1. It really sad this mission never flew
    LGAS would be the predecessor for Lunar Prospector and SMART-1
    That Little ESA probe made the mission LAGS was design for:
    fly back pack on Ariane 5G with two Com Sat to GTO and coast to Moon on ion Engine
    It successor SMART-2 got relabel to LISA Pathfinder, after it got new purpose (not the moon)

    it's shame that Moon is not visit by more mini probes like SMART-1
    They are low cost alternatives to big birds like SELENE or LRO
    I wonder if the Cubsats will inherit one day the legacy of LGAS and SMART-1
    and fly on into unknown on behalf of SpaceX, Blue Origin and the Planetary Society

    1. Michel:

      SMART-1 was pretty cool - it reminded me a little of the first New Millennium mission, which was an electric propulsion test-bed. That paved the way for Dawn, as I expect you know. New Millennium died with the Mars Surveyor Program, but it was having problems before that - everyone insisted that it should carry science instruments along with new technology, which drove up the cost. Confusion about what it was for - tech development, which implied the potential for failure - left it open to attack when the New Millennium-2 Mars penetrators were lost.

      Have any other ESA technology test-beds like SMART-1 flown?

      I think Cubesats are already cheap enough that high schools could potentially fly them - or they soon will be. I was a little skeptical about their value at one time, but I am coming around now.


    2. >Have any other ESA technology test-beds like SMART-1 flown?

      yes, Allot
      ESA has the MicroSat program:
      LISA Pathfinder (formally know as SMART-2) sensor testbed for ESA planned gravitational wave observatory
      PROBA series (Project for On-Board Autonomy) build by Belgium for ESA
      Young Engineers' Satellite 1 & 2
      OPS-SAT a cubsat testbed for more powerful on-board computers

      bigger hardware
      Intermediate eXperimental Vehicle experimental suborbital re-entry vehicle
      Lunar Lander a Testbed for soft lunar landing

      At ESA under study is Asteroid Impact & Deflection Mission (AIDA)
      four satellite: one impactor DART, one observer AIM and two cubsats 1&2

    3. Michel:

      That's an impressive list of ESA technology test flights. Thanks for providing it.


  2. Hi David.
    I wonder what was the reason for not flying this satellite. Was it as simple as too much competition for the available places? It seems like it would have been a valuable test of an innovative technology.

    1. Hi, Kerrin:

      Many times when folks propose a mission design they don't necessarily expect it to fly. They are putting forward a concept that they want people to think about - that is, trying to promote a concept they feel has promise. I am not an engineer, but when I look at the date on this paper - 1987 - and think about the State of the Art at that time, I can't help but wonder whether this is one of those cases. Electric propulsion still hasn't received enough attention, in my opinion, though Dawn has thoroughly demonstrated its potential.



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