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NASA To Apply Lessons Of Low-Cost Moon Mission

NASA’s first use of a low-cost, modular spacecraft design will be put into an unusually low orbit of the Moon to sample its atmosphere and dust and create a profile that will be useful for studies throughout the Solar System.

The Lunar Atmosphere Dust Environment Explorer (Ladee) is still in the early days of mission and science planning but is booked for an Oct. 28, 2012, liftoff on an Orbital Sciences Minotaur V from NASA’s Wallops Island, Va., space complex. It will be the debut of the five-stage solid propellant Minotaur V as a low-cost alternative for planetary missions.

Ladee is the first application of NASA Ames Research Center’s Modular Common Bus, a tiered satellite development program that aims for all-inclusive costs of as little as $50 million for simple missions (AW&ST Jan. 5, 2009, p. 32). With four tiers, Ladee is more complex than that; its budget is $200 million.

The project will rely heavily on the commercial off-the-shelf (COTS) systems and instruments approach that is basic to the low-cost/quick-build common bus concept.

The first contract has gone to Space Systems/Loral to build a propulsion system derived from its signature 1300-series communications satellite platform. Ladee will circle the Moon’s equator at a 5-deg. inclination at a nominal altitude of just 50 km. (31 mi.), lower than any previous lunar satellite’s planned orbit.

However, mission developers have not settled on an orbit profile. Operational altitudes could vary widely as scientists let some orbits decay to as low as 20 km. in order to skim the bottom of the atmosphere to build a profile of its molecular and dust constituents, says Co-Principal Scientist Gregory Delory of the University of California-Berkeley’s Space Sciences Laboratory.

Even if the Moon’s gravity has only one-sixth the strength of Earth’s, this passive approach will need active positioning burns to prevent a crash.

“The threshold as to when to kick it back up is still under discussion,” he says. “There is a lot of thinking going into the orbital planning. But, in general, the propulsion system will be used actively to raise orbits.” Scientists are expected to ask that some orbits venture up to the 75-80-km. range.

While past missions have helped draw a good profile of the anomalies in the Moon’s gravitational field, much of the gravitational influences Ladee faces will depend on the timing of its orbital insertion, Delory says.

The decision to orbit low will take its toll. Ladee’s nominal mission duration is just 100 days, although if all goes well another 30 days is likely. Left unmaintained, the spacecraft could crash within a week. On the other hand, orbits can be stable for up to a month. “So it’s a unique challenge,” says Delory.

The propulsion system will occupy the bottom two tiers, with other systems added in the top two. COTS systems buys are expected for them as well, including the spacecraft’s avionics, communications, inertial measurement unit, star tracker and solar arrays, says Deputy Project Manager Stevan Spremo. Ladee’s dry weight has been set at 130 kg. (286 lb.)

The spacecraft’s three science instruments will be managed by the Goddard Space Flight Center and are mounted on Ladee’s top two tiers. An ultraviolet spectrometer will top the fourth tier. It is a variant of an instrument flown on Ames’ Lunar Crater Observation and Sensing Satellite (Lcross) and will be steered by spacecraft positioning. The only instrument to be making remote measurements, it will examine lunar dust using solar occultation.

Mounted with it is the Lunar Dust Experiment (LDEX) from the University of Colorado’s Laboratory for Atmospheric and Space Physics. Unique for this mission, it derives from similar dust samplers on the Highly Eccentric Orbiting Satellite-2, Galileo, Cassini and Ulysses.

The Neutral Mass Spectrometer (NMS) from Goddard is mounted outside the third tier and is derived from one to be flown on the Mars Science Laboratory.

LDEX and NMS are pointed in the spacecraft’s ram, or forward, direction. When a dust particle flows through their apertures, it is vaporized into a charged cloud so its mass can be measured.

Also on board, but as a piggyback payload, is the technology demonstration from MIT’s federally funded Lincoln Labs. A companion to NMS in the third tier, the lasercom will test transmission rates to Earth of as high as 600 mbps. While not central to Ladee’s mission, scientists are welcoming it on board given its potential for boosting data transfer rates from deep space. Delory notes that Ladee’s transmission rates are a mere 10 kbps.

The ancients wondered whether the Moon had even an atmosphere—Galileo thought so—but they had no way of understanding interactions between space’s environment, the lunar surface and its exosphere, or atmosphere. Samples taken by Apollo crews brought science’s first measurements of the exosphere’s atomic composition and chemistry.

The exosphere is fairly airless and certainly tenuous, but it is not asleep, nor well understood, Delory explains. Molecules do not collide there the way they do in Earth’s atmosphere. That means the exosphere’s lower boundary is actually the lunar surface itself and it is from this rock and dirt that it is derived.

Exposed as it is to space weather, the surface is bombarded by ultraviolet radiation, charged particles and the impacts of micrometeorites—activities bound to jostle compounds and dust from the surface into the atmosphere.

These forces can be quite dynamic. An electrostatic process can produce 4,000-volt jolts. “It could be that electrostatic charges are what kicks dust up,” says Delory. “Finding out is a key goal for Ladee.”

As dust and chemicals are tossed into the atmosphere, some bounce like balls, others drift into space. But how many?

Much of the mission’s focus will be on the terminator line between the Moon’s dark and light sides. “The terminator is like a mini-laboratory of the exosphere,” he explains, “so the more often we cross it the better it is to watch.”

For instance, argon, which originates in the Moon’s interior, is frozen on the dark side. But once the Moon’s slow rotation exposes it to the Sun’s energy, argon hops off the surface, swirling in its own wind around the terminator and rising into the atmosphere as much as 30-50 km.

Other chemicals produce different height signatures. The Ladee team expects to encounter sodium, potassium and helium as high as 1,000 km.

So for scientists, the lunar atmosphere and its dust environment are interesting in and of themselves. But they also are thought to be quite similar to the atmospheres of other bodies in the Solar System from Mercury, the smallest planet, to Ceres, the largest asteroid, and from many other moons too, whether circling Jupiter or Saturn.

“This is a great mission to get that [science] done,” says Delory.

While not a direct successor to Ames’ Lcross, Ladee will build on its results, which, so far, have presented the most compelling direct evidence of water on the lunar surface. Discussions about other chemical finds are pending publication in science journals.

Just where the water came from is a mystery. Some suggest its origins date to comets that struck the surface, just as comets may have contributed water on Earth. Or perhaps hydrogen from the solar wind activates oxygen on the Moon to produce a water cycle.

“If we see water in the atmosphere it will tell us something about where the water may be coming from, or where it is going,” Delory says. “Lcross is going to tell us what’s there; Ladee will tell us the how and why” of its arrival.