A novel mission concept involving two CubeSats connected by a thin, miles-long tether could help scientists understand how the Moon got its mysterious “tattoos” — swirling patterns of light and dark found at more than 100 locations across the lunar surface.
NASA’s Planetary Science Deep Space SmallSat Studies, or PSDS3, program recently selected a team at the Goddard Space Flight Center in Greenbelt, Maryland, to further develop a mission concept called the Bi-sat Observations of the Lunar Atmosphere above Swirls, or BOLAS. The study, led by Goddard Principal Investigator Timothy Stubbs, could lead to the first tethered planetary CubeSat mission, Stubbs said.
“This is an exciting concept,” said Michael Collier, a BOLAS co-investigator who has studied tether-based missions for gathering difficult-to-obtain lunar measurements since 2015. “Candidly, I think it’s groundbreaking. Tethered satellites are a very natural approach for targeting lunar science.”
As currently conceived, the mission would involve two 12-unit CubeSats, whose individual units would measure just four inches on a side. Once the pair reached a low-maintenance, quasi-stable orbit about 62 miles above the Moon’s surface, the two, connected by a 112-mile-long thin tether, would separate. The top satellite would climb 118 miles above the surface, while the lower, nearly identical twin would plunge to an altitude of about six miles above the surface.
“The tension in the tether keeps the CubeSats in vertical alignment as they orbit,” Stubbs said. “The configuration, with the center-of-mass in a quasi-stable orbit, should enable the lower CubeSat to fly for long durations at low altitudes.”
Without a tether system, a comparable low-altitude mission would need prohibitive amounts of fuel to maintain its orbit. NASA’s Lunar Reconnaissance Orbiter, or LRO, for example, flew in a circular orbit 31 miles above the surface early in its mission. If NASA hadn’t executed propulsive maneuvers to maintain this orbit, the spacecraft would have smashed into the surface.
This is due to the large concentrations of mass lurking on the lunar surface. These “mascons” change the gravity field and can either pull the spacecraft or push them off course, dooming them to a mission-ending collision.
“For planetary objects that lack an atmosphere, tethering is an innovative approach to the technical challenge of low-altitude measurements using minimal propellant,” Collier said, adding that a CubeSat couldn’t carry the amount of fuel needed to carry out periodic station-keeping maneuvers.
For scientists attempting to better understand the Moon’s odd, airless environment, the lower they can deploy the CubeSat, the better.