Four and a half months after it was launched, a nanosatellite called Capstone has begun circling the moon — in a peculiar type of orbit where no probe has gone before.
The complex path, known as a near-rectilinear halo orbit, is the same type of trajectory that NASA hopes to use for crewed missions to the moon starting in the mid-2020s. Capstone is an acronym, standing for “Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment.” But it’s also a metaphorical capstone for the Artemis moon program’s mission architecture.
Capstone is owned and operated by Colorado-based Advanced Space, which is conducting the mission on NASA’s behalf. The CubeSat was designed and built by Terran Orbital — with support from development partners including Bothell, Wash.-based Tethers Unlimited, which provided the probe’s cross-link radio.
Rocket Lab sent the probe on its way from its New Zealand launch pad in June atop an Electron launch vehicle. Because the Electron isn’t nearly as powerful as, say, a Saturn V rocket, Capstone had to follow a circuitous, gravity-assisted path to lunar orbit, known as a ballistic lunar transfer or BLT. Along the way, the mission experienced a worrisome communications breakdown — but fortunately was able to recover.
NASA reported that Capstone fired its thrusters to enter its halo orbit around the moon at 4:39 p.m. PT Nov. 13.
“We did it! This was the most critical event of the entire mission,” Bradley Cheetham, Advanced Space’s CEO and the Capstone mission’s principal investigator, said in a news release. “Waiting with the team to get confirmation of the insertion maneuver was both a privilege and extremely stressful. It is an honor beyond description to be a part of returning the U.S. to the moon.”
Over the next few days, the probe will execute two additional maneuvers to fine-tune its orbit.
Capstone’s orbit traces the same route that NASA will use for the Gateway, a moon-orbiting space station that will support crewed lunar missions. The orbit takes advantage of gravitational pulls from Earth and the moon to put the spacecraft on a path that swoops within 1,000 miles of the lunar surface but also swings as far away as 43,500 miles.
Such an orbit could facilitate the transport of crews and supplies to and from the lunar surface. For example, supply ships could dock with the Gateway at the farthest-out point of the lunar orbit, and ride with the Gateway down to the nearest point to drop off their cargo. That would reduce the amount of propellant needed to make the trip.
Halo orbits would provide a wider range of perspectives for lunar observation, and eliminate blackouts in Earth-moon communications. But there’s also a potential downside.
“Finding a lunar orbit for the Gateway is no trivial thing.” Markus Landgraf, an architecture analyst at the European Space Agency, explained in a primer on near-rectilinear halo orbits. “If you want to stay there for several years, the near-rectilinear halo orbit is slightly unstable, and objects in this orbit do have a tendency of drifting away.”
In the coming weeks, mission managers at Advanced Space and NASA will test a navigation system that would allow future spacecraft to determine their location in space without relying exclusively on tracking from Earth. Capstone’s primary mission is due to last six months, but the mission could be extended for a year longer.
