NASA’s CAPSTONE CubeSat launched Wednesday morning and is on its way to test a longer, more fuel-efficient route to the Moon, powered mostly by gravity.
NASA’s CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment) blasted off from New Zealand aboard a Rocket Labs rocket on Wednesday morning. As of Wednesday evening, the CubeSat is safely in low-Earth orbit and on its way to test a longer, more fuel-efficient route to the Moon and a revolutionary new lunar orbit, both of which will enable future crewed Moon missions.
What’s New – CAPSTONE will go where lots of spacecraft — and several humans — have gone before, but it will get there in a completely new way. The CubeSat mission will test a very long but very fuel-efficient route to the Moon, called a ballistic lunar transfer trajectory.
It will also test a previously-untried lunar orbit, called a near rectilinear halo orbit: a stretched-out oval of an orbit that passes over the Moon’s north and south poles. This very stable orbit will let future missions like Gateway stretch their fuel supplies further and stay in constant radio contact.
While in lunar orbit, CAPSTONE will also test a communications system that will help future lunar missions monitor their positions in space without relying on Earth-based tracking. CAPSTONE will work with NASA’s Lunar Reconnaissance Orbiter to constantly measure the distance between the two spacecraft, then use that as a basis for calculating the ships’ positions.
Why It Matters – Everything CAPSTONE is doing will directly pave the way for future crewed missions to the Moon.
The near-rectilinear halo orbit is the same orbit NASA plans to use for the Gateway space station, which will eventually orbit the Moon with a full-time crew of astronauts to provide a docking station for lunar landers and supply missions, as well as communications and other support for Artemis missions on the Moon.
And the ballistic lunar transfer orbit will help keep Artemis and Gateway up and running. Crewed Artemis missions will take the more direct route, which means they’ll fire their thrusters to transfer the spacecraft from Earth orbit to lunar orbit. But for uncrewed missions, like supply deliveries to future lunar outposts, the ballistic lunar transfer orbit will be more fuel-efficient and less expensive.
Digging Into The Details – When the Apollo spacecraft traveled to the Moon, it was roughly a three-day trip, but CAPSTONE’s journey will take four months on the ballistic lunar transfer trajectory. The route is “gravity-driven,” as NASA puts it, which means CAPSTONE will mostly use gravity, not its own thrusters, to change its speed and position.
Six days from now, CAPSTONE’s Lunar Photon third-stage booster will release the satellite on a long cruise out into deep space. Eventually, Earth’s gravity will pull the spacecraft back toward Earth and the Moon — the “ballistic” part of the route’s name.
Once it’s 1.5 million kilometers from home, the Sun’s gravity will boost CAPSTONE into an extremely wide orbit around Earth; the lowest point in the orbit will cross paths with the Moon. As CAPSTONE swings back toward Earth and the Moon, it will need only a few small nudges from its thrusters to stay on course. One last small maneuver will change CAPSTONE’s speed so that the Moon’s gravity can capture it into lunar orbit — that’s the “transfer” part.
On paper, the physics work beautifully, and NASA engineers flew tens of thousands of simulations to plan and practice CAPSTONE’s long but surprisingly fuel-efficient path to the Moon. But the spacecraft is flying this route to ensure it works as well in practice.
And once it arrives at the Moon, CAPSTONE will settle into a bizarre orbit around the Moon’s poles. It’s called a near rectilinear halo orbit, or NRHO: “near rectilinear” because the orbit is such a long, stretched oval that its sides are nearly straight, and “halo” because it’s orbiting the Moon’s poles instead of its equator. At its closest approach, CAPSTONE will pass about 1,600 km above the Moon’s north pole; at the other end of its orbit, the spacecraft will be about 76,000 km from the Moon’s south pole.
Gravitational influence from both Earth and the Moon will help keep the spacecraft stable in this orbit, so CAPSTONE won’t need to fire its thrusters very often or very long.
“The burns will be timed to give the spacecraft an extra boost as it naturally builds momentum — this requires a lot less fuel than a more circular orbit,” Elwood Agasid, deputy program manager for Small Spacecraft Technology at NASA’s Ames Research Center, said in a statement.
That means a spacecraft can stay in orbit much longer with the same amount of fuel than it could in a more conventional orbit. NASA estimates that a big spacecraft like Gateway will be able to stay in lunar orbit for about 15 years.
Meanwhile, orbiting the Moon’s poles will keep CAPSTONE — and later, Gateway — in constant radio contact with Earth and with the lunar surface. During the Apollo missions, the command module lost communications with both Mission Control and the astronauts on the lunar surface for about 48 minutes at a time while its orbit carried to the far side of the Moon. Gateway crews won’t have that problem.
What’s Next – CAPSTONE will spend the next four months on its way to the Moon. For six months after that, the little spacecraft will send home data about orbital dynamics and its communications project with LRO, along with a few other experiments.
NASA has scheduled the first components of Gateway to tentatively launch in late 2024. NASA will start with the power and propulsion model and the Habitation and Logistics Outpost, which will combine crew quarters, science facilities, docking ports, and command and control for the rest of the station.
Meanwhile, NASA hasn’t yet set a date for the Artemis 1 launch; engineers are still reviewing data from a key pre-launch test on June 20.
