If CAPSTONE's goals are successful, much larger lunar orbiters could follow.

By Rahul Rao | Published Jun 23, 2022 6:00 PM

In a few years, if all goes well, NASA astronauts will ride to the moon aboard an Orion capsule, an 8.5-ton shelter that fills up a large room. But on the other end of the size spectrum—yet, in many ways, no less important to those lunar exploration goals—sits a spacecraft that could fit, neatly, on an office desk.

That craft is the Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment—CAPSTONE, for short. It will launch for the moon in late June, potentially becoming the first lunar satellite of its class. And it’s going on a test run where future, perhaps shinier missions are planned to follow. CAPSTONE may help NASA create a communications hub that, not too far in the future, will circle the moon. 

Despite its size, CAPSTONE is remarkable for a few reasons, many of which have to do with the satellite’s class: CubeSat.

CubeSats are, well, cubic: The common base models are about 4 inches to a side and weigh no more than 4.5 pounds. You could hold one in your hand; you might even build one by hand, too, since most use off-the-shelf components. You can stack them into larger satellites. CAPSTONE combines 12 CubeSats, shy of the largest to date (which used 16). 

From 1998 to the start of June 2022, 1,862 CubeSats have launched—and that number is set to more than double by 2028. CubeSats’ low cost means that they’re within reach of amateurs, university groups, fledgling startups, small developing countries, and others who lack the resources of SpaceX or the world’s big space agencies.

But CubeSats’ low cost has made them appealing for other missions, too. In 2019, NASA contracted private firm Advanced Space to build CAPSTONE for $13.7 million. (For comparison, even the most rudimentary large lunar probe can cost an order of magnitude more.) Advanced Space chose to use CubeSats to put the probe into space cheaply and quickly.

[Related: This satellite has high hopes—the transformation of Finland’s space industry]

The vast majority of CubeSats live in Earth orbit. Only a few have gone beyond that. In 2018, two arrived at Mars alongside NASA’s Mars InSight mission. Absolutely none have gone to CAPSTONE’s destination in the moon’s orbit.

“To date, there have not been lunar cubesats,” says Jekan Thanga, an engineer at the University of Arizona, who isn’t involved with CAPSTONE. “CAPSTONE is actually going to be a first in that respect.”

Other CubeSats are riding with the Artemis 1 uncrewed test flight. Depending on when they launch—currently scheduled for no earlier than August—they may outrace CAPSTONE to the moon.

CAPSTONE will launch from the Mahia Peninsula in New Zealand on an Electron rocket, built by private space company Rocket Lab, who mainly launch little satellites into Earth orbit. CAPSTONE will be Electron’s first attempt to reach for the moon. “That’s also a bit of precedent,” says Thanga.

In early November, after a 3.5-month-long voyage, CAPSTONE will insert itself into a peculiarly elongated loop around the moon, called a near-rectilinear halo orbit (NRHO). This swings from 1,000 miles above one pole to 43,500 miles above the other pole. Entering NRHO is more than just a fun curiosity. CAPSTONE will test this orbit for the future Lunar Gateway, a moon-orbiting space station planned as part of the Artemis program.

“There’s no real uncertainty that the math works,” says Cheetham, but CAPSTONE will give spacecraft operators practice for getting into that orbit.

While it’s orbiting the moon, CAPSTONE will try to do something else: talk to a spacecraft without contacting ground control on Earth. CAPSTONE’s onboard computer will try to link with the Lunar Reconnaissance Orbiter, an earlier NASA spacecraft that’s been mapping the moon’s surface since 2009, and calculate the positions of both spacecraft. When communication from Earth to the moon, even at light speed, takes more than 1 second, being able to chat with local satellites is a useful ability.

Future CubeSats, Thanga says, might be able to make that ability more permanent. For instance, it would enable easier communication to the lunar far side, currently out of Earth’s reach. When the Chinese lander Chang’e-4 touched down on the moon’s far side last year, it needed another satellite to relay messages to and from Earth. 

Lunar satellites that talk with each other can more easily avoid collisions, and they won’t have to hail Earth’s ground control for their every need. “What we want to do is prioritize that ground contact,” Cheetham says, removing routine location checks in favor of transmitting important operational data,

The world’s attention will likely be on the crewed Artemis fights—whenever they actually get off the ground, with the first set for 2024. But small-scale missions like CAPSTONE are necessary to lay the groundwork (or spacework, as it were) for those astronauts.

More moon missions are in the pipeline, potentially launching as soon as the end of this year. NASA has tapped a handful of companies to build an armada of lunar landers—fitted with science experiments for measuring things like subsurface water, the composition of the moon’s surface, and the strength of its magnetic field—that test the prospects for future lunar living.

[Related: We could actually learn a lot by going back to the moon]

As more and more Artemis flights and astronauts make it to the moon, they’ll rely on infrastructure like the Lunar Gateway, which will act as a communications center and a delivery hub for astronauts on the surface. That plan has faced criticism—some commentators have suggested sending moon landings through Gateway will make missions require more energy and expensive fuel.

But Gateway is only the start. The space agencies and their partners behind Artemis are planning everything from lunar mines to lunar satnav to lunar nuclear power plants.

“The feeling is there’s going to be a lot more traffic to the moon,” says Thanga, “and that requires a lot more infrastructure, including systems like the Gateway.”

Correction (June 24, 2022): The CAPSTONE launch location was changed from the Chesapeake Bay to Mahea, New Zealand. Also, the company behind the Electron rocket is called Rocket Lab, not Rocket Labs.

Rahul Rao is a former intern and contributing science writer for Popular Science since early 2021. He covers physics, space, technology, and their intersections with each other and everything else. Contact the author here.

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