June 28, 2022

— A small private spacecraft is now on its way to the moon to test new technologies and pave the way for a NASA human-tended space station to be deployed into a special lunar orbit.

CAPSTONE, or Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment, lifted off early Tuesday morning (June 28) on a four-and-a-half month journey to enter a near rectilinear halo orbit (NRHO) around the moon. The same orbit will be used by Gateway, NASA's planned lunar platform from which Artemis missions will be able to reach any site on the moon's surface, including landings at the lunar south pole.

"We view the CAPSTONE mission as a valuable precursor, not just for Gateway, but also for [the crewed spacecraft] Orion and the human landing system in the larger architecture," Nujoud Merancy, chief of the exploration mission planning office at NASA's Johnson Space Center in Houston, said in a pre-launch briefing. "Artemis teams for Gateway and Orion will use the data from CAPSTONE to validate our models, which will be important for operations and planning for future missions."

CAPSTONE launch to the moon. Click to view and enlarge video in new window. (NASA)

Designed and built by Tyvak Nano-Satellite Systems, a subsidiary of Terran Orbital, and owned and operated by Advanced Space, the CAPSTONE cubesat was launched atop an Electron two-stage booster from Rocket Lab's Launch Complex 1 (LC-1) on the Mahia Peninsula of New Zealand at 5:55 a.m. EDT (0955 GMT or 9:55 p.m. local time). Nine minutes later, CAPSTONE separated from Electron's upper stage into Earth orbit.

At 6:05 a.m. EDT (1005 GMT), Rocket Lab's Photon spacecraft bus fired its HyperCurie engine for the first of six planned burns that over the next five days will raise CAPSTONE's altitude. A final burn on Monday (July 4) will set the satellite on its course for the moon.

Unlike the Apollo lunar missions, which took a free return trajectory to the moon, Photon's fuel-efficient ballistic lunar transfer will take advantage of the gravitational pull from the Sun, making it possible to send CAPSTONE into a distant lunar orbit using a small launch vehicle. The 55-pound (25-kg), microwave-size cubesat will reach the moon on Nov. 13.

"It's a much more efficient transfer in terms of fuel usage, but it trades efficiency for time," Chris Baker, NASA's program executive for small spacecraft technology, told reporters.

A near rectilinear halo orbit is a significantly elongated path located at a balance point between the gravities of Earth and the moon. The orbit provides stability for long-term missions like Gateway and requires minimal energy to maintain.

CAPSTONE's orbit will bring it within 1,000 miles (1,600 km) of one lunar pole on its near pass and 43,500 miles (70,000 miles) from the other pole at its peak every seven days. The same orbit will enable spacecraft flying to and from Gateway and the moon's surface to require require less propulsion capability than is needed in other circular lunar trajectories.

"So it has the benefit of the low energy to get into and out of, but you are then riding this balance point between the gravitational pull of Earth and gravitational pull of the moon," said Baker. "And that's where it gets into this kind of complexity, making sure we know how much energy it is going to take to stay in that balance point. We're getting that kind of operational experience before we go and do that with Gateway."

CAPSTONE will circle the moon for at least six months, validating the power and propulsion requirements for maintaining its specific orbit. Another benefit of NRHO is that the satellite will never be out of sight of Earth.

"One of the key things of the specific operational NRHO we will be flying is that this is an eclipse-free NRHO. It was designed to avoid eclipses from Earth, which is a very important consideration," said Brad Cheetham, Advanced Space's CEO and principal investigator for the CAPSTONE mission. "This unique orbit is always in view of Earth, so we are never behind the moon as you look from Earth [and] we're never flying through the shadow of Earth."

"This is the same approach that Gateway is taking, In fact, we have designed all of our CAPSTONE mission operations around the orbit that is planned for Gateway," Cheetham said.

In addition to testing NASA's models for NRHO, CAPSTONE will also demonstrate a new approach to spacecraft-to-spacecraft navigation, as well as capabilities to communicate with Earth. Using NASA's Lunar Reconnaissance Orbiter (LRO) as a reference point, CAPSTONE will employ a dedicated flight computer and radio to determine where it is in its orbital path.

CAPSTONE will communicate directly with LRO and utilize the data obtained from this crosslink to measure how far the two spacecraft are apart and how fast the distance between the two is changing, which in turn will be used to determine CAPSTONE's position in space. If successful, this software, referred to by the first half of the mission's name — Cislunar Autonomous Positioning System, or CAPS — will enable future spacecraft to determine their location without having to rely on tracking from Earth.

CAPSTONE will be the first spacecraft to enter an NRHO around the moon and only the third mission to test a multi-body orbit. Previously, NASA's ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) probes (not to be confused with the Artemis program) and China's communication relay satellite, Queqiao, entered multi-body orbits.

CAPSTONE's launch is the first to use Rocket Lab's Electron booster and Photon bus to reach the moon, although it is not the first small rocket to send a satellite or probe on an interplanetary mission. NASA's Lunar Prospector reached the moon flying atop a Lockheed Martin Athena II in 1998.