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"This first of a kind maneuver from the X-37B is an incredibly important milestone."
Artist's illustration of the X-37B conducting an aerobraking maneuver using the drag of Earth’s atmosphere. Credit: Boeing Space
After more than nine months in an unusual, highly elliptical orbit, the US military's X-37B spaceplane will soon begin dipping its wings into Earth's atmosphere to lower its altitude before eventually coming back to Earth for a runway landing, the Space Force said Thursday.
The aerobraking maneuvers will use a series of passes through the uppermost fringes of the atmosphere to gradually reduce its speed with aerodynamic drag while expending minimal fuel. In orbital mechanics, this reduction in velocity will bring the apogee, or high point, of the X-37B's orbit closer to Earth.
Bleeding energy
The Space Force called the aerobraking a "novel space maneuver" and said its purpose was to allow the X-37B to "safely dispose of its service module components in accordance with recognized standards for space debris mitigation."
While the reusable Boeing-built X-37B spaceplane is designed to land like an aircraft on a runway, the service module, mounted to the rear of the vehicle, carries additional payloads. At the end of the mission, the X-37B jettisons the disposable service module before reentry. The Space Force doesn't want this section of the spacecraft to remain in its current high-altitude orbit and become a piece of space junk.
"Once the aerobrake maneuver is complete, the X-37B will resume its test and experimentation objectives until they are accomplished, at which time the vehicle will deorbit and execute a safe return as it has during its six previous missions," the Space Force said.
The Space Force has identified mobility in orbit as a key focus for its next-generation space missions. This would allow satellites to more freely move between altitudes and orbital inclinations than they can today. Commanders don't want a spacecraft's movements to be constrained by the amount of fuel it carries, allowing satellites to "maneuver without regret."
Space Force leaders have discussed in-orbit refueling, more efficient propulsion technologies, and other ways to achieve this end. Aerobraking is another way to lower a spacecraft's orbit without using precious propellant.
"This first-of-a-kind maneuver from the X-37B is an incredibly important milestone for the United States Space Force as we seek to expand our aptitude and ability to perform in this challenging domain," said Gen. Chance Saltzman, the Space Force's chief of space operations.
Officials did not say when the X-37B spaceplane, also called the Orbital Test Vehicle, will end its mission, which began on December 28 with a launch on a SpaceX Falcon Heavy rocket. The Space Force also didn't say what orbit the X-37B will end up in after the aerobraking maneuvers, but the spaceplane will presumably settle into a low-Earth orbit, where all of its previous missions flew.
There are two X-37Bs in the Pentagon's inventory. The spaceplanes have solar arrays to generate electricity and enough fuel to remain in orbit for years. The longest X-37B flight to date lasted more than 908 days. The vehicles have payload bay doors that open in space, revealing a cargo bay about the size of a pickup truck bed. The spacecraft measures 29 feet (9 meters) long, about a quarter the length of a NASA space shuttle orbiter. It is not designed to carry people.
An X-37B spaceplane with its payload bay doors open, revealing a solar array folded inside.
Credit: Boeing
An X-37B spaceplane with its payload bay doors open, revealing a solar array folded inside. Credit: Boeing
NASA has used aerobraking at Mars to reshape the orbits of its scientific probes surveying the red planet. In 2014, the European Space Agency executed a series of aerobraking maneuvers at Venus with its Venus Express spacecraft. Precise navigation is crucial for aerobraking—coming in too high won't produce enough air resistance to bleed off velocity, while dipping too low could cause the spacecraft to reenter the atmosphere.
The Space Force said it is leveraging experience from civilian science missions to carry out the X-37B's aerobraking maneuvers.
It seems like military officials have been planning this kind of maneuver with the X-37B for at least several years. In 2019, former Air Force Secretary Heather Wilson said the spaceplane can fly in an orbit that "looks like an egg," presumably referring to an elliptical orbit like the one the current mission is flying.
"When it’s close to the Earth, it’s close enough to the atmosphere to turn where it is," she said. "Which means our adversaries don’t know—and that happens on the far side of the Earth from our adversaries—where it’s going to come up next. And we know that that drives them nuts. And I’m really glad about that."
Breaking the silence
The Pentagon rarely releases an update on the X-37B spaceplane in the middle of a mission. During previous flights, military officials typically provided some basic information about the mission before its launch, then went silent until the X-37B returned for landing. The military keeps specifics about the spaceplane's activities in orbit a secret.
This made the Space Force's announcement Thursday somewhat of a surprise. When the seventh flight of the X-37B launched, there were indications that the spacecraft would soar into a much higher orbit than it did on any of its six prior missions.
In February, a sleuthing satellite tracking hobbyist spotted the X-37B in orbit by observing sunlight reflected off of the spacecraft as it flew thousands of miles above Earth. Follow-up detections confirmed the discovery, allowing amateur observers to estimate that the X-37B was flying in a highly elliptical orbit ranging between roughly 300 and 38,600 miles in altitude (186-by-23,985 miles). The orbit was inclined 59.1 degrees to the equator.
On its previous missions, the X-37B was confined to low-Earth orbit a few hundred miles above the planet. When it became apparent that the latest mission was cruising at a significantly higher altitude, analysts and space enthusiasts speculated on what the secret spaceplane was doing and how it would come back to Earth. A direct reentry into the atmosphere from the spaceplane's elliptical orbit would expose the craft's heat shield to hotter temperatures than any of its previous returns.
Now, we have an answer to the latter question.
As for what it's doing up there, the Space Force said the spaceplane on this mission has "conducted radiation effect experiments and has been testing space domain awareness technologies in a highly elliptical orbit." The orbit brings the X-37B through the Van Allen radiation belts and crosses several orbital regimes populated by US and foreign communications, navigation, and surveillance satellites.
Military officials have said previous X-37B flights have tested a Hall-effect ion thruster and tested other experimental space technologies without elaborating on their details. X-37Bs have also secretly deployed small military satellites in orbit.
Stephen Clark is a space reporter at Ars Technica, covering private space companies and the world’s space agencies. Stephen writes about the nexus of technology, science, policy, and business on and off the planet.
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