The X-37B Orbital Test Vehicle (OTV) has remained cloaked in enigma since its debut flight in 2011. Crafted by Boeing and overseen by the United States Space Force (USSF), this autonomously piloted, reusable spacecraft operates within Low-Earth Orbit (LEO), spanning altitudes of 240 to 800 kilometers (150 to 500 miles) above the Earth. Its mission is to evaluate renewable spaceplane technologies that bolster enduring space objectives.

On December 29, 2023, the X-37B embarked on its seventh journey (OTV-7), reportedly engaging in studies of space radiation impacts and testing Space Domain Awareness (SDA) tools.

In this undertaking, the X-37B is poised to execute groundbreaking orbital modifications. These involve the spacecraft skimming Earth's upper atmosphere to decelerate and lower its altitude while conserving fuel – a technique identified as aerobraking.

This marks the inaugural aerobraking attempt by the X-37B, aiding it in eluding detection by adversarial entities and enabling covert low-altitude passes over Earth in future operations.

According to the USSF, this maneuver builds on six previous successes in LEO and insights accrued from decades of lunar and Martian missions. For instance, in 1997, NASA’s Mars Global Surveyor (MGS) used its solar panels as aerodynamic surfaces to navigate Mars' sparse upper atmosphere, gradually reducing its orbit.

Similarly, aerobraking was employed by the Mars Odyssey in 2001, the Mars Reconnaissance Orbiter in 2006, and the European Space Agency’s ExoMars Trace Gas Orbiter during 2017-2018.

"This pioneering and resourceful series of maneuvers underscores the Space Force’s commitment to cutting-edge innovation while executing national security initiatives in space," stated Secretary of the Air Force Frank Kendall.

"This unique maneuver by the X-37B signifies a critical milestone for the United States Space Force as we broaden our expertise and capability in this demanding domain," added Chief of Space Operations Gen. Chance Saltzman. "The achievement reflects the dedication and tenacity of our team."

Beyond these trials, details regarding the X-37B's functions and objectives remain scarce. However, during the Aspen Security Forum in 2019, former U.S. Air Force Secretary Heather Wilson elaborated on the X-37B's elusiveness, noting:

"[The X-37B is] captivating [because it] executes an orbit resembling an egg, coming close enough to the atmosphere at one point to alter its trajectory. This prevents adversaries from predicting its location since such adjustments occur out of their view, driving them to distraction – and I’m delighted by that."

As Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, explained to Military.com:

"[Wilson’s remarks hint at] a once-classified orbital capability. By dipping into the atmosphere, it shifts the timing of its next appearance, forcing trackers to recalibrate predictions and reassess its position. Even minor timing disruptions create substantial challenges for adversaries relying on consistent orbital forecasts."

Once aerobraking concludes, the X-37B will proceed with its experiments, including exploration in novel orbital zones, advancing SDA technologies, and analyzing radiation’s effects on plant seeds provided by NASA in the Seeds-2 experiment.

The spacecraft will also dispose of service module components per established space debris mitigation protocols. Afterward, it will de-orbit and return to Earth, mirroring its six previous missions.