The X-37 is called the "mini-shuttle" because it is about 29 feet long, has a 15-foot wingspan, and weighs just 11,000 pounds, and there have been five Earth orbit missions since 1999 at altitudes between 200 and 250 miles. The current two reusable X-37Bs look like scale-model military "spacefighters" but are officially designated as robotic "temporary satellites" to avoid violating the Outer Space Treaty that forbids weapons platforms at altitudes over 62 miles above the Earth.

The Air Force 's top-secret X-37B Space Plane that appears to be the prototype for the world's first stealth outer space fighter just set a new record of 700 days in orbit.

Formally titled the "Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies," the United Nations agreement prohibits nuclear weapons in space and mandates that all celestial bodies be limited to peaceful purposes and that no nation can claim sovereignty over anything in space.

The treaty became effective in October 1967 with the signatures of the United States, the Soviet Union, and the United Kingdom. Subsequently, 106 other nations signed, including Taiwan, which ratified the treaty in 1969. When the U.N. transferred the "China" seat to the People's Republic of China in 1971, Taiwan's signature for "China" was also officially transferred. But the PRC never actually signed the Outer Space Treaty.

The current X-37B mission began atop a SpaceX Falcon 9 rocket that lifted off from Launch Complex 39A at NASA's Kennedy Space Center in Florida on Sept. 7, 2017. But despite a sophisticated global network of academic and amateur astronomers scanning the heavens, only one photograph was snapped of a stealthy X-37B in space.

The secret to the X-37s escaping visual detection in orbit was outed last week by former secretary of the U.S. Air Force from 2017 through 2019 Heather Ann Wilson in a speech at the Aspen Security Forum on space situational awareness and deterrence.

Wilson called the X-37B "fascinating" because it "can do an orbit that looks like an egg and, when it's close to the Earth, it's close enough to the atmosphere to turn where it is." She added, "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."

Harvard-Smithsonian Center Astrophysics researcher Jonathan McDowell commented that Wilson was referring to the X-37B's previously secret capability to unpredictably alter its elliptical orbit without a detectable rocket burn. Apparently, the alteration movements are due to flying low with stubby wings, creating varying atmospheric drag.

Engineers normally seek to avoid any atmospheric drag, because it is associated with slowing speeds that create orbital decay that causes the satellites to fall back to Earth. But McDowell told Military.com, "The dip into the atmosphere causes a change in the timing of when it next comes overhead. So [trackers'] predictions are off, and [they] have to search for it all over again."

To potential adversaries like China and Russia that are racing to develop integrated radars and telescopes for targeting, the United States may have developed orbital "eccentricity" that changes the timing for when the vehicle will be in a particular point in space. According to McDowell, such stealth orbit changes make more work for an enemy.

Given that the payload and most of its activities remain classified, it was surprising the military authorized Wilson to disclose the X-37B's stealth features just as it set a record for reusable vehicle space flight duration. But it has been disclosed that the vehicle is carrying the Air Force Research Laboratory Advanced Structurally Embedded Thermal Spreader to "test experimental electronics and oscillating heat pipe technologies."

Modern electronics design is limited by potentially catastrophic waste heat and temperature fluxes associated with denser electronics packaging. If scaled up and militarily equipped with breakthrough thermal heat spreaders, future X-37s could be weaponized with directed energy laser canons that could knock out opponents' satellites and swat away ground-launched intercontinental missiles in boost phase.

The X-37 program began in the late 1990s as a NASA vehicle to service the International Space Station, but it was transferred to the Defense Advanced Research Projects Agency in 2004. Boeing has built at least two X-37 "B" spaceplanes, but the company is developing 52-foot long X-37C spaceplanes that could carry six astronauts.