An Air Force fact sheet described it as ``experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the US Air Force''.
The secretive nature of the equipment on the X-37B has led to speculation over its true nature, with some experts saying it could eventually be designed to tamper with satellites from rival nations.
Built by Boeing's secretive Phantom Works in Huntington Beach, Calif., the Air Force X-37B spacecraft is rumored to be everything from a space bomber to a satellite-killer or a test-bed for advanced spy satellite sensors.
So, what exactly does the X-37B do?
And what doesn't it do?
China and other adversaries "see it as yet another example of the U.S. developing space weapons, contrary to the U.S. public position - that it doesn't have any space weapons," said Brian Weeden, technical adviser to Secure World Foundation, which promotes peaceful uses of outer space.
"There is no proof to support their claims, but that doesn't really matter. Just as some in the U.S. have misrepresented or exaggerated Russian and Chinese programs to advance certain political positions, Russia and China are going to do the same."
Dave Webb, chairman of the Global Network Against Weapons & Nuclear Power in Space, says the X-37B "is part of the Pentagon's effort to develop the capability to strike anywhere in the world with a conventional warhead in less than an hour."
On-orbit sensor platform and test bed, with the ability to return payload.
"What it offers that we have seldom had is the ability to bring back payloads and experiments to examine how well the experiments performed on-orbit," said Gary Payton, the undersecretary of the Air Force for space programs. "That's one new thing for us."
Given the R&D that likely was put into the X-37B, this approach probably isn't very cost-effective, but Weeden said this is the most likely use the spaceplane. X-37B payload bay could hold various sensors used for intelligence collection of the Earth from space, potentially including radar, optical, infrared, and signals/electronic intelligence suites to flight-test and evaluate new sensors and hardware.
Deployment platform for operationally responsive space satellites.
Weeden said this has a midrange chance of being X-37B's mission, and he quotes Payton: "We could have an X-37 sitting at Vandenberg or at the Cape, and on comparatively short notice, depending on warfighter requirements, we could put a specific payload into the payload bay, launch it up on an Atlas or Delta, and then have it stay in orbit, do the job for the combatant commander, and come back home. And then the next flight, we could have a different payload inside, maybe even for a different combatant commander."
But given it still would be dependent on the availability of EELV, it may not have a very quick response time for launch.
On-orbit repair vehicle.
Weeden said this option has a fairly low chance of being X-37B's real mission. While it could be used to rendezvous with malfunctioning satellites and repair or refuel them, the X-37B is limited in altitude (it has been rumored that it will have a maximum altitude range of 700 or 800 km (about 500 nautical miles), potentially high enough to access most Sun-synchronous satellites, but this is unconfirmed, plus not many existing operational military satellite components will fit in the X-37B cargo bay. And as the engineers who tried to figure out how to fix the Hubble Space Telescope robotically, without humans, on-orbit repair is extremely difficult, if not impossible.
On-orbit inspection of satellites.
This option has a low potential, as well. The X-37B could be used to rendezvous and inspect satellites, either friendly or adversary, and potentially grab and de-orbit satellites. However, the X-37B cargo bay is much smaller than many operational satellites, and most of the space in the bay is likely to be filled by the required robotic arm and other gear.
Conventional Prompt Global Strike (CPGS) weapon or delivery system. Weedend says that chance of this being X-37B's mission is zero. It could be launched in response to a pending crisis and remain on orbit for a length of time to respond to high value/very time sensitive targets. However, since the X-37B re-enters like the space shuttle and lands at an estimated 200 mph (321 kph), this means it travels in the atmosphere much slower than a ballistic arc or a hyperkinetic weapon, so it would need to carry conventional explosives to do any significant damage. Also, after re-entry would be a slow moving, not-very-maneuverable glide bomb, easy prey for any air defense system along its path to the target.
So, then, what is the X-37's mission?
Gary Payton, deputy undersecretary of the Air Force for Space Programs and a former shuttle astronaut, says the spacecraft is primarily a technology demonstrator and a platform for space experiments.
"The primary objectives of the X-37 are (developing) a new batch of reusable technologies for America's future, plus learning and demonstrating the concept of operations for reusable experimental payloads," Payton told reporters prior to the X-37B's inaugural launch.
"Take a payload up, spend up to 270 days on orbit. They'll run experiments to see if the new technology works, then bring it all back home and inspect it to see what was really going on in space," Payton said.
The first X-37B mission was a 224-day shakedown cruise - a flight primarily aimed at "proving that the vehicle itself can get up in space, do a job, get back down," Payton said.
Then "the most important demonstration" would be determining whether the vehicle could be prepped for another flight rapidly, and at low cost, he said.
Objectives of the second flight - the first flight of the Air Force's second X-37B vehicle - probably were similar.
Weeden said the spacecraft could serve as test-beds for advanced sensors for spy satellites.
"I think it is very likely, and this may be the primary mission of the X-37B," Weeden said. "As far as the types of sensors, it could be virtually anything, but my guess would be radar, hyperspectral or infrared."
Pike remains skeptical.
"Over time, most of the money got spent just to keep the program going," Pike said.
"It acquired a life of its own. And now to the extent that it might be said to have any larger purpose, it would be to bewilder the Chinese."
The Middle East
Although its orbits may be difficult to predict in advance, its tracks show where the X-37B has been and its likely purpose, says Brian Weeden, a technical advisor at the Secure World Foundation, a Washington-based foundation that focuses on space issues.
For example, he says, the X-37B flew at inclination of 42.79 degrees, which tells you how far north and south in latitude the spacecraft can see. "The tradeoff is that something at a 90 degree polar orbit covers the whole world, but its frequency is less; it may arrive only every couple of days," he says. "If something is 40 or 45 degrees, it would be covering a smaller portion [of the earth], but more often. "
At a 42.79 degree inclination, the X-37B would be useful for looking at a geographic region such as the Middle East, says Weeden, pouring cold water on one theory that that it was used to spy on China's spacelab, Tiangong-1. And given the current political context, he says, the Middle East "makes sense".
Weeden also suggests the X-37B's orbit may indicate that the military is trying out a new sensor system, such as radar imaging or hyperspectral sensors, which collect information across different wavelengths. He suggests this could be the case, because unlike satellites collecting light in the visible wavelength, the X-37B's orbits are not synchronized with the sun, a trick used to maintain a predictable angle between the sun, satellite and ground.
But, like with many of the theories surrounding the X-37B, he warns, "it is just speculation."
Electronic Signals Collection
Publicly, the Air Force says that this is a chance to fine-tune "an affordable, reusable space vehicle."
That is no small feat. In the past, US space "flight" has been less like flying and more akin to billiards, says James Lewis, director of the Technology and Public Policy Program at the Center for Strategic and International Studies in Washington. "It hasn't really been flight because in a flight you can maneuver, you can pull on the controls."
But the X-37B really does more closely resemble space flight. "A maneuverable spacecraft is unique," says Dr. Lewis. "It's really hard to do."
The aircraft is designed to complete a mission that lasts as long as nine months, according to the Air Force. After that time, the OTV will automatically reenter the atmosphere, and descend and land horizontally on a runway at Vandenberg Air Force Base in California.
"This mission will further expand the test envelope" of the OTV, and verify that it can demonstrate "repeatability and reliability," according to an Air Force statement.
The length of the mission indicates that the OTV has a more covert mission as well. There wouldn't be a need to stay in orbit for months otherwise, Lewis says. "It's not like it's just a bus designed to take things into space and bring them back."
This experiment show that the US can use space "as a platform for sensors that can collect on things in a way other countries really can't stop," he adds.
In the case of the X-37B this likely includes collecting "electronic signals of all kinds," whether it's microwave communications or the ability to measure data from a distance.
"In the case of, say, the recent North Korean missile launch this could include messages going back and forth between the ground control and the missiles, as well as measuring the heat signature and the flight path of the launch.
"There's lots of good stuff that you can collect to give you an idea of what the other side is up to," says Lewis.
These are details that the Air Force will not share. "I can't go into the details of the mission any further," says Plante.
What is clear is that it has long been a goal of NASA to lower the cost of putting things into orbit. "A lot of the time people talk about America losing its lead in space, but a maneuverable spacecraft is unique," Lewis says. "Now America is doing something no one else can do."
- Christian Science Monitor
Game Changing Technology
The head of the Air Force Space Command recently made cryptic comments about some aspects of one of the military's most cutting edge and secret weapons, the X-37 space plane.
Air ForceGen. William L. Shelton, who heads the Air Force's space operations, said during a March 26 breakfast meeting with defense writers that the X-37, currently on orbit, is performing its mission.
"It is doing very well on orbit, and we don't have an exact re-entry date for it. But it's had a successful mission, and we are very happy with the performance," he said.
Asked what the space plane was doing well, the four-star general declined to specify.
Gen. Shelton said there are no plans at the present to increase the number of spacecraft from the current two, and he also defended the secrecy of the program's budget.
"If you reveal budgets, you reveal sometimes the capabilities, the amount of technology that's inserted into a program," he said. "I think in this case, it's just a good strategic national security decision. Like we do other things of that ilk. Keep that budget to ourselves."
Pressed for details on the secret craft, Gen. Shelton said only that the system is "game-changing," often a reference to strategic capabilities that can make a difference between winning and losing in a conflict.
A defense official said the X-37 is a key element of the Pentagon's new Air Sea Battle Concept to closely link Air Force and Navy capabilities for defeating China's advanced military systems such as anti-satellite weapons, anti-ship ballistic missiles and cyberwarfare capabilities.
In a future conflict with China, the X-37 is slated to play a key role in knocking out Chinese space sensors that would provide key targeting data for the DF-21D anti-ship missile.
Pentagon officials, however, refuse to say publicly that the X-37 is part of future space warfare systems and insist it is a test bed for research.
Chinese military writers have said the X-37 is part of secret U.S. plans for space warfare against China.
China's Version of the X-37B
Last year several Chinese media outlets reported a test flight of the Shenlong space plane that apparently included its airdrop from an H-6 bomber. But the nature of the Shenlong project's testing, as well as what the robot vehicle truly represents, remains sketchy.
Several China watchers in the U.S. have taken a stab at what the Shenlong (Mandarin for "Divine Dragon) might mean, with some experts conjecturing that the craft is simply a tit-for-tat response to the unmanned X-37B space plane.
"Shenlong is China's effort to develop a re-entering aerodynamic spacecraft, similar to the space shuttle or the X-37B but much smaller than either," said Mark Gubrud, a postdoctoral research associate in the Program on Science and Global Security at Princeton University.
Gubrud told SPACE.com that if China space designers are successful in their Shenlong work, the country may attempt to develop a larger version. [Winged Spaceships: Space Plane Evolution (Infographic)]
"However, the economic rationale for the [NASA] shuttle was never realized, and it is not clear what advantages the X-37B offers the U.S. military over conventional upper stages, satellite buses and re-entry capsules," Gubrud said. The Air Force's robotic plane would appear to serve the U.S. primarily as a sign that American space power endures despite retirement of the NASA space shuttlefleet, he said.
"Appropriately enough, Shenlong may also be little more than a symbol of China's ability to challenge U.S. assumptions of primacy and technological dominance," Gubrud said.
Erickson and Collins also wrote that, depending on its precise nature, Shenlong's reported test may turn out to be part of a larger trend: a shrinking time gap between when the U.S. discloses a prototype military system and when China publicly shows a system similar in type —if not equal in capabilities or immediately operational.
"For previous aerospace developments, China typically revealed its systems' existence at least 15 years after the U.S. first showed its analogous platforms," Erickson and Collins observed.
The bottom line, the two researchers said, is that foreign policymakers need to take China's ambitions in space seriously.
But other analysts say the X-37B is no space bomber.
"Absolutely not," said Weeden, a former Air Force official with experience in space and ballistic missile operations. "The laws of physics are a pretty harsh mistress and make such systems impractical and not very useful."
Some question whether the X-37B itself might be a delivery system for a nuclear bomb - whether the spaceship is intended to re-enter Earth's atmosphere on autopilot and dive-bomb an enemy target.
That's highly unlikely. Just like NASA's shuttle orbiters, the X-37B is an unpowered glider during reentry.
"It would be a very expensive, sitting duck for any air-defense system in the world," Weeden said.
Some surmise the X-37B is a satellite-tracker or a satellite-killer. Or both.
But the U.S. military knows the X-37B is "a very high-interest object" for amateur satellite observers and military officials in Russia and China.
Top Secret Missions of the X-37B
Orbital Test Vehicle mission 4 (May 2015). The Air Force revealed that OTV4 will host a Hall thruster experiment onboard the X-37B flight vehicle.
The Hall thruster that will fly on the X-37B experiment is a modified version of the units that have propelled SMC's first three Advanced Extremely High Frequency military communications spacecraft. A Hall thruster is a type of electric propulsion device that produces thrust by ionizing and accelerating a noble gas, usually xenon. While producing comparatively low thrust relative to conventional rocket engines, Hall thrusters provide significantly greater specific impulse, or fuel economy. This results in increased payload carrying capacity and a greater number of on-orbit maneuvers for a spacecraft using Hall thrusters rather than traditional rocket engines.
This experiment will enable in-space characterization of Hall thruster design modifications that are intended to improve performance relative to the state-of-the-art units onboard AEHF. The experiment will include collection of telemetry from the Hall thruster operating in the space environment as well as measurement of the thrust imparted on the vehicle. The resulting data will be used to validate and improve Hall thruster and environmental modeling capabilities, which enhance the ability to extrapolate ground test results to actual on-orbit performance. The on-orbit test plans are being developed by AFRL and administered by RCO.
Air For Material Command
Produced by Aerojet Rocktdyne, the AEHF satellites' Hall thrusters are 4.5-kilowatt units that use electricity and xenon to produce thrust for maneuvering satellites in space. The novel electric propulsion system produces a whisper-like thrust by ionizing and accelerating xenon gas.
Unlike conventional chemical engines that deliver substantial boosts with each brief firing, the electric system needs the stamina to operate for exceptionally long periods of time to harness its 0.06-pound-thrust into orbit-changing power.
The divergent systems have their advantages and drawbacks. Although typical engines can maneuver satellites rapidly, they use large amounts of heavy fuel that in turn require a bigger, more expensive rocket to carry the spacecraft. Electric propulsion gives up timeliness for efficiency since its xenon fuel weighs a mere fraction of conventional hydrazine, but you must have patience to reap the rewards.
Each of the AEHF satellites, valued at over $1 billion, is a nuclear-survivable spacecraft that would ensure American leadership with communications in the most hellish scenarios of war imaginable. Three such satellites have been deployed to date, with three more planned.