The mysterious X-37B has ended its fourth mission, and for the first time on this flight, it has been officially unmasked. Gliding swiftly to a daylight landing at the Kennedy Space Centre after 718 days in orbit, boffins have been rewarded with detailed images and video of the robot spaceplane.

That's been typical of previous missions, but it's a major unmasking for this flight. The landing marks the first time that the general public have been allowed to see the vehicle on this mission.

Previously, the US Air Force (owners of the spacecraft) released nice images of X-37B before it was fully encapsulated in the payload fairing of the Atlas V rocket used to launch it. We could see the outside of X-37B but nothing inside its shuttle-style payload bay.

Clamshell doors protected its contents, which presumably included some secret experiments, from outside view. That was fair enough. We knew a lot about the design and appearance of X-37B, as it originated in an experimental program operated by NASA. Covering up the vehicle didn't really help to keep anything secret. That horse had already bolted.

Why, then, did the US Air Force refuse to release images of the latest mission before launch? It only added to the mystery of an already mysterious program. This analyst speculated that the new spacecraft was possibly a modified X-37B, with new aerodynamic surfaces or coatings.

It was also possible that some form of secondary payload was attached to the outside of the vehicle. Looking at the latest images of the vehicle tends to nullify those theories. It looks just like previous X-37B spacecraft, and it is believed to be a previously flown vehicle.

Additional security protocols could be one explanation. The landing also caught analysts by surprise, with essentially no advance notice or warning signs. Enhanced security could simply satisfy bureaucratic requirements, even if it produces no practical benefits.

It's reminiscent of security clampdowns on announcing the exact launch time of some shuttle missions, even when the launch time could be calculated through orbital mechanics. Missions launched to the International Space Station were fairly constrained in their launch windows.

It's also possible that there could have been additional payloads carried just beneath the X-37B, stuck like barnacles to the payload adaptor. There were no tracking reports of other satellites being released from this launch, but it's still possible that something small and very stealthy was released. More likely, the additional payloads (if any) remained attached to the rocket.

These could have been cameras or other sensors that monitored the X-37B during and soon after separation. They could also have served as observation targets or transponders for some experiment carried aboard X-37B.

The photography blackout could have served to keep these secondary payloads secret instead of the X-37B itself. Knowing about them would not just tell us about the secondary payloads. It would provide clues to the secret objectives of the mission.

The latest flight of X-37B has again extended the record for a mission of this spaceplane. That's consistent. Each subsequent flight has gone longer than its predecessor. But the extension was relatively modest when compared to some of the previous leaps.

That suggests that the endurance of the spacecraft was approaching its limits. Fuel reserves are probably the greatest limitation on mission duration. It burns fuel every time it changes it orbit, and it must maintain a considerable reserve for its re-entry burns.

Thus, we have gained some answers to the nature of this latest X-37B mission. Some theories can be discarded. But most of the mysteries remain.

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Newest Secret US Spacecraft Returns to Earth After Over 700 Days in Space
Kennedy Space Center FL (Sputnik) May 08, 2017
After over 2 years in space, advanced US re-entry X-37B Orbital Test Vehicle spacecraft successfully landed at NASA's Kennedy Space Center. According to the US Air Force, the X-37B Orbital Test Vehicle is "the newest and most advanced re-entry spacecraft." This was the fourth flight of this vehicle. Boeing started the secret X-37B Orbital Test Vehicle project under NASA's aegis in 19 ... read more

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