The third launch of the US Air Force's X-37B robot spaceplane opens new challenges to this robust spacecraft. For the first time, an X-37B mission is underway with a used vehicle. The spaceplane that's now in orbit flew the first X-37B mission in 2010, and already had more than 224 days of flight time before its second launch.

Few observers would doubt that the vehicle can survive a lot longer in orbit. The second X-37B mission remained aloft for 469 days. The first vehicle also has the advantage of being examined and possibly repaired after its first return to Earth. Nobody knows how long this third mission will last, but we can reasonably expect that it will be in space for longer than its first mission.

The greatest challenge this spacecraft will face will be during re-entry. The spacecraft's thermal protection system will be subjected to the rigours of atmospheric friction for a second time.

Images of both previous returns of X-37B vehicles suggested that the Shuttle-like thermal tiles that cover the spacecraft were slightly discoloured from their ordeal, but looked very healthy. A second baptism of fire will show just how re-useable this vital spacecraft sub-system can be.

The overall goals of this third mission probably remain the same as the previous flights. The US Air Force is using the spacecraft as a technology test bed, with a variety of innovative new parts on board.

Some of these are openly disclosed, such as the "robot pilot" avionics that will guide the vehicle to a runway landing. Some remain well hidden inside the spacecraft's small payload bay, which exposes its contents to space when two clamshell doors open in orbit.

We know that there's a boom with a deployable solar panel inside the payload bay, and this unfurled soon after X-37B reached orbit. But it's widely suspected that more interesting things are also inside the bay.

This analyst has long advocated that each of the three X-37B flights has carried payloads on behalf of the National Reconnaissance Office (NRO), the organisation responsible for America's spy satellites. But the contents are probably not as sexy as some conspiracy theories have suggested.

The NRO is probably testing materials and small electrical and mechanical parts that it hopes to use in future NRO satellites. Flying these on board X-37B offers a rare chance to test the parts in space, expose them to the full rigors of the space environment, then return them to Earth for study.

It's possible that some of the parts in the bay were carried on previous missions. Others have probably been changed around on different flights. There could even be a separate communications payload inside the bay that reports on the health of these parts through a secure channel, bypassing the normal telemetry links that control the rest of the spacecraft.

As usual, we do not even have a rough estimate of the duration of the flight. It's probable that the Air Force, and their NRO partners, have an open plan.

The performance of the spacecraft and its payloads will be monitored throughout the mission, and they will extend the flight for as long as everything seems healthy. The mission should last at least nine months.

Beyond this, anything is possible. Amateur satellite observers will have fun tracking the X-37B and watching for any changes in its orbit. These will probably be rare. Mission controllers will need to husband the spacecraft's fuel reserves carefully if they want a highly extended mission.

We will probably receive few updates on this semi-secret mission for months, either through official or unofficial channels. It will be a long wait for the next major phase of the mission, which will probably be the spacecraft's return to Earth.

Will this vehicle show any more weathering from its second endurance flight? In the short term, this is probably the only piece of information that analysts will be able to glean from open sources.

X-37B will soon slip from headlines in the media. But its third mission has advanced the program to a highly advanced state. Each mission seems to be breaking new ground. Who knows what the third flight will really achieve?

Dr Morris Jones is an Australian space analyst who writes for SpaceDaily.com Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.