* Astronomy

Hope that Japan's Hayabusa spacecraft will return to Earth is fading as mission controllers remain unable to regain complete control of its orientation.
The spacecraft was designed to bring the first-ever asteroid samples back to Earth for analysis. But recent data suggest that, during a landing attempt on 26 November, it did not fire metal pellets into the 600-metre-long asteroid Itokawa to draw up material for collection.
Now mission controllers have little hope the spacecraft will be able to get back to Earth - even without its quarry - because of continuing problems with its fuel thrusters.

"The situation is not optimistic" - Jun'ichiro Kawaguchi, Hayabusa's project manager.

The spacecraft was supposed to begin its return journey by mid-December to take advantage of an ideal alignment between the Earth and the asteroid. The next such alignment will not occur for another three years.
The fuel thrusters have been used to point the spacecraft because two of its three stabilising reaction wheels failed in July and October 2005. But after the landing attempt on 26 November, one thruster on the spacecraft's upper panel sprang a leak, forcing the craft into an emergency shutdown of all its non-essential systems.
As mission controllers worked to return the craft to normal operations, other thrusters failed - possibly because their valves got locked shut or because their hydrazine fuel froze.
As a result, ground controllers were not able to accurately orient the spacecraft to ensure its solar panels faced the Sun. Around 1 December, the resulting loss of battery power forced most of the onboard instruments to shut down or restart incompletely.
As an emergency measure to restore power, ground controllers programmed the craft to vent the xenon gas from its ion engines. These engines - which use electric fields to accelerate a beam of ions - were designed to propel the craft on its 2 billion-kilometre round trip to Itokawa.
Firing the xenon thrusters successfully pointed the craft's solar panels towards the Sun on 5 December - when the least manoeuvring was needed. But it was not an easy feat.

"It is not a strong thruster and is very easily subject to disturbance" - Jun'ichiro Kawaguchi

According to a recent mission status report, there is enough xenon fuel both to point the spacecraft and return it to Earth. But the ion engines are currently off and it is not clear how healthy the engines are. A plan to reignite them on 14 December will likely be postponed.

"We are in a very tough spot. We are waiting for what we can do next" - Jun'ichiro Kawaguchi .

The mission has been plagued by problems, including the loss of a robot that was due to hop around and explore the asteroid. Mission members say using commands from both human ground controllers and the spacecraft's autonomous landing protocols sent the robot into space when it was too far away from the asteroid.
Poor planning of spacecraft commands also appears to have been responsible for the failure to collect samples during the spacecraft's two landings on 20 and 26 November.
During the first landing, Hayabusa bounced twice on the asteroid then sat on its surface for 30 minutes. But it did not fire a pellet - apparently because two of its autonomous landing systems sent it conflicting commands. One detected a potentially damaging boulder on the asteroid's surface and commanded the spacecraft to rise away from the asteroid, while another found that the spacecraft was not in the correct orientation for an emergency ascent.
A similar problem may have occurred on 26 November, when the spacecraft touched the asteroid for just one second. Telemetry sent back just after the landing showed a command to fire the pellets had been made.
But other data sent back on 6 December suggest the pellets' control mechanism had been accidentally "disarmed", preventing the pellets from firing.

As has been reported, it is estimated that part of a series of attitude and orbit control commands to restore the Hayabusa from its safe-hold mode have not gone well, and the functions of its major systems, including its attitude and communication network, have significantly deteriorated. However, on Nov. 29, a beacon line through low gain antenna was restored.

On Nov. 30, JAXA started a restoration operation by turning on and off the radio frequency modulation through the autonomous diagnostic function.
Subsequently, on Dec. 1, telemetry data was acquired at 8 bits per second through the low gain antenna, although the line was weak and often disconnecting. According to the data transmitted so far, the attitude and orbit control commands sent on Nov. 27 did not work well due to an unknown reason, and either major attitude control trouble or a large electric power loss seems to have occurred.
It is estimated that the overall power switching systems for many pieces of onboard equipment were reset as their temperature dropped substantially due to the evaporation of leaked propellant, and also because of a serious discharge of electricity from the batteries of many sets of onboard equipment and systems due to declining power generation. Details are still under analysis.

On Dec. 2, JAXA tried to restart the chemical engine, but, even though a small thrust was confirmed, they were not able to restore full-scale operations. Consequently, the cause of the anomaly on Nov 27 is still under investigation, and they suspect that one of the causes could be the malfunction of the chemical engine.

On Dec. 3, JAXA found that the angles between the axis of the onboard high gain antenna (+Z angle) and the Sun, and also that with the earth, had increased to 20 to 30 degrees. As an emergency attitude control method, they decided to adopt a method of jetting out xenon for the ion engine operation. Accordingly, they immediately started to create the necessary operation software. As they completed the software on Dec. 4, they changed the spin speed by xenon jet, and its function was confirmed.
Without delay, they sent an attitude change command through this function.

As a result, on Dec. 5, the angle between the +Z axis and the sun, and the earth, recovered to 10 to 20 degrees, and the telemetry data reception and acquisition speed was restored to the maximum 256 bits per second through the mid gain antenna.

After that, they found that there was a high possibility that the projectile (bullet) for sampling had not been discharged on Nov. 26, as they finally acquired a record of the pyrotechnics control device for projectile discharging from which they were not able to confirm data showing a successful discharge. However, it may be because of the impact of the system power reset; therefore, they are now analysing the details including the confirmation of the sequence before and after the landing on Nov. 26.

On Dec. 6, the distance between the Hayabusa and the Itokawa was about 550 kilometres, and that from the earth is about 290 million kilometres. The explorer is relatively moving from the Itokawa towardthe earth at about 5 kilometres per hour.

JAXA are now engaging in turning on, testing, and verifying onboard equipment of the Hayabusa one by one to start the ion engine. They currently plan to shift the attitude control to one using the Z-axisreaction wheel, and restart the ion engine. The restart is expected to happen no earlier than the 14th. JAXA are currently rescheduling the plan for the return trip to earth. They need to study how to relax the engine operation efficiency. They will try to solve the problem with the attitude control (such as the restoration of the chemical engine), then find a solution for the return trip.

Since Nov. 29, their reports have been limited due to difficulties in confirming telemetry data.

It now looks like that the Hayabusa probe may not be able to return to Earth because of the thruster problems.

"If we cannot revive the thrusters, of course a return journey is impossible" - Junichiro Kawaguchi, Project leader.

JAXA is not certain, even if the thrusters can be persuaded to function, that there is sufficient fuel left for the return leg.

The spacecraft is still in "safe"mode, after a problem with one or two of its 12 fuel thrusters.
The onboard computer has shut down all unnecessary systems to protect it from damage.
JAXA controllers are still working to bring it out of this mode by testing how it responds to various commands.

"The big question is can they bring the spacecraft back to normality, identify which of the thrusters have a problem and perhaps shut them down. I don't think it's a big problem" - Don Yeomans, mission team member, NASA's Jet Propulsion Laboratory in Pasadena, California, US.

JAXA controllers still have to receive the telemetry signals that will confirm whether the pellets were fired and whether the spacecraft was oriented so that it could collect the resulting debris.
That data will be received after the probe switches out of safe mode, in the next few days.
It will only be certain they have samples when they open the capsules on Earth.

If everything did go to plan on Saturday, mission controllers will cancel a possible second sampling attempt being considered for 1 December.

The deadline the spacecraft must keep is to leave Itokawa by mid-December, and take advantage of gravitational assists from other bodies in the solar system to return to Earth using as little power as possible.
It will use an ion engine to propel itself during the 18-month trip.