NEXT month, space shuttle astronauts will cut the power on the Hubble Space
Telescope for the first time since it went into orbit. But no-one is sure if
Hubble will come back to life when the power is switched back on again.
The telescope cannot survive long in orbit without power, and shutting down
the craft鈥檚 systems completely isn鈥檛 something Hubble鈥檚 designers ever
anticipated. The shutdown is necessary because its solar panels are being
replaced with more efficient arrays that will generate extra power for new
cosmos-probing instruments. To handle it, the ageing power control unit that
regulated Hubble鈥檚 electricity supply is being replaced at the same time.
Hubble鈥檚 original power control system was never designed to come out at all.
To do the job, NASA has had to devise a complex set of tools for the astronauts
to remove 36 hard-to-reach power connectors. Power must be shut down during the
operation to protect the astronauts and instruments from power surges. But all
such operations are risky in zero g, as turning a nut, for instance,
tends to turn the astronaut in the opposite direction.
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Hubble needs power to maintain its orbit and orientation and to keep
instruments at their safe operating temperature. But even the back-up batteries
that normally keep the craft鈥檚 systems ticking over safely when it鈥檚 in the
Earth鈥檚 shadow will be switched off as new power systems are fitted. The Space
Agency is taking no chances鈥攂efore the shut-down, astronauts will run
extra power to warm up delicate instruments so they won鈥檛 get too cold while the
power is off.
Mission planners are setting aside all seven hours of the third of five space
walks for the task, with clean-up set for the following day. Clean-up time
leaves a little leeway if more time is needed to complete the mission.
Astronauts have been training extensively with mock-ups on the ground. 鈥淲e鈥檙e
prepared for anything, but we like to have luck on our side,鈥� says Rud Moe,
Hubble mission manager at the NASA Goddard Space Flight Center in Greenbelt,
Maryland. NASA won鈥檛 start the power system change-over if there鈥檚 any sign of a
problem with the shuttle that might mean aborting the repair halfway
through.
The Hubble servicing mission is due to lift off on 28 February, on shuttle
Columbia鈥檚 27th mission. It will capture Hubble with the shuttle鈥檚 robot arm on
2 March, with the delicate power control unit replacement scheduled for 5 March.
The Hubble Service Mission is at http://sm3b.gsfc.nasa.gov/