The Hubble Space Telescope is one of NASA's most beloved projects. After
over 30 years in service, it's now facing one of its greatest challenges, as
a technical glitch has left it in safe mode for over a month. On Wednesday,
NASA said it may have tracked down the source of the issue.
The Hubble team had been looking at the payload computer -- hardware dating
back to the 1980s -- as the potential source of a memory problem. "A series
of multi-day tests, which included attempts to restart and reconfigure the
computer and the backup computer, were not successful, but the information
gathered from those activities has led the Hubble team to determine that the
possible cause of the problem is in the Power Control Unit," NASA said.
As with the payload computer, the PCU is part of Hubble's Science Instrument
Command and Data Handling unit. The PCU is responsible for supplying a
constant and steady source of electricity to the computer and its memory.
Herein lies the possible problem.
"The team's analysis suggests that either the voltage level from the
regulator is outside of acceptable levels (thereby tripping the secondary
protection circuit), or the secondary protection circuit has degraded over
time and is stuck in this inhibit state," NASA said.
Hubble is equipped with a lot of backup systems, including a backup PCU. On
Thursday, NASA will attempt the switch to the other hardware in an effort to
restore the telescope to normal operations. Since the issue cropped up on June
13, Hubble's science work has been stalled.
The PCU also resides on the SI C&DH unit. It ensures a steady voltage
supply to the payload computer’s hardware. The PCU contains a power regulator
that provides a constant five volts of electricity to the payload computer and
its memory. A secondary protection circuit senses the voltage levels leaving
the power regulator. If the voltage falls below or exceeds allowable levels,
this secondary circuit tells the payload computer that it should cease
operations. The team’s analysis suggests that either the voltage level from
the regulator is outside of acceptable levels (thereby tripping the secondary
protection circuit), or the secondary protection circuit has degraded over
time and is stuck in this inhibit state.
Because no ground commands were able to reset the PCU, the Hubble team will
be switching over to the backup side of the SI C&DH unit that contains
the backup PCU. All testing of procedures for the switch and associated
reviews have been completed, and NASA management has given approval to
proceed. The switch will begin Thursday, July 15, and, if successful, it
will take several days to completely return the observatory to normal
science operations.
The team performed a similar switch in 2008, which allowed Hubble to
continue normal science operations after a Command Unit/Science Data
Formatter (CU/SDF) module, another part of the SI C&DH, failed. A
servicing mission in 2009 then replaced the entire SI C&DH unit,
including the faulty CU/SDF module, with the SI C&DH unit currently in
use.
If the plan works, it could still take several days to get the telescope
back up and running. There has been concern for the aging telescope, which
is a joint project from NASA and the European Space Agency. Hubble's
successor, the much-delayed James Webb Space Telescope, is still here on
Earth, waiting for a possible October launch.
Hubble has weathered many technical glitches in its time, and it may yet
survive this latest one. The good news is NASA has a plan, and the hope to
go along with it.