To
keep the Hubble Space Telescope going, officials are changing how it operates
and contemplating other actions for the aging observatory.
Engineers
recently shut
down one of the orbiting observatory's three operational gyroscopes in an
effort to preserve the operating life of the third gyro, thereby pushing Hubble's
science observations into mid-2008.
Other
life-extension ideas are being studied - even downshifting Hubble onto one-gyro
mode.
Scientists
and engineers remain hopeful that the telescope will once again get a servicing
makeover by astronauts. But such a shuttle mission depends on the health of
that human spaceflight program. The shuttle is headed for retirement in 2010,
with a vaguely defined Crew Exploration Vehicle to be its replacement.
Meanwhile, it is not clear when the next
flight will take place nor whether a trip to Hubble will be possible.
Keeping
Hubble alive and scientifically valuable has become a race against the clock that
involves aging hardware and dwindling battery life while solar activity that
eats away at satellite's orbit -- and of course, budget considerations.
Shuttle stopover
Hubble
packs six gyroscopes and four free-spinning steering devices called reaction
wheels. This hardware is used to point the telescope for observations.
Late
last month, Hubble Space Telescope engineers purposely shut down one of the
three operational gyroscopes aboard the observatory. The system was originally
designed to operate on three gyros, with another three in reserve.
The "two-gyro science mode" is expected to preserve the operating life of the
third gyro and stretch out the telescope's science observations halfway into
2008.
"The
best idea for extending life beyond two gyros is to get up there with the
shuttle and service it. And that's what we're working on," said Preston Burch,
program manager for the Hubble Space Telescope at NASA's Goddard Space Flight
Center in Greenbelt, Maryland.
While
such a mission is not manifested for the shuttle program at the moment, Burch
said his program is shooting for a Hubble servicing mission in December 2007.
Preliminary discussions, he told SPACE.com, also point to shuttle
Atlantis being tapped for that servicing stopover.
Science discovery factor
There
are two new instruments ready and waiting to be plugged into Hubble:
Wide
Field Camera 3 that sees in both infrared and ultraviolet wavelengths and far
more sensitive in the infrared than Hubble's Near Infrared Camera and
Multi-Object Spectrometer (NICMOS).
Cosmic
Origins Spectrograph that is capable of studying the chemical composition of
far-distant interstellar gas and replaces Corrective Optics Space Telescope
Axial Replacement (COSTAR) hardware.
"What
makes Hubble worthwhile," Burch said, "is putting in new science instruments
that have state-of-the-art detectors to really make the leaps forward...what we
call the 'science discovery factor.' Those are the things that are really going
to make Hubble a useful and desirable space research vehicle beyond the 2008
time frame."
A
Hubble Servicing Mission 4 would have the visiting shuttle crew also boost
Hubble into a higher orbit, replace a fine-guidance sensor, and place
protective material on top of torn insulation.
Life-extension initiatives
Meanwhile,
Hubble engineers have scoped out a non-shuttle priority list of things to do
that can maintain the telescope's well-being. There's a list of roughly two
dozen items that would be worth considering, Burch said. After the Columbia
accident, he added, a Hubble life-extension initiatives program was put into
force.
The
two-gyro science mode was at the top of that list. New pointing algorithms had
to be developed. Simulations were done on the ground to prove the idea
workable. Then the concept was tested on Hubble itself.
"It
worked far better than we had expected," Burch said. Now, both government and
industry teams are looking at the feasibility of a one-gyro science mode. "That
could be very key in terms of keeping the science going if a shuttle
[servicing] launch date drifts to the right a lot," he added, say until late 2008
or perhaps early 2009.
Other
life enhancing thoughts include tracking the number of cycles on Hubble
transmitters and hours of operational use.
Endurance run
A
key to Hubble's endurance run are rechargeable nickel-hydrogen batteries,
energized by the observatory's solar panels. The telescope has its original
batteries. They date back to 1990 and are deteriorating in power levels with
age. When the batteries are no longer able to hold a charge, Hubble becomes
inoperable.
Burch
said that a Hubble battery test bed at NASA's Marshall Space Flight Center in
Huntsville, Alabama is helping to forecast battery lifetime. That test rig uses
one of the Hubble flight-rated battery spares to assess power trends. Other
Hubble-unique facilities try different charging and energy management schemes,
he said.
"The
projected life of Hubble batteries has been extended. We used to say we thought
they were good until 2008-2009. Now we're thinking 2010," Burch said. "We think
Hubble will remain serviceable until well into 2010...based on latest projections
and test data we have."
How
productive Hubble will be at that time from a science perspective is a
different matter.
"That's
driven principally by the gyros. And right now, we're saying we think we're
good until the middle of 2008," Burch said. "Hubble will still be worth
servicing as long as the batteries hold out."
Good news
With
the stretch out of Hubble servicing via shuttle astronauts, the vigor of the
telescope's batteries and gyros are central to extending the observatory's
life.
Going
to the two-gyro science mode is great news, said James Crocker, vice president
of civil space programs at Lockheed Martin Space Systems Company near Denver,
Colorado. The firm is part of an on-going industry and government partnership
to keep Hubble alive and well.
"The
team is used to doing miracles," Crocker told SPACE.com. The placing of
one gyro into storage and saving it was a well-researched and tested plan. As
for the gyro, he added: "When it's running, it is wearing. When it is off, it's
not."
Moreover,
Hubble batteries are also a source of good news. "They are not degrading as
fast as we had feared," Crocker said. Also, past experience with batteries in
space suggest that they tend to degrade gracefully, he said, perhaps allowing
use of select Hubble instruments instead of all of them at some point.
Working in the cathedral
The
Hubble operating and servicing team has been a steady-state activity since the
observatory was shuttle-deployed in 1990.
"People
are constantly pulling rabbits out of the hat. Hubble is like working in the
cathedral. There's a very dedicated group of people who just aren't going to
let Hubble go quietly in the night," Crocker said.
Crocker
said that NASA chief, Michael Griffin, has the Hubble team marching toward a
shuttle servicing mission - given a couple of good shuttle flights. "From an
execution point of view, we're continuing to hold the option open," he said,
"and we're doing all the steps that we need to do to be ready to go."
Given
Hubble's batteries and the gyro fixes, "I think we've got a nice window" for
shuttle servicing, Crocker added, probably no sooner than December 2007 - with
a goal of perhaps pulling it in a few months earlier if necessary.
"Everything
is being done to get shuttle back into a position where it can service the
space station and Hubble," Crocker explained.
Partial termination
Meanwhile,
work on a Hubble deorbit module
has been cancelled, Burch said, a task that was underway at Lockheed Martin.
A
"Dear Lockheed letter" advising of the stop work on the deorbit module was
dated Sept. 2, Burch said. However, that communique also called for "partial
termination" of some related work by the company.
The
contract was not totally cancelled, Burch noted. Lockheed Martin expertise is
being requested for both sensor technology ideas and an attachment fixture to
be outfitted to Hubble. This fixture could be secured to the telescope by
either astronauts or by a robotic mission, he said.
"We
thought it smart to do something while we're up there to make it a lot easier
to rendezvous with and grapple Hubble for deorbiting," Burch said. That might be
accomplished by either the Crew Exploration Vehicle meant to replace the
shuttle, or by robotic means, he noted.
Furthermore,
quite a bit of work had been done by Lockheed Martin on the Hubble deorbit
module. The company has been informed that all propulsion hardware that is 75
percent complete or greater should be wrapped up and turned over to the Lunar
Reconnaissance Orbiter (LRO) project.
"Scrapping
hardware that is three-quarters complete is, I think, sinful," Burch said. So
the plan is to use deorbit module-related valves, thrusters, and other items
for the Goddard Space Flight Center-managed LRO project, he said, with the LRO
office reimbursing the Hubble project for that hardware.
"Instead
of [LRO] building from scratch and having all of our stuff wind up in a scrap
pile, we thought that this made some sense," Burch said.
Eventual destiny
At
some point in the future, Hubble will meet its ultimate fate - taking a
destructive dive into the Earth's atmosphere by natural forces or under
controlled ditching.
The
telescope is not likely to fall back to Earth on its own prior to 2020,
according to Nicholas Johnson, NASA Orbital Debris Program Manager and Chief
Scientist for Orbital Debris at the NASA Johnson Space Center in Houston,
Texas.
The
telescope's reentry could happen a few years earlier, but that depends on the
Sun's activity in future years, Johnson said. Increased output from the Sun expands
the Earth's atmosphere. That creates added drag on Hubble and will hasten its fall
from orbit.
Johnson
said if another servicing mission is carried out, Hubble would likely be given
a small boost in altitude before the shuttle departs the scene. This would
further delay a natural reentry of the telescope, he explained.
Burch
of Goddard Space Flight Center said that a very conservative estimate of
Hubble's reentry is 2021. Other models predict a historically low solar cycle,
putting an on-its-own tumble of the telescope to Earth in 2025, he said, and
perhaps out to 2030.
"I
think two more decades of flight is a real possibility for Hubble," Burch
advised. "The biggest unknown is how long Hubble is going to live...to continue
to produce useful science.
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