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 communiqué 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.