Posted on 08/15/2003 6:36:49 AM PDT by bedolido
The Hubble Space Telescope, astronomy's vaunted time machine, was granted a conditional reprieve yesterday when an expert panel recommended that NASA consider sending a crew of astronauts at the end of the decade to extend its career, rather than dropping it out of orbit, as has been planned.
But the committee said its recommendation should be carried out only if the science to be performed in those additional years was able to beat competing proposals for new NASA science projects.
For the last 13 years, floating above the Earth's murky atmosphere, the telescope has beamed down crisp images of galaxies still forming at the dawn of cosmic time, peering into the hearts of galaxies and quasars in search of black holes, and investigating the mysterious "dark energy" that seems to be wrenching the cosmos apart.
"By any standards the H.S.T. has been a spectacular success — one of the most remarkable facilities in the entire history of science," said the committee, whose chairman is Dr. John Bahcall of the Institute for Advanced Study in Princeton, in a report posted yesterday on the Web site of the National Aeronautics and Space Administration.
The committee members and other astronomers pointed out, however, that since the breakup of the Columbia shuttle, the telescope's future has been threatened because it is hostage to the ability of a space shuttle to pay periodic visits for maintenance and to replace old instruments with new ones.
Those repair missions would take the shuttle too far from the orbit of the International Space Station in case of trouble. As a result, the space agency should be prepared for a range of possibilities, the report said, from no more shuttle missions to two.
More is likely to be heard on that score in a couple weeks when the report of the Columbia Accident Investigation Board is released. But ultimately, Dr. Bahcall said, Congress, the White House and the NASA administrator will decide whether the shuttle may visit the telescope.
Dr. Anne Kinney, who is in charge of astronomy and physics in the space agency's office of space science, said that the Bahcall committee's report was "a good report."
"It reminds us that we need to be flexible," Dr. Kinney said. But she added that there was no budget for the extra mission and no precedent for the kind of competition that Dr. Bahcall and his colleagues had proposed.
"It's going to be a challenge," she said.
Astronomers were generally pleased with the report. Dr. Wendy Freedman, director of the Carnegie Observatories in Pasadena, Calif., called it "balanced and thoughtful."
Dr. George Rieke, an infrared astronomer at the University of Arizona, said the idea of a competition was "a sensible way to deal with limited resources."
Dr. Steven V. W. Beckwith, director of the Space Telescope Science Institute, praised the report, saying, "I couldn't be happier."
He added:"Everyone here is tremendously grateful to NASA for its support of the Hubble. We're delighted to have the chance to compete to continue this extraordinary story."
The story of the $1 billion Hubble, launched in 1990, is one of the great comeback stories in modern science. It was designed to take advantage of an orbital vantage point above the Earth's atmosphere, which smears images and blocks some wavelengths of light from reaching ground-based telescopes.
Once it was in orbit, however, astronomers were devastated to discover that the telescope had a flawed mirror.
The flaw was corrected in 1993 by sky-walking astronauts who, in effect, fitted the telescope's instruments with corrective lenses, enabling Hubble to attain the glory for which it was designed.
NASA has long planned to end Hubble's spectacular run and bring it down to make way in the budget for the James Webb Space Telescope, now scheduled to be launched in 2011. But some astronomers have urged that Hubble's life be extended, arguing that the telescope has grown even more productive in its years in orbit, and that the Webb could be delayed.
Moreover, the Webb is being designed for the infrared wavelengths that very distant galaxies would be emitting as they sped away in the expanding universe, not the visible wavelengths that Hubble sees so exquisitely.
The panel that NASA asked to review the issue also includes Dr. Barry Barish of the California Institute of Technology, Dr. Jacqueline Hewitt of the Massachusetts Institute of Technology, Dr. Christopher McKee and Dr. Charles Townes, both of the University of California, and Dr. Martin Rees of Cambridge University in England.
One of the main elements in their thinking, Dr. Bahcall said, was the realization that NASA might have to mount a mission to the telescope anyway at the end of the decade to attach a rocket to bring it out of orbit safely. The telescope is too big to be left to tumble out of orbit by itself.
The space agency had originally hoped to grab it with the shuttle, bring it back to Earth and put it in the Smithsonian Air and Space Museum, but the Columbia disaster scuttled that option.
NASA is looking into developing a robotic spacecraft that could attach itself to the telescope, but many astronomers argued that the job could be done more reliably by astronauts. And if the astronauts are there anyway, they said, the telescope could be spiffed up for another few years of science.
Another element was the inspirational qualities that Hubble has both for astronomers and for the public, Dr. Bahcall said.
Should really be:
Government bureaneers beg the government to save their jobs.
The external forces (that cause a torque of the vehicle) our satellite “feels,” are mainly from atmospheric drag for the low Earth orbiters and the solar radiation pressure for the higher ones. Other torques may include magnetic fields, gravitational field gradients, solar wind particles, etc.
Also, once you start adding angular momentum to a wheel, isn't it hard to subtract exactly the same amount of momentum?
On some satellites, that is exactly what the torque rods are used for. To unload the wheel momentum.
(Hmmm... maybe that is why the wheels need to be normally still, so you can just apply a brake to stop a turn of the telescope
The wheels are used constantly to keep the satellite pointed properly. The onboard control laws perform the “dance” between the wheels, satellite, and torque rods. :-)
Here is a really nice write up on a reaction wheel strategy for the ESA’s Integral Gamma Ray Telescope:
The stable Lagrange Points are much further out. They fall within the orbit that the moon sweeps out. So those points are on the lunar orbit track both leading and laging the moon by 60 degrees.
:-)
Lets get even pickier. Add the effects of both Jupiter and the Sun. :-)
What Vade was pointing out was a glaring error in my statement about the center of mass for the Earth/Moon system. It is not exactly at the center of the Earth due to the large mass of the moon in Earth's orbit.
It so happens I worked on a satellite that did that very thing. No matter what the orientation, there was enough solar cell area to power the satellite. Often though due to weight, power, and stability reasons, panels are just sail areas that need to be orientated.
I assumed you guys would be way ahead of me. At least I'm not in the same league as the writer of a famous letter to the London Times, who seriously suggested that moon shots should be scheduled for dates when there is a full moon, so we'd have a bigger target to aim for.
ROFL! I had forgotten that. :-)
Thanks! :-)
Your welcome :-)
(You liked my "note to self") :-)
Let me tell you an anecdotal story. This is from a time not too long ago, when I worked at another place. They had recieved a Small Explorer Mission grant from NASA, worth about 3 million. They selected a company of some reknown to build the satellite bus and software (not one who's name I thankfully remember). It wasn't anyone too well-known, since it was a small satellite and a small university. The company had made prior satellites, but bad luck had pretty much followed them the entire way before this. Their first satellite had minor attitude issues, which were believed to have been caused by problems with coming out of the nosecone . They fixed them, I believe, and the satellite worked, sort of. The second satellite never made it out of the Pegasus rocket vehicle's nosecone, and ran out of batteries without ever seeing the sun. Finally, the last satellite was built for the group, and it passed all of the tests necessary for the SMEX. It was launched into space, and it was the first satellite made by that particular company to make it out of the nosecone of the rocket cleanly. Unfortunately, the satellite had one solar panel on one side of the spacecraft bus, instead of panel wings extending from the outside of it. The satellite came out of the cone, and as it turned to orient the solar panel, oriented itself so that the panel was directly away from the sun.
With the solar panel not recieving insolation, the spacecraft quickly ran out of batteries and died. The spacecraft began to tumble, and every time the panels recieved solar power, it would immediately turn away from the sun, and the batteries would die, and it would begin to tumble again. A team of engineers were turned loose on the problem, and after about 15 hours, they discovered that the attitude control code had a sign error in it, and the satellite would turn away from the sun while trying to align the panels to the sun. It turned out that that particular instance of code had been in all of the other satellites, but it was never discovered because of all of the other calamities. I felt bad for a graduate student who had helped with every stage of the process, but had to find a different topic because of a lack of satellite to take any data. How depressing is that?
Remember the Lewis satellite? There waq a flaw in the Attitude Control System. They did not take into accout the natural spin axis of the spacecraft body. It entered a flat spin that resulted in no solar power to the panels and a fatal battery discharge as well. It too was a total loss of mission.
The companion Clark satellite (yes they were named for the famous Lewis and Clark) was canceled and never launched.
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