As of this writing the astronauts have completed their work, coping with several unexpected problems along the way, and released Hubble to fly again on its own. Over the next few weeks scientists at the Space Telescope Science Institute will painstakingly test and calibrate the space telescope, and only when they're done will we know whether Discovery's mission was a success. The preliminary tests indicate that the astronauts performed their difficult and exacting mission with flair and precision, and we eagerly await the new marvels we'll see through Hubble's new and sharper vision.

The Crew

Dicovery's crew trained for their mission for two years. Any mistake could be a disaster, and absolute precision and care would be required.

NASA selected a highly experienced astronaut crew for this critical mission. Astronaut Gregory Harbaugh was a back-up crewperson and Pilot Kenneth Bowersox flew as pilot during the first servicing mission. Astronaut Steven Hawley, who will operate the Shuttle's 50-foot robot arm during the mission, flew the Hubble deployment mission in 1990.

The other astronauts include: Scott "Doc" Horowitz, who served as a pilot on STS-75, which featured the second deployment of the Tethered Satellite System; Mark Lee, who in 1994 deployed and retrieved a solar science satellite and conducted the first untethered spacewalk; Joseph R. Tanner, who flew a mission in 1994 designed to study the Earth's atmosphere; and Steven Smith, who flew STS-68 during which the crew studied the Earth's surface and atmosphere, creating radar images and mapping global carbon monoxide pollution.

The team trained in NASA's deep water buoyancy tanks, which simulate the weightlessness of space, and practiced procedures using tools specifically designed for the mission. Astronaut Story Musgrave, a mission specialist on the first servicing mission, also helped with the training to make sure the procedures and equipment worked properly.

The Mission

The Discovery astronauts had many tasks to perform to service the Hubble Space Telescope, including important repairs to the spacecraft's systems. But most exciting for astronomers are the two new instruments installed aboard Hubble.

The Space Telescope Imaging Spectrograph (STIS): A spectrograph separates the light gathered by the telescope into its spectral components so that the composition, temperature, motion, and other chemical and physical properties can be analyzed. The existing spectrographs aboard Hubble analyze the spectrum of just one point in the sky. But the new STIS is a two-dimensional spectrograph--it can create images we can view and analyze the light spectrum at each point in the image. The new detectors allow the instrument to gather 30 times more spectral data and 500 times more spatial data than the existing spectrographs.

STIS will search for massive black holes by studying the star and gas dynamics around galactic centers. It will measure the distribution of matter in the universe by studying quasar absorption lines, use its high sensitivity and spatial resolution to study star formation in distant galaxies and perform spectroscopic mapping of solar system objects.

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) promises to gain valuable new information on the dusty centers of galaxies and the formation of stars and planets. NICMOS consists of three cameras. It will provide the capability for infrared imaging and spectroscopic observations of astronomical targets.

NICMOS will give astronomers their first clear view of the universe at near-infrared wavelengths between 0.8 and 2.5 micrometers-longer wavelengths than the human eye can see. The expansion of the universe shifts the light from very distant objects toward longer red and infrared wavelengths.

NICMOS's near infrared capabilities will provide views of objects too distant for research by current Hubble optical and ultraviolet instruments. NICMOS's detectors perform more efficiently than previous infrared detectors.

(The new instruments were built by Ball Aerospace, and you can find detailed information by visiting their Web site.

Several major spacecraft components were refurbished as well, not only increasing the operational life of Hubble but providing it with finer control over the direction it points. Hubble will get a refurbished Fine Guidance Sensor (FGS), an optical sensor that is used on HST to provide pointing information for the spacecraft and as a scientific instrument for astrometric science. The modification to this FGS spare will add the capability for ground-controlled alignment corrections.

The addition of the Optical Contol Electronics Enhancement Kit (OCE-EK) will provide the electronic pathway for commanding the alignment mechanisms. The Solid State Recorder (SSR) replaced one of HST's three Engineering Science Tape Recorders (ESTR). The SSR provides much more flexibility than an ESTR, which is a reel-to-reel recorder and can store ten times more data. One of the other ESTRs will also be replaced but with a spare ESTR unit. Future missions may see all the reel-to-reel units replaced with solid state recorders.

One of Hubble's four Reaction Wheel Assemblies (RWA) was replaced by a refurbished spare. The RWA is part of Hubble's Pointing Control System. Spin momentum in the wheels moves the telescope to a target and maintains it in a stable position.

Originally there were to have been four EVAs (Extravehicular Activities, or spacewalks), but then it was discovered that the foil insulation protected Hubble from the heat of the sun and the chill of the night had become scratched and "weathered" by exposure to space, and that in some places it was even ripped by micrometeorites. A fifth spacewalk was added, on the night of 17 February, and the crew of Discovery improvised patches from blankets and parachute cord carried aboard the Shuttle.

After the crew had finished this fifth spacewalk and were starting to reenter Discovery, ground controllers discovered that another RWA was behaving erratically, and might have to be replaced on a sixth spacewalk. After a couple of hours ground control was able to get the RWA spinning properly, and the sixth EVA was avoided (no doubt much to the disappointment of the crew).

Additional Resources

The description of the new instruments was adapted from a NASA Fact Sheet.

There is a great deal of background material on the mission at the NASA Shuttle Web

"Everything You Always Wanted to Know About the Hubble Space Telescope Second Servicing Mission (SM-2)" at the Goddard Space Flight Center has some background material and links to other Web sites.