SM2 (STS-82) February 11 – 21, 1997

The second Hubble servicing mission involved replacing the FOS with an instrument called the Space Telescope Imaging Spectrograph (STIS), and replacing the GHRS with an instrument called the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

The STIS was a combination spectrograph and camera designed to cover a broad spectrum of light from near-infrared to ultraviolet wavelengths. It was equipped with special two-dimensional detectors that were capable of collecting “30 times more spectral data and 500 times more spatial data than the previous spectrographs on Hubble,” according to NASA.

The second new instrument, NICMOS, was a cryogenically cooled instrument consisting of three cameras, all designed to operate simultaneously and focus their images in the same plane. Unfortunately, stress and deformation problems with the cryogenic storage dewar prevented one of the cameras – NIC3 – from focusing properly and created a heat sink that depleted the nitrogen coolant quicker than expected. Despite these problems, NICMOS still provided valuable images of the universe in the near-infrared range.

SM3A (STS-103) December 19 – 27, 1999

The third servicing mission was an unscheduled emergency repair mission hastily assembled and executed following the failure of three of HST’s six gyroscopes. When a fourth gyro failed on November 13, 1999, putting Hubble to sleep, the wisdom of splitting SM3 into two parts became evident. For Christmas that year, Hubble got six new gyros, a more powerful main computer, another solid-state data recorder, a more sophisticated FGS, battery system improvements, and better thermal insulation.

SM3B (STS-109) March 1 – 12, 2002

The second half of SM3 took place in March 2002. Astronauts aboard the Space Shuttle Columbia replaced the original instrument aboard Hubble, with a new device called the Advanced Camera for Surveys (ACS). The ACS featured a large detector area and three cameras capable of recording wavelengths from ultraviolet to near infrared. The Wide Field Camera, which is equipped with a 16- megapixel detector that operates in the 350 – 1100 nm spectral range, is designed to search for galaxies that date back to some of the earliest days of the universe. The High-Resolution Camera (HRC) is designed to take finely detailed high-resolution images of things like galaxies, star clusters and gaseous nebulae. The Solar Blind Camera increases its sensitivity to ultraviolet light, particularly in the 1150 to 1700 Angstrom range, by blocking visible light using a Multi-Anode Microchannel Array (MAMA) similar to the one used in the STIS.

The ACS functioned well for almost five years, but beginning in mid-2006, a series of electrical problems forced NASA to come up with a variety of creative workarounds to keep the instrument operational. A short circuit in the unit’s backup power supply in January 2007, however, temporarily killed the HRC.

In addition to installing the ACS, new solar arrays were installed, Hubble’s power control unit (PCU) was replaced, and NICMOS was retrofitted with a new experimental cryogenic cooling system, returning it to service.

SM4 (STS-125) May 11 – 24, 2009

The final Hubble servicing mission, originally scheduled for February 2005, might not have happened at all due to safety concerns following the Space Shuttle Columbia disaster. There was serious debate over the risk vs. value of one last mission to Hubble, but in the end it was approved by NASA administrator Michael D. Griffin and scheduled for October 2008.

On September 27, 2008, just weeks before the mission’s scheduled launch, Hubble’s primary data handling unit, called the Science Instrument Command and Data Handling (SI C&DH) module, failed. This module, together with HST’s data management unit, controlled the processing, storage and communication of all science and engineering data collected by Hubble. NASA engineers successfully transitioned Hubble to the backup unit and postponed SM4 until a new SI C&DH module could be prepared.

On May 11, 2009, Space Shuttle Atlantis took off for the final trip to HST. In addition to replacing the faulty data handling unit, the crew installed two new instruments: the Wide Field Camera 3 (WFC3), which replaced WFPC2, and the Cosmic Origins Spectrograph (COS), which they installed in the space formerly occupied by COSTAR. In addition, they repaired the STIS and ACS. These repairs were crucial to Hubble’s mission because the ACS and WFC3 were designed to complement each other, as were the STIS and COS. WFC3 added ultraviolet and infrared capability to the visible light spectrum normally covered by the ACS. COS added the ability to observe pinpoint sources of light, such as those typically emitted by stars and quasars, to STIS’ ability to observe the broad spectrum of light typically emitted by larger bodies such as galaxies and nebulae.

In addition, the crew also replaced all six of the HST’s nickel-hydrogen (NiH2) batteries, installed a refurbished fine guidance sensor and new outer blanket layer insulation panels, and installed a device called the Soft Capture and Rendezvous System (SCRS), which will allow a future manned or robotic mission to recover the HST when it reaches the end of its life.

The Future

Nobody knows how much longer Hubble will continue to operate. NASA’s original projection for its life expectancy was 15 years, so it has already exceeded all expectations. But nothing lasts forever. Knowing this, in 1996, NASA began planning Hubble’s successor. Originally called the Next Generation Space Telescope (NGST), it was officially renamed the James Webb Space Telescope (JWST) in 2002 in honor of NASA’s second administrator.

The JWST is designed to pick up where Hubble left off, looking farther back in time through the universe. Where Hubble was designed to operate primarily in the visible and ultraviolet spectrum, JWST will operate primarily at infrared wavelengths of 0.6 to 28 microns. The best that Hubble’s instruments can do in the infrared spectrum is 0.8 to 2.5 microns.

The JWST will also have a much larger primary mirror than Hubble – 6.5 meters vs. 2.4 meters – and whereas Hubble orbits the Earth at an altitude of approximately 350 miles, the JWST will orbit the sun at the Earth-Sun L2 Lagrange point. This should give the JWST the ability to see the earliest stars and galaxies formed in the universe.

Hubble’s Legacy

Like the mythical Phoenix, Hubble rose from the ashes of what could have been an embarrassing failure to become one of NASA’s greatest accomplishments. What it has contributed to our knowledge and understanding of the universe is invaluable.

For example, data collected by Hubble on the expansion of the universe helped astronomers calculate its age to be about 13.7 billion years. Not only did Hubble discover that the universe is expanding, it also determined that the rate of expansion has recently begun to accelerate, leading astronomers to wonder why. It was Hubble that also provided compelling evidence of the existence of supermassive black holes and gave astronomers vital clues as to how planets are formed.

And let us not forget the thousands of breathtaking, historic images captured by Hubble such as the Hubble Deep Field and the iconic Pillars of Creation.

According to data provided by NASA, since 1990, Hubble has made well over 1 million observations of 38,000 celestial bodies. It typically collects data at a rate of 844 gigabytes per month, meaning that over its 25-year career it has contributed more than 100 terabytes of knowledge to our understanding of the universe. By any measure, that has to be considered an unqualified success.

For more information on the Hubble Space Telescope, visit www.nasa.gov/hubble .