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Space Exploration

Hubble Space Telescope (HST)

galaxy M87, Voyager spacecraft, arcsecond, space shuttle Columbia, space shuttle Discovery

Hubble Space Telescope (HST), the first general-purpose orbiting observatory. Named after American astronomer Edwin P. Hubble, the Hubble Space Telescope was launched on April 24, 1990. The HST makes observations in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum. The primary mirror of the HST has a diameter of 94.5 in (240 cm), and the optics of the telescope are designed so that, theoretically, when making a visible-light observation, the telescope can resolve astronomical objects that are at an angular distance of 0.05 arcsecond apart. In comparison, traditional large ground-based telescopes, under very good sky conditions, have an image resolution of about 0.5 arcsecond. Originally, the HST was equipped with five detectors: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph, the High-Resolution Spectrograph, and the High Speed Photometer. It also has three fine guidance sensors that can be used for precision astronomy measurements such as determining the distances of stars from Earth.

After the HST was launched, scientists discovered that its primary mirror had a systematic aberration, the result of a manufacturing error. A service mission was carried out in December 1993 using the space shuttle Endeavour. A corrective optical device, called the Corrective Optics Space Telescope Axial Replacement (COSTAR), was inserted in the slot for the High Speed Photometer, which had to be removed to make room for COSTAR. The Wide-Field Planetary Camera, which had a different optical path from the other four instruments, was replaced with a second camera, which has a built-in correction for the aberration in the primary mirror. The service mission, which involved numerous intricate procedures, was successful.

Even before the aberration was corrected, the HST produced many valuable images, such as images showing mysterious dark structures in the spiral galaxy M51. Now that the HST has the resolving power it was designed to have, it is capable of performing research such as that necessary to significantly improve the calculation of the rate at which galaxies are receding from the Milky Way as a function of their distances. This data could then be used to calculate the age of the universe. In June 1994 a team of American scientists announced that the HST had provided the first convincing evidence of the existence of a black hole: The acceleration of gases around the center of the galaxy M87 indicates the presence of an object with a mass 2.5 billion to 3.5 billion times greater than that of the Sun. In addition, the HST provided one of the best available views of the planet Jupiter when fragments of Comet Shoemaker-Levy 9 bombarded the planet in July 1994. The HST’s detailed images of the collisions provided scientists with data for a spectral analysis of the chemical makeup of Jupiter’s atmosphere.

Astronauts aboard the space shuttle Discovery participated in another servicing mission for the telescope in 1997. They replaced the High-Resolution Spectrograph and Faint Object Spectrometer with an instrument called the Space Telescope Imaging Spectrograph (STIS). The STIS took over the work of both of the old instruments and incorporates newer technology. The HST also gained a new infrared telescope, called the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). NICMOS expanded the range of wavelength that the HST could study. During the servicing mission, astronauts found that some of the HST’s insulation had been damaged, so they tied insulating blankets over the holes with wire and string to protect the telescope.

The new instruments helped the telescope continue its record of important discoveries. In 1998 the HST brought astronomers images of colliding galaxies and host galaxies of quasars (distant but very bright astronomical objects). The HST’s NICMOS revealed light emitted by galaxies more than 12 billion light-years away. These galaxies are farther away and their light is older than any before seen. Astronomers used this information to increase the estimate of the number of galaxies in the observable universe to 125 billion, up from the previous estimate of 80 billion. The HST also showed that by 1998, Neptune’s moon Triton had warmed by about 2°C (36°F) since the Voyager spacecraft visited it in 1989. In late 1998 NICMOS ran out of coolant and was shut down.

By April 1999, three of the six gyroscopes that help navigate the HST had stopped working. NASA scheduled a servicing mission for late 1999 to replace the ailing gyroscopes and other equipment and to install an advanced computer. In November 1999, before the servicing mission was launched, a fourth gyroscope failed, and the HST was put into a safe mode that does not require gyroscopes. The servicing mission was completed successfully in December, and the HST was put back into full operation. An additional servicing mission in 2002 replaced the telescope’s solar panel array and the cooling system for NICMOS.

The loss of the space shuttle Columbia in 2003 led Sean O’Keefe, then the head of NASA, to cancel a planned fifth and final service mission to the HST. O’Keefe cited safety concerns for the astronauts. Because the space shuttle and the International Space Station would be moving with such different velocities and orbital inclinations (angles of orbit) around the Earth, a shuttle sent to the HST would not be able to change its orbit to safely rendezvous with the International Space Station in case the shuttle suffered damage that prevented reentry to Earth. Scientists, the public, and prominent politicians spoke out in favor of extending the life of the HST, which would become unusable if its gyroscopes and batteries failed. Plans to send a robot to service the HST were dropped as too complex to develop in time to rescue the orbiting telescope.

However, after a series of successful space shuttle flights in 2005 and 2006, Michael D. Griffin, who succeeded O’Keefe as chief administrator of NASA, reviewed the astronaut safety concerns and approved new plans for a shuttle mission to service the HST. The mission, announced in October 2006, is scheduled for 2008 and will replace gyroscopes and batteries and install new scientific instruments. If the service mission is successful, the HST is expected to function until at least 2013.

One of the most important instruments on board the HST, the Advanced Camera for Surveys (ACS), stopped functioning in January 2007. Installed during the 2002 service mission to the HST, the high-resolution camera became the most in-demand instrument on the telescope. Using the ACS, astronomers were able to photograph extremely distant galaxies that formed one to two billion years after the big bang that began the universe, providing the deepest images of the cosmos ever taken. Current plans for the 2008 service mission to the HST do not include replacing or repairing the ACS. Other new instruments to be installed on the mission should provide nearly equal image capability. The ACS was apparently disabled by a short circuit, one of a number of recent power and equipment problems the aging HST has suffered.


Kondo, Yoji, B.A., M.S., Ph.D.

Professor, Senior Facility, George Mason University. Adjunct Professor of Astronomy and Astrophysics, University of Pensylvania. NASA Project Scientist, International Ultraviolet Explorer and Extreme Ultraviolet Explorer Satellites. Astrophysicist, Goddard Space Flight Center.

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