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Last Updated on Friday, 16 April 2010 19:20
Published on Sunday, 27 February 2005 00:00


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Scheduled Launch time: 11 Dec 1999, 05:13 GMT
Shuttle: Discovery
Mission Duration: 9 Days
Space Walks: 4 scheduled
Commander: Curtis L. Brown
Pilot: Scott J. Kelly
Target Landing time: 20 Dec 1999, 15:15 GMT


The Hubble Space Telescope was launched in April 1990. It is designed to be visited by the space shuttle fleet approximately once every 3 years for general repairs and installation of new, state-of-the-art astronomical instruments. The first HST servicing mission (December 1993) was a resounding success, with the restoration of the telescope's full optical capabilities. The HST focuses extremely sharp images of the heavens onto its suite of on board instruments. These science instruments allow astronomers to dissect the light with unprecedented detail which then provides clues to the mass, temperature, chemistry and dynamics of the stars and galaxies that make up our Universe. The second servicing mission in February 1997 has proved to be a stunning success. Two new complex science instruments were installed which have heralded new discoveries in their short two year lifetime so far. During this third servicing no new science instrumentation will be fitted, put a series of critical repairs will be carried out.


The HST carries six gyroscopes which allow the telescope to point precisely at astronomical targets. A minimum of three gyros must be working for the pointing to be accurate enough for science operations. At the time of the second servicing mission in February 1997, all six gyros were functioning normally. However between 1997 and January 1999, three gyros failed. In anticipation of another failure, and the HST being reduced to an inoperable state, NASA divided the upcoming third servicing mission into two parts. Servicing mission 3A (SM3A) to replace the gyros was scheduled urgently for late-1999. Mission SM3B, to install new instruments, was re-scheduled for spring 2001. A fourth gyro did indeed fail on November 15th 1999, and the HST was placed immediately into a safe hold. This protective safe-hold mode allows control from the ground, but with only two gyros functioning the telescope cannot be pointed with precision to astronomical objects and hence science operations have been suspended. NASA reports that there is no risk to the telescope itself during this safe-hold mode. The servicing mission SM3A to replace the six gyros has become critical, and launch is scheduled for December 11th.


NASA engineers believe they have identified the failure mechanism responsible for the first three gyro breakdowns, with the fourth under analysis. The mission will replace all six gyros allowing the telescope to return to normal science operations and providing redundancy for the rest of the telescope's lifetime. In addition, other hardware maintenance will take place. This will include replacement of the telescope's central computer with a faster, larger memory machine, repair of the thermal insulation jacket surrounding the telescope structure, replacement of some data storage and transmission hardware, and new voltage-temperature control kits for Hubble's batteries. The mission will last 9 days, and 4 spacewalks (each lasting 6 hours) are planned. The tasks that are carried out during each spacewalk (or EVA - "Extra-vehicular activity") are arranged in order of importance and efficiency, hence the highest priority job of replacing the broken gyros is scheduled for the first EVA. The mission commander Curtis L. Brown will be joined by the pilot Scott J. Kelly and five mission specialists , including NASA astronaut Michael Foale, born in Louth England and educated at the University of Cambridge. Foale, a veteran of four previous space flights will carry out 2 of the four planned spacewalks. The estimated total cost of this servicing mission is $200 Million.


Dr. Stephen Smartt and Dr. Rachel Johnson UK HST Support Unit
Institute of Astronomy, Cambridge
Phone: 01223 337528
E-mail This email address is being protected from spambots. You need JavaScript enabled to view it.
(Dr. Smartt out of office hours phone: 01223 473198,
6th-15th December only)

UK HST Support Unit press web page (with useful links):

Other Useful Web links:

- Media reference Guide produced by NASA (3 Mbyte pdf format)

- Servicing mission home page at Goddard Space Flight Centrer

- Today at NASA

- Space shuttle Media resources at J.F. Kennedy Space Center

- Hubble Space Telescope Home Page


The above Press Information Note has been issued by the UK Hubble Space Telescope Support Unit, and is forwarded on its behalf. Jacqueline Mitton (Royal Astronomical Society Press Officer)

Note: contact details are given at the end.

Cambridge, UK 7 December 1999