Hubble's Universe Unfiltered

  • November 17, 2008

    Episode 3: Hubble Servicing Delayed

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    On September 27, 2008, NASA was two weeks away from a space shuttle mission to upgrade and repair the Hubble Space Telescope. That night, Hubble suffered an electronics failure and went into its protective safe mode. The servicing mission was soon put on hold as engineers scrambled to diagnose the problem and activate hardware that had not been used in eighteen years. Managers also had to consider whether the failure left Hubble without a backup for a critical system. Now, a month later, Hubble's vision has been restored, and the servicing mission, while delayed, has been expanded to include a complete fix for the problems encountered.

    • The next servicing mission to Hubble is commonly referred to as Servicing Mission 4, or SM4. It will be, however, the fifth servicing mission to Hubble. The third servicing mission was split into two pieces: SM3A occurred in 1999, while SM3B took place in 2002. The numbering of the servicing missions has stuck with the original plans and was not updated to reflect the true count of the missions.

    • In contrast with Hubble's low Earth orbit of about 600 km above the planet's surface, most communications satellites reside much higher in the sky. Communications satellites are generally in geosynchronous orbits, an orbit where the orbital period matches Earth's rotational period. Satellites in these orbits stay over the same spot on Earth and can therefore be found at the same position in the sky all the time. This feature allows you to point your satellite dish once and leave it pointed. The height of a geosynchronous orbit is about 36,000 km above Earth's surface. That is 60 times the height of Hubble's orbit.

    • The September 2008 rollout of space shuttles Atlantis and Endeavor was not the first time that two shuttles had been at their launch pads at the same time. It happened as recently as July 2001, and there have been about 15 more occasions with two shuttles on launch pads simultaneously. Coincidentally, when Discovery launched Hubble into space on April 24, 1990, Columbia was also on the launch pad. You can see Columbia in the foreground of the Hubble launch picture noted below.

    • The failure in Hubble's electronics occurred in the CU/SDF (Control Unit / Science Data Formatter), which is one part of the SIC & DH (Science Instrument Command and Data Handler). For simplicity, we did not differentiate between the component and the overall unit. There are two CU/SDF units on the SIC & DH unit, and we call one CU/SDF side A, and the other side B. More details on all the events can be found in the NASA SM4 press release archive at

    • An astute viewer pointed out that while there are four main bolts holding the SIC & DH unit, there are also several other bolts or screws that need to be unfastened during the spacewalk. He suggested that the total number was around ten. Still, this is much, much less than the more than 100 screws that will need to be taken care of on a different spacewalk repair during Servicing Mission 4. Thanks for the correction.

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