Servicing Mission 4
(SM4)

The Hubble Space Telescope was reborn with Servicing Mission 4 (SM4), the fifth and final servicing of the orbiting observatory. During SM4, astronauts installed two new scientific instruments – the Cosmic Origins Spectrograph (COS) and Wide Field Camera 3 (WFC3). Two failed instruments, the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS), were brought back to life by the first ever on-orbit repairs. These efforts brought Hubble to the apex of its scientific capabilities.

To prolong Hubble's life, astronauts installed new batteries, new gyroscopes, a new science computer, a refurbished fine guidance sensor, and new insulation on three electronics bays. Additionally, astronauts attached a device to the base of the telescope that will facilitate de-orbiting when the telescope is eventually decommissioned.

Quick Facts

COS is an ultraviolet spectrograph with one overriding objective: to collect as many ultraviolet photons of light as possible over a wide field of view. Scientifically, its primary goals include studying of the origins of large-scale structure in the universe, the formation, and evolution of galaxies, and the origin of stellar and planetary systems and the cold interstellar medium. It can effectively study of the huge, dark reservoir of gas that exists between the galaxies both near and far. This dark reservoir represents the largest-scale structure in the universe, the so-called "cosmic web." 

COS and the Space Telescope Imaging Spectrograph (STIS) were designed to complement each other. COS seeing ultraviolet light and STIS seeing in ultraviolet through optical to near-infrared light.

SM4 astronauts mounted COS in the space occupied by the Corrective Optics Space Telescope Axial Replacement (COSTAR), which became redundant when SM3B astronauts replaced the Faint Object Camera in 2002.

Four large flexible solar array (SA) panels provide power to the observatory.

With a "panchromatic" sensitivity extending from the ultraviolet through the visible and into infrared wavelengths of light, Hubble’s WFC3 is an extremely powerful imaging instrument that extended Hubble's capabilities to see deeper into the universe. WFC3 was a bridge to the infrared observations made by the James Webb Space Telescope.

WFC3 replaced the Wide-Field Camera 2 (WFPC2), which SM1 astronauts installed in 1993. WFPC2 performed extraordinarily for nearly sixteen years. WFC3 has a 7-16 times advantage over WFPC2 in discovery efficiency –the product of its wider field-of-view and greater sensitivity.

WFC3 is optimized for ultraviolet and infrared wavelengths. This complements the Advanced Camera for Surveys (ACS), which is optimized for visible light between blue and red wavelengths. The WFC3 also provides a capable backup to the ACS at visible wavelengths, should the latter suffer an operational failure. Both create the kinds of images for which the Hubble is best known.

Designed with servicing in mind, Hubble’s instrument bay doors hold handrails and are relatively easy for astronauts to open with customized tools. The same is not true for the instruments themselves.

Installed on the telescope during SM2 in 1997, the Space Telescope Imaging Spectrograph (STIS) worked brilliantly for 7.5 years before the failure of a power supply in August 2004 interrupted its operation, sending it into "safe mode."

Repairing STIS required replacing a power supply board located within the Side 1 electronics box. Developing this repair program represented a significant step forward in the ability to perform on-orbit spacecraft servicing.

In January 2007, the Advanced Camera for Surveys (ACS) suffered a severe electronics failure, rendering the two most-used of its three science channels inoperable. Since its installation during SM3B in 2002, ACS quickly became Hubble’s most heavily used instrument. It was responsible for many of the telescope’s most popular and dramatic images, including the Hubble Ultra Deep Field.

With only two years until the final servicing mission, there was little time to develop the procedures and tools needed to repair ACS. Fortunately, the team was already preparing for a very similar task involving the repair of STIS. Designing a repair process for ACS became achievable by adapting the processes already under development for repairing STIS. The ACS repair required replacing four boards in the electronic box and a new power supply box.

NASA scientists, technicians, and engineers were able to generate all the hardware along with some very ingenious tools to perform both repairs. The astronauts repeatedly tested and refined the repair procedures during the two years that proceeded the mission launch. The spacewalking astronauts made it look easy, though they had to overcome frozen bolts, stripped screws, and stuck handrails. Tests performed immediately after the repairs confirmed that the procedures worked. Both instruments were indeed operational once again.

Since SM4, Hubble has charted a new path of discovery through the universe. Among many key revelations in the last 15 years, Hubble has