Servicing Mission 4

Hubble precisely measured the age of the universe. It found evidence of dark energy. It brought you images of distant galaxies in the young universe. And now, with the state-of-the-art instruments delivered by Servicing Mission 4 (SM4), the Hubble Space Telescope will look onto the universe with new eyes, surpassing even its previous vision.

Hubble was designed to be repaired and upgraded by astronauts, and these servicing missions have occurred several times since Hubble's launch in 1990. NASA has selected a crew for the upcoming servicing, and the astronauts are currently training.

In late September, Hubble experienced a malfunction in one of the systems that commands the science instruments, and stores and transmits their data to Earth. The malfunction was corrected by switching to a backup system that was built into the telescope to compensate for this kind of problem.

Because this is a critical system, NASA is unwilling to leave the telescope without another backup in case of future problems. Fortunately, the system was designed to be removed and replaced, and NASA has a spare system waiting on Earth.

Engineers are investigating the situation and deciding what measures need to be taken. The most likely scenario is the installation of a replacement system during the servicing mission. NASA has evaluated the back-up system, and as a result the launch date will be moved to early 2009, with a specific date pending.

Veteran astronaut Scott D. Altman will command the final space shuttle mission to Hubble. Navy Reserve Capt. Gregory C. Johnson will serve as pilot. Mission specialists include veteran spacewalkers John M. Grunsfeld and Michael J. Massimino, and first-time space fliers Andrew J. Feustel, Michael T. Good and K. Megan McArthur. Grunsfeld, Massimino and Altman have visited Hubble on previous servicing missions.

SM4 has an ambitious program of activities and three main objectives. The following list of objectives states the mission's current priorities. These will be adjusted once engineers finish their investigation into the malfunctioning of the data storage and transmission system.

The first objective is to extend Hubble's operational life by at least five years. Over a series of five spacewalks, astronauts will replace all six gyroscopes, install new batteries, and exchange a degraded Fine Guidance Sensor with a new one. They will also install replacement thermal insulation on critical component bays of the telescope, and attach a mechanism that will aid in Hubble's final de-orbiting.

The second objective is to enhance Hubble's scientific power. Astronauts will install two new instruments, the Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS). WFC3, which sees in visible, infrared and ultraviolet light, will improve Hubble's sensitivity 10-30 times because of improvements in technology and design that have occurred since the last instruments were installed.

COS, Hubble's new spectrograph, will improve Hubble's sensitivity at least 10 times. Spectrographs are instruments that break light into its component colors, revealing information about the object emitting the light. COS sees ultraviolet light, which is particularly important because most of the ultraviolet light from space is absorbed by the Earth's atmosphere, making ground-based telescope observations impossible.

The third objective is to repair Hubble's out-of-commission instruments, the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS). STIS stopped working in 2004 and ACS failed in 2007.

ACS is Hubble's most prominent camera. Its wide field of view and ability to see in wavelengths from ultraviolet to visible light allows it to conduct broad surveys of the universe, study the nature and distribution of galaxies, and examine some of the universe's earliest activity. ACS was responsible for the Hubble Ultra Deep Field Image, NASA's deepest view of the cosmos.

STIS is a spectrograph. It separates light into its component colors, allowing scientists to examine the object's temperature, chemical composition, density and motion. STIS can see in ultraviolet, visible and near-infrared and has been used to examine black holes, quasars and planets.

If these objectives can be successfully carried out during the servicing mission, then Hubble will be at the apex of its scientific capability, with six working, complementary science instruments. These upgrades will keep Hubble functioning at the pinnacle of astronomy well into the next decade.