STS-109

STS-109 was a Space Shuttle mission that serviced the Hubble Space Telescope in March 2002. Flown by the orbiter Columbia, it was the fourth Hubble servicing mission and the fifth Shuttle flight to rendezvous with the observatory. NASA designated the flight Servicing Mission 3B because its objectives had originally formed the second portion of the reorganized third servicing mission.

The crew installed new solar arrays, replaced the telescope's central electrical distribution unit, fitted the Advanced Camera for Surveys, and restored the disabled Near Infrared Camera and Multi-Object Spectrometer with a mechanical cooling system. Five extravehicular activities, totaling 35 hours and 55 minutes, were conducted on consecutive days.

Mission context

Hubble had been placed in orbit by STS-31 in April 1990 and was designed for periodic servicing by Shuttle crews. The first three servicing flights corrected the telescope's initial optical defect, replaced aging spacecraft equipment, and introduced progressively more capable scientific instruments. The work assigned to STS-109 originated in the plan for Servicing Mission 3, which NASA divided into two flights after several Hubble gyroscopes approached the ends of their operating lives. The urgent gyroscope replacement became Servicing Mission 3A, flown by STS-103 in December 1999, while the remaining upgrades became Servicing Mission 3B.

STS-109 was the 108th flight of the Shuttle programme despite the number in its designation. Mission numbers functioned as identifiers within NASA's flight sequence and did not invariably correspond to the number of missions already launched.

The flight was also Columbia's first mission after an extensive orbiter maintenance period that began in 1999. Changes made during that interval included installation of a multifunction electronic display system in the flight deck and reductions in the mass of several internal systems. Although Columbia was heavier than the other operational orbiters and was not assigned to assembly flights for the International Space Station, its orbital performance was sufficient for the lower-inclination Hubble mission.

Crew

The mission used an eight-member crew divided among flight-deck operations, robotic-arm work, and telescope servicing.

Altman and Carey controlled the rendezvous, proximity operations, and return phases. Currie-Gregg and Watanabe operated Columbia's Canadarm, captured Hubble, and directed the arm positioning that carried astronauts and replacement hardware between the payload bay and the telescope. Grunsfeld, Newman, Linnehan, and Massimino formed the two alternating spacewalking teams.

Rendezvous and capture

Columbia entered an orbit with an inclination of 28.5 degrees, corresponding to Hubble's orbital plane. The Shuttle approached the telescope over two days through a sequence of orbital adjustment burns. During final proximity operations, Altman maneuvered Columbia beneath Hubble while the telescope was placed in the attitude required for capture.

The robotic arm grappled Hubble on 3 March and lowered it onto the Flight Support System in Columbia's payload bay. This rotating and tilting platform mechanically restrained the observatory while allowing different equipment bays to be presented to the spacewalkers. Electrical connections from the Shuttle supplemented the telescope's internal power arrangements during servicing, although the replacement of its power control unit later required a complete shutdown.

Servicing operations

Solar-array replacement

The first two spacewalks replaced Hubble's second-generation flexible solar arrays with third-generation rigid arrays derived from communications-satellite technology. Grunsfeld and Linnehan conducted the first excursion on 4 March, removing one existing array and installing its replacement. Newman and Massimino completed the second side during the following day's excursion and also replaced a reaction-wheel assembly used in the telescope's attitude-control system.

Each new array had less surface area than the structure it replaced but converted incident sunlight more efficiently. Together, the arrays increased available electrical power while producing less vibration during transitions between sunlight and orbital darkness. The reduction in mechanically induced motion improved the stability of the observatory during long exposures.

The replacement demanded coordination between the spacewalkers and the arm operators because the folded arrays were too large for unaided transport through the payload bay. Currie-Gregg positioned the astronauts near the telescope attachment points, while Watanabe directed the transfer of the released assemblies and supported the controlled installation of the new hardware. The older arrays were secured in the payload bay for return to Earth.

Power control unit

During the third spacewalk on 6 March, Grunsfeld and Linnehan replaced Hubble's original power control unit. The unit accepted electricity from the solar arrays and distributed regulated power throughout the spacecraft. Its replacement permitted Hubble to use the greater output provided by the new arrays.

Unlike most earlier servicing tasks, the operation required Hubble to be completely powered down. It was the first deliberate full shutdown of the observatory since its deployment in 1990. The astronauts disconnected and reattached 36 electrical connectors within a restricted equipment bay, after which controllers restored power and confirmed normal distribution through the replacement unit. The task occupied most of the six-hour, 48-minute excursion and represented the mission's most direct intervention in Hubble's central spacecraft systems.

Advanced Camera for Surveys

Newman and Massimino installed the Advanced Camera for Surveys during the fourth spacewalk on 7 March. The instrument replaced the Faint Object Camera, which had been carried into orbit with Hubble and had already exceeded its intended operational lifetime.

The new camera contained separate optical channels intended for wide-field imaging, high-resolution observations, and ultraviolet-sensitive work. Its wider field of view and improved detector efficiency substantially increased Hubble's imaging rate for survey programmes. Installation involved removing the older instrument as a complete orbital replacement unit, transferring it to Columbia, and placing the new camera in the vacated axial instrument bay.

The same excursion included an attempt to fit an experimental cooling connection to the telescope's electronics. The spacewalk lasted seven hours and 30 minutes, making it the longest of the mission's five excursions.

NICMOS cooling system

The final spacewalk, conducted by Grunsfeld and Linnehan on 8 March, installed the NICMOS Cooling System. NICMOS had entered service during STS-82 in 1997, but its solid-nitrogen coolant had been depleted prematurely after an internal thermal contact increased the rate of heat transfer into the instrument.

The replacement system used a closed-cycle mechanical cryocooler rather than a finite stored refrigerant supply. It circulated neon through the instrument and transferred waste heat to an external radiator attached to Hubble's support structure. Power and data interfaces connected the cooler to the telescope, while external lines linked it to the instrument's thermal hardware.

Installation restored NICMOS to an operating temperature suitable for infrared observations. The instrument resumed scientific operations later in 2002 and remained usable beyond the lifetime originally determined by its stored coolant.

Release and return

Following functional checks, the crew used the robotic arm to raise Hubble from the servicing platform and release it on 9 March. Columbia then performed a separation maneuver that prevented recontact and placed the Shuttle on a trajectory suitable for deorbit preparations. Ground controllers subsequently completed the commissioning of the new instruments and aligned the telescope's optical systems.

Columbia landed at the Kennedy Space Center on 12 March after completing 165 orbits. The returned hardware included the Faint Object Camera, the second-generation solar arrays, the original power control unit, and the removed reaction-wheel assembly. Their recovery allowed direct engineering examination of components that had spent approximately a decade in low Earth orbit.

Scientific and operational results

STS-109 altered both Hubble's scientific capability and its available spacecraft resources. The Advanced Camera for Surveys increased the efficiency of wide-field optical imaging, while the restored NICMOS extended observations into near-infrared wavelengths. Increased electrical output from the solar arrays provided sufficient power for these instruments and for subsequent additions made during later servicing.

The mission also demonstrated that major internal electrical equipment could be replaced after launch. Hubble's modular construction had anticipated exchange of instruments and selected spacecraft components, but replacement of the power control unit required interruption of every active onboard system. Its successful reactivation established that the observatory could survive a controlled shutdown and retain functional continuity after extensive rewiring.

The telescope remained in service through the loss of Columbia on STS-107 in 2003 and the resulting suspension of Shuttle flights. NASA initially cancelled further Hubble servicing because the telescope's orbit did not permit the crew to reach the International Space Station in an emergency. The agency later approved STS-125, which completed the fifth and final servicing mission in May 2009.

See also

• List of Space Shuttle missions
• Hubble Space Telescope servicing missions
• STS-61
• STS-82
• STS-103
• STS-125
• Extravehicular activity
• Scientific instruments of the Hubble Space Telescope
• Space Shuttle Columbia