STS-109 PAYLOADS
Hubble Space Telescope Servicing Mission 3B

Payload Bay

The Advanced Camera for Surveys
The Advanced Camera for Surveys, Hubble's new scientific instrument, will give astronomers the opportunity to discover celestial objects far beyond the reach of current instruments in a fraction of the time, unlocking more of the universe's secrets. The new camera, which is also known as ACS, is a large phonebooth-sized instrument consisting of three different, specialized channels. Each channel plays a unique imaging role, enabling ACS to contribute to many different areas of astronomy and cosmology. It will become Hubble's new workhorse, surveying far regions of the universe, searching for extra-solar planets and observing weather and other features on planets in our own solar system. With its wider field of view, superb image quality and exquisite sensitivity, ACS will take full advantage of Hubble's unique position as a space-based telescope.

During STS-109's fourth spacewalk, Mission Specialists James Newman and Michael Massimino will install ACS into the location currently held by the Faint Object Camera -- the last of Hubble's original instruments.

Hubble's New Solar Arrays
When STS-109 visits Hubble, spacewalkers will install new solar arrays that will give the orbital observatory a new look and boost its power. STS-109 astronauts will install the new arrays during the mission's first two spacewalks.

Unlike Hubble's first two pairs, which are so flexible that they roll up like window shades, Hubble's newest solar arrays are flat, rigid and they fold up. The new arrays have one-third less solar cell area, but produce at least 20 percent more power than the current set. The added power enables all the science instruments to be powered and ready to operate simultaneously, allowing for more discoveries in less time.

Also, the new arrays, which are made of aluminum-lithium, are easier for the astronauts to work around during servicing missions. In addition, their smaller size decreases on-orbit drag and slows the rate at which Hubble's orbit decays. Over time, all low-Earth orbiting satellites feel the effects of atmospheric drag and lose altitude.

The European Space Agency (ESA) built Hubble's first two sets of solar arrays. For the newest pair, ESA designed, developed and tested the Solar Array Drive Mechanisms, which maneuver the arrays to keep them constantly pointed at the Sun.

The NICMOS Cryocooler
During STS-109, the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which has been dormant since January of 1999, will receive a new cooling system called the NICMOS Cryocooler.

Installed on Hubble in February 1997, NICMOS used infrared vision to explore dark, dusty regions of space with precise optical clarity. Since NICMOS operates at cold temperatures, it was encased in a container filled with solid nitrogen ice to keep the detectors cold. A small heat leak caused the nitrogen ice to consume more quickly than planned.

Scientists and engineers at NASA's Goddard Space Flight Center in Greenbelt, Md., devised the NICMOS Cryocooler, a state-of-the-art cryogenic cooler that is expected to return NICMOS to active duty and re-cool the NICMOS infrared detectors to about minus 203 degrees Celsius (minus 334 degrees Fahrenheit or 70 degrees Kelvin).

This closed system, which uses non-expendable neon gas as a coolant, delivers high cooling capacity, extremely low vibration and high reliability. It uses a miniature cryogenic circulator to remove heat from NICMOS and transport it to the cryocooler.

This new technology was successfully demonstrated in 1998 aboard the Space Shuttle Discovery on STS-95.

The Power Control Unit
The Power Control Unit, or PCU, is Hubble's power switching station. As the central controller of the telescope's electrical system, it regulates and distributes the power Hubble needs to operate.

The new PCU that will be installed in Bay 4 during STS-109's third spacewalk is going to replace Hubble's original PCU, which has degraded with age. A fresh PCU will enable Hubble to remain healthier and more productive throughout its lifetime. The new PCU is the original flight spare built for Hubble, but this unit has been modified and retested to ensure optimal performance and to enhance ease of installation by the astronauts.

Reaction Wheel Assembly
One of four reaction wheel assemblies, or RWAs, is scheduled to be replaced during STS-109's second spacewalk. RWAs are an important part of Hubble's Pointing Control Subsystem that use rotational momentum to move the telescope from one target to another and to keep it pointed steadily once the target is acquired in the aperture of the observing scientific instrument. Even though only three RWAs are required for scientific operations, it was decided to add a spare RWA to the manifest of STS-109 because of the importance of the RWAs to the telescope's performance and the amount of time between this servicing mission and the next.