Galaxies from the early universe. The birthplaces of planets. Dark matter. Dark energy. Since its launch in 1990, the Hubble Space Telescope has brought these mysteries into focus, its powerful gaze scanning the universe for the details planet-bound telescopes find impossible to detect.

Far above the Earth's surface, Hubble floats clear of the planet's light-distorting atmosphere, beaming back images that have transfixed humanity and changed the scientific world.

Hubble's triumphs continue to accumulate thanks to a unique design that allows astronauts to repair and upgrade the telescope while it remains in orbit. Repairs keep the telescope functioning smoothly, while upgrades to the instruments bring a slew of new discoveries and science.

On October 10, 2008, astronauts will board the Space Shuttle Atlantis for Servicing Mission 4 (SM4), the final trip to the Hubble Telescope. Over the course of five spacewalks, they will install two new instruments, repair two inactive ones, and perform the component replacements that will keep the telescope functioning at least into 2014.

The effort-intensive, rigorously researched, exhaustively tested mission also involves diverse groups of people on the ground throughout the country.

Ready, Set, Go
The mission's planning is years in the making, and its success will be the product of months of intensive preparation and the work of hundreds of people at NASA and in academia and industry.

Astronauts train at NASA's Johnson Space Center in Texas, where they learn to deal with weightlessness in a giant water tank in the Neutral Buoyancy Lab, and at Goddard Space Flight Center in Maryland, where they enter Goddard's huge intensively filtered "clean room" to work with the actual equipment they'll use and install aboard Hubble.

Goddard prepares and tests instruments and hardware, while Kennedy Space Center in Florida prepares the space shuttle for the mission.

During SM4, teams at Johnson and Goddard work around the clock to make sure the mission succeeds. Johnson's Mission Control Center monitors the space shuttle and astronauts, and supervises spacewalks, procedures, crew activities and health, as well as shuttle systems and experiments.

Hubble personnel, including managers, scientists, and engineers from Goddard and the Space Telescope Science Institute in Maryland, will be split between supporting the mission at Johnson and those working at Goddard.

At Goddard, where the Hubble program is administered, the Space Telescope Operations Control Center controls the telescope itself, giving the commands that prepare the telescope for the astronauts' activities and test the newly installed equipment. Goddard's Control Center closes the door that protects the telescope's delicate optics, and maneuvers Hubble into position as the shuttle Atlantis approaches.

When the shuttle is about 200 feet (60 m) away from Hubble, Goddard will command the telescope to execute a roll that brings it into position for grappling. Astronauts use the shuttle's robotic arm to capture Hubble and dock it in the shuttle's cargo bay.

Next, the astronauts begin their series of five six-and-a-half-hour spacewalks. Two astronauts work outside on Hubble at a time. One mainly handles the free-floating tasks. The other is connected by a foot restraint to the robotic arm, which is operated by a third astronaut from within the shuttle.

To keep themselves from accidentally floating away, the astronauts attach safety tethers to a cable that runs along the cargo bay. Hubble was built with handrails that also make it easy for astronauts to cling to the telescope.

Out With the Old
Many of the telescope's components, especially the instruments, were designed to be easily removed and replaced during servicing missions. This mission's primary scientific priority is the installation of Hubble's new instruments, Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS).

Wide Field Camera 3 will be the power behind studies of dark energy and dark matter, the formation of individual stars and the discovery of extremely remote galaxies previously beyond Hubble's vision. WFC3 sees three different kinds of light: near-ultraviolet, visible and near-infrared, though not simultaneously. The camera's range is much greater than that of the instruments currently aboard.

Astronauts will remove Hubble's Wide Field and Planetary Camera 2 (WFPC2) to make room for WFC3. WFC3 has a higher "resolution," or ability to distinguish details, and a larger "field of view," or area the camera is able to see, than WFPC2.

Galaxy evolution, the formation of planets, the rise of the elements needed for life, and the "cosmic web" of gas between galaxies will be some of the areas of study for the Cosmic Origins Spectrograph (COS). A spectrograph is an instrument that breaks light into its component colors, revealing information about the object emitting the light.

COS sees exclusively in ultraviolet light and will improve Hubble's ultraviolet sensitivity at least 10 times, and up to 70 times when observing extremely faint objects.

COS will take the place of the device installed in Hubble during the first servicing mission to correct Hubble's flawed mirror, the Corrective Optics Space Telescope Axial Replacement (COSTAR). Since the first servicing mission, all of Hubble's replacement instruments have had technology built in to them to correct Hubble's marred vision, making COSTAR no longer necessary.

The Fix is in
Two of Hubble's instruments, the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS), are in need of repair. ACS, which partially stopped working in 2007 due to an electrical short, is the "workhorse camera" responsible for some of Hubble's most spectacular images.

STIS is a spectrograph that sees ultraviolet, visible and near-infrared light, and is known for its ability to hunt black holes.

While COS works best with small sources of light, such as stars or quasars, STIS can map out larger objects like galaxies. STIS suffered a power failure in 2004 and was put into hibernation to preserve the possibility of its repair.

Astronauts plan to fix both - a challenging prospect since these repairs are beyond the scope of Hubble's serviceable design. Hubble's creators envisioned astronauts swapping out components, not performing delicate surgeries during spacewalks.

An interior electronics box of ACS that supplies power for ACS detectors, contains equipment affected by an electrical short. However, its location makes it inaccessible to astronauts. So instead of trying to reach the problem area, astronauts will attempt to bypass those power-shorted components entirely.

The failed power supply is connected by cables to a series of electronics boards, which are within reach but have no power because of the damaged box.

Astronauts will install a new power supply to a handrail on the ACS outer enclosure, remove the electronics boards and install different ones that are compatible with the new power supply, and connect them to the new supply with exterior cables. The arrangement simply cuts the damaged box out of the equation.

STIS needs a new power supply circuit board. The repair would be relatively easy but for the electronics access panel, which was never meant to be opened and is attached to STIS by 111 small screws.

The screws are hard to grasp with the astronauts' gloved hands, and could create problems if they were to escape and float around the electronics. So engineers have created a "fastener capture plate" that fits over the top of the panel.

When the astronauts remove the screws, they will be trapped in the plate. Astronauts will then switch out the power supply circuit board and close off the open electronics with a new, simpler panel that attaches easily with two levers.

As each of Hubble's instruments and components is repaired or installed, Goddard performs tests to ensure that everything is working correctly. Once the work is completed, Hubble will be ready for peak performance with a grand total of five instruments: WFC3, COS, ACS, STIS and the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS).

Routine Maintenance
But before it can try out its new equipment, the telescope needs maintenance. Hubble's batteries store the energy that powers the telescope during the "nighttime" portion of its orbit, when the Earth blocks the Sun's rays. Astronauts will replace all six of Hubble's 125-pound batteries with new, more effective versions.

The telescope's six gyroscopes are part of the system that points the telescope. When all six gyroscopes are functioning, three gyroscopes are used for pointing, and the other three are held in reserve.

Time has degraded the gyroscopes to the point where three have failed, two are in use, and a third is turned off to be used as an emergency backup. Astronauts will install six new gyroscopes.

Hubble's Fine Guidance Sensors (FGS) lock onto guide stars, helping the telescope point. They can also be used as instruments to measure the position of stars in relation to other stars. Astronauts will replace one worn-out FGS with a refurbished model that was removed during a previous servicing mission.

Hubble's insulating blankets, which maintain the telescope's normal operating temperature, eventually break down because of their exposure to space. Astronauts will cover key Hubble equipment bays in fresh insulation, also called a New Outer Blanket Layer (NOBL).

Finally, they will install a new device, the "soft capture mechanism." This simple device will allow a robotic spacecraft to attach itself to Hubble someday, once the telescope is at the end of its life.

When the astronauts have finished all of their tasks, they will use the robotic arm again to release the telescope, and Goddard will issue the commands to bring the telescope back into operation. But before Hubble's science mission can resume, the telescope will undergo a several-month-long testing and calibrating period. The first new images from the telescope will be released in early 2009.

Restored and updated, Hubble will continue on its journey around the Earth, its new components merging seamlessly with the old, a rejuvenated telescope ready for years of groundbreaking revelations from the universe.