Spaceflight Now STS-109

Hubble to undergo risky makeover by astronauts
Posted: February 25, 2002

The Hubble Space Telescope has cost U.S. taxpayers some $6.9 billion in the quarter century since the project was approved. But to astronomers around the world, the 24,000-pound satellite is, quite simply, priceless.

The Hubble Space Telescope has been in orbit for 12 years. Photo: NASA
Since its famously flawed optical system was repaired by spacewalking astronauts in 1993, the Hubble Space Telescope has become an international icon of science, one of the most productive astronomical observatories ever built and the flagship of NASA's exploration of the universe.

The remotely-controlled spacecraft has helped astronomers confirm the existence of black holes, zero in on the true age of the universe and spot the faint glimmer of stars in galaxies born within a billion years or so of the big bang birth of the cosmos.

Its spectacular photographs have charted the life cycles of distant suns in glorious detail, providing unmatched views of stellar nurseries and the explosive end results of stellar evolution.

It has catalogued myriad infant solar systems in the process of forming planets and provided flyby-class views of the outer planets in Earth's own solar system, routinely capturing phenomena as common as dust storms on Mars to the once-in-a-lifetime crash of a comet into giant Jupiter.

While huge ground-based telescopes now rival and in some areas exceed the power of Hubble's relatively modest 94.5-inch primary mirror, the space telescope, operating 350 miles up, high above the turbulence of Earth's atmosphere, remains in a class by itself.

"I think everyone would agree the Hubble Space Telescope has been one of this country's most valuable scientific assets for its 12-year operating life," said Phil Engelauf, a shuttle mission manager at the Johnson Space Center in Houston.

But Hubble's ability to continue making world-class observations, he said, "is really enabled by our ability to keep the spacecraft healthy and scientifically relevant by updating and servicing that spacecraft on orbit."

And so, at 6:48 a.m. EST (1148 GMT) Thursday, NASA plans to launch the shuttle Columbia on the fourth of five planned Hubble servicing missions, the most technically challenging - and risky - overhaul and upgrade the space agency has ever attempted.

During five back-to-back spacewalks, four astronauts working in two-man teams plan to install a set of smaller-but-more-powerful solar arrays, a power control unit to more efficiently route that power to Hubble's subsystems and a new gyroscopic reaction wheel assembly to help the telescope move, or slew, from one target to another.

While installation of the new solar arrays and the reaction wheel assembly are relatively straight forward, the power control unit, or PCU, was not designed to be serviced in orbit and its replacement marks the riskiest part of Columbia's mission.

Not only is the replacement physically difficult, ground controllers will have to completely shut Hubble down for the first time since launch in 1990 before the astronauts can begin the critical transplant.

"That scares me a lot, it kind of violates a long-standing policy in the space business that if something's working well you turn it off and just hope it comes back on," said Edward Weiler, NASA's associate administrator for space science.

"We're not doing that cavalierly, we fully anticipate that everything will work just fine," he said. "But it is a risk that we've never faced before. So this mission is no cakewalk."

On the scientific front, the astronauts will install a $75 million 870-pound digital camera called the Advanced Camera for Surveys, or ACS, with twice the resolution and five times the sensitivity of the upgraded Wide Field-Planetary Camera - WFPC-2 - that currently is in place.

"With ACS, Hubble will detect more faint stars and galaxies during its first 18 months than have been detected with all of the previous Hubble instruments," said principal investigator Holland Ford of Johns Hopkins University.

"For astronomers, those stars and galaxies in the data archive are money in the bank."

The crew emblem for STS-109. Photo: NASA
The Columbia astronauts also will attempt to revive Hubble's Near Infrared Camera and Multi-Object Spectrometer, or NICMOS, an instrument designed to operate at near absolute zero to detect the faint heat of stars and galaxies in the remotest depths of space and time.

Installed during the second Hubble servicing mission in 1997, NICMOS was victim of an internal "thermal short" that caused its nitrogen ice dewar to come in contact with surrounding structure. As a result, the instrument's nitrogen ice coolant sublimated away faster than expected, leaving NICMOS dormant after just two-and-a-half years.

To repair the instrument, the Columbia astronauts will install an experimental "cryocooler" that uses neon gas and three tiny turbines spinning at an astonishing 400,000 rpm to chill NICMOS to about 75 degrees above absolute zero.

But it will not be easy. The astronauts will have to work deep inside the telescope's lower instrument bay to connect the cryocooler to NICMOS and attach a 13-foot-long radiator to the side of the telescope to dissipate the unwanted heat. Cables and coolant lines from the radiator to the cryocooler will be snaked through a small vent opening in the telescope's aft bulkhead.

"We believe we now have in hand a new technology developed jointly by NASA and the Air Force, which gives us the first really good shot at a reliable, mechanical cooler in space on an infrared instrument," said David Leckrone, Hubble project scientist at the Goddard Space Flight Center.

"So a very important objective of Servicing Mission 3B is, as an experiment, to try this new technology and see if we can bring the NICMOS instrument back from the dead."

But because of the experimental nature of the cryocooler, the NICMOS repair falls to the bottom of NASA's list of priorities for Hubble Servicing Mission 3B and as such, it will not be attempted until the fifth and final spacewalk.

The top scientific priority, as one might expect, is installation of the Advanced Camera for Surveys. The ACS, about the size of a phone booth, will be installed in place of a no-longer-used instrument called the Faint Object Camera.

The ACS actually includes three cameras sensitive to a broad range of wavelengths, from the ultraviolet to the far infrared.

To visualize the power of the ACS, it helps to recall one of the telescope's most famous photographs, the so-called "Hubble Deep Field," one of the most remarkable images ever produced by the space telescope.

In December 1995, Hubble was aimed at a presumably empty patch of sky near the Big Dipper about the size of a rice grain held at arm's length. The spot was chosen specifically because it appeared, for all practical purposes, to be devoid of stars and galaxies.

Over the next 10 days, WFPC-2 took 342 images that later were digitally combined. In the resulting Hubble Deep Field image, amazed astronomers counted some 1,500 discernible galaxies, or fragments of galaxies, dating back to a billion years or so of the big bang.

"The Hubble Deep Field, one of humanity's and Hubble's singular achievements, can be done in two days instead of 10 days," said Ford. "A ten-fold increase is especially important for finding rare objects such as the first galaxies and distant supernovae."

Working in tandem with a revived NICMOS, the Advanced Camera for Surveys also will play a major role in one of the hottest fields in modern astronomy, the ongoing search for exploding stars called type 1A supernovae.

Consider a binary star system that includes a compact white dwarf. The smaller white dwarf's gravity may pull in gas and dust from the companion star. If the white dwarf's mass builds up to about 1.4 times that of the sun, catastrophic fusion reactions begin and the star explodes in a type 1A supernova.

By definition, all type 1A supernovae involve stars of roughly the same mass and the intensity of the emitted light follows a well-defined "light curve," brightening and then fading away. The intensity of the light can be used to infer the supernova's distance from Earth and spectroscopic analysis can provide a measure of its recession velocity.

Astronomers have long assumed the expansion of the universe is slowing down. And based on the presumed rate of that deceleration, one would expect to find certain brightness levels for type 1A supernovae at various points in time and space after the big bang.

But to the surprise of everyone in the astronomical community, researchers in the late 1990s discovered type 1A supernovae at extreme distances appeared dimmer than one would expect based on their observed recession velocities.

As it turned out, the only reasonable - though counterintuitive - explanation was to assume the expansion of the universe is actually accelerating. The nature of the "dark energy" powering that acceleration is a complete unknown and determining its nature is a top astronomical priority.

"This is an amazing time for both physics and astronomy," Leckrone said. "We've come to start to realize over the past few years that we do not understand 95 percent of the content of the universe in which we live.

"Between dark matter and dark energy, those two things together constitute approximately 95 percent of the total energy and mass balance, or budget, of the universe," he said. "These are very challenging physical problems.

"What is the nature of dark energy, that may well be the most important question in the physical sciences today. The beauty of (ACS and NICMOS) is they give us very powerful tools for beginning to address these very fundamental and revolutionary new issues in physics."