Scientists are bracing for the loss of two key instruments aboard NASA's infrared Spitzer Space Telescope, which will considerably diminish the observatory's ability to image infant stars but unlock more time for more thorough studies of mysterious worlds outside the solar system.

An artist's concept of the Spitzer spacecraft. Credit: NASA/JPL-Caltech

Officials expect Spitzer will drain its supply of liquid helium some time next month, reducing the telescope's sensitivity to infant stars and distant galactic nuclei.

"According to our estimates, we have about a pound (of liquid helium) left," said Michael Werner, Spitzer's project scientist at NASA's Jet Propulsion Laboratory.

Engineers have calculated the helium will probably run out around May 12, according to Robert Wilson, the mission's project manager at JPL.

Spitzer, the last of NASA's Great Observatories designed to revolutionize astronomy, was launched in 2003 with about 95 gallons of frigid liquid helium chilled to about -457 degrees Fahrenheit. That number equates to 1.2 degrees Kelvin, nearly as cold as the point of absolute zero.

The spacecraft was launched at room temperature and the cryogenic helium began cooling the 33.5-inch-diameter telescope before operations began in late 2003.

Sensors for the observatory's three science instruments are mounted inside a structure called the cryostat, which is actively cooled by vapors vented from the adjacent liquid helium tank.

The instruments must be maintained at such cold temperatures to see through relatively cool dust clouds and observe star-forming regions hidden from optical telescopes.

When Spitzer empties the liquid helium tank, the spacecraft will automatically detect the slight warming of the cryostat, according to Wilson.

"The spacecraft will go into a planned anomalous condition, basically a standby mode," Wilson said.

It will take about a day to recover from the fault, and then the Spitzer ground team will begin monitoring the telescope as the cryostat warms to about 30 degrees Kelvin, still a bone-chilling -405 degrees Fahrenheit.

Assuming Spitzer runs out of helium in the middle of May, scientists hope to begin "warm" operations by the end of June.

Scientists will only be able to use part of the capacity of Spitzer's Infrared Array Camera, an instrument with four detector channels ranging from near-infrared to mid-infrared wavelengths.

Warm observations will be limited to two wavelengths in the near-infrared spectrum, Werner said.

"There are many things we won't be able to do," Werner said.

Spitzer's Infrared Spectrograph and Multiband Imaging Photometer will be unusable as temperatures inside the telescope rise. Both instruments require temperatures below 11 degrees Kelvin, according to Wilson.

Those instruments are designed to study composition and gather far-infrared imagery to see cooler material in star-forming regions throughout the Milky Way galaxy, Werner said.

The Spitzer team has already selected investigations for the first round of warm mission observations.

"There are a number of large projects which have already been identified," Werner said. "They range from new estimates of what's called the Hubble constant, which is the rate of expansion of the local universe, to studies of galaxies in the more distant universe, to wide-area surveys looking for unusual types of objects, both nearby and distant."

The telescope's curtailed capabilities will also allow scientists to devote more time to detecting and analyzing targets much closer to home.

"We hope to be executing the same numbers of observations, but since we're only using a part of one instrument, each one of our programs will be a deeper, more substantial program than were most of the programs in the cryo mission," Werner said.

One such project will focus on observing near Earth objects, asteroids circling the sun in orbits close to Earth.

An artist's concept of an extrasolar planet with hypothetical water-bearing moons. Credit: NASA/IPAC/R. Hurt

Other scientific programs will study extrasolar planets, worlds orbiting other stars outside the solar system.

Spitzer has already achieved several firsts in the field. The telescope made the first direct measurements of light from exoplanets and detected chemicals and temperatures in their atmospheres.

Officials were surprised by such discoveries because exoplanets were not a primary objective of Spitzer's mission.

Scientists will turn Spitzer's infrared camera toward exoplanets again during the warm mission to collect even more data.

"It's amazing how much progress we've made given that the first of these measurements weren't made until a couple of years into the Spitzer mission," Werner said. "So I think that would be at the top of most everybody's list."

Astronomers have detected more than 340 exoplanets so far, but that number is expected to balloon when NASA's Kepler planet-hunting telescope begins combing through nearby stars searching for new worlds.

Kepler was launched last month and should begin regular science operations in the next few weeks. Spitzer will be there to help in the pursuit.

Exoplanet studies could be Spitzer's No. 1 legacy so far, according to Werner.

Spitzer has also proven to be an important ally of other NASA missions, including the Hubble Space Telescope, another member of the Great Observatories program.

Spitzer, Hubble, the Compton Gamma Ray Observatory and the Chandra X-ray Observatory were developed to give scientists a clear picture of the universe across a wide swath of light spectra.

"I think the other thing that we've done that's been very surprising was to show that Spitzer can look as far back in space and time as Hubble can," Werner said. "The combination of the Spitzer and Hubble observations of very distant galaxies tells us a lot about how galaxies formed and evolved in the early universe. We'll be doing more of that type of science as well during the warm mission."

Unlike Hubble, Spitzer is circling the sun in an Earth-trailing orbit that gradually takes it further from the planet each year.

The craft is currently about 51 million miles from Earth, a distance that increases by more than 10 million miles each year, officials said.

"Spitzer is getting further away from the Earth every day," Werner said. "At the end of 2013, we're so far away that we're not able to communicate under safe mode conditions. If there was an anomaly or something, we would pretty much have our hands tied."

But operating through 2013 would require new funding for Spitzer, which is currently only approved for the next two years.

The telescope has cost more than $1.1 billion so far, including about $400 million in operations costs, Wilson said.

Managers plan to cut the mission's staff in half for the warm mission to help save money.

The James Webb Space Telescope, conceived as a successor to both Spitzer and Hubble, should be able to take over observations by 2013.

"It would be a natural end for the Spitzer mission," Werner said.