The 16-year-old orbital observatory is expected to function for only two or three more years without a servicing call by space shuttle astronauts.

Equipment upgrades could keep the world's premier observatory operating until at least 2013, scientists say.

"We haven't even approached the limits of what we can do with Hubble," said senior project scientist David Leckrone with NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Using the orbiting Hubble telescope, which has captured some of the most spectacular images of the universe ever recorded, scientists recently found between eight and 16 new planets near the center of the Milky Way.

The U.S. space agency had planned a fifth servicing call to the telescope to install two new science instruments and to replace spent batteries and faulty steering gyroscopes. But NASA canceled the servicing after 2003 Columbia disaster.

The space shuttle was destroyed and its seven crew members killed close to landing because of undetected damage to Columbia's heat shield during launch.

Safety upgrades after that included in-flight heat shield inspections and a plan to shelter shuttle astronauts aboard the International Space Station in the event of similar damage. Crews heading to Hubble's orbit, however, cannot reach the station.

Shuttle managers have plans to quickly launch a second shuttle for an emergency rescue if a Hubble repair crew runs into trouble. There are no guarantees a rescue mission would work, however.

"You just don't have the orbital lifetime on a Hubble mission to be able to get another vehicle launched. It's going to be very tough," deputy shuttle program manager John Shannon said last month.

Cancellation of the Hubble servicing drew harsh public criticism and NASA later vowed to reconsider its decision.

Hubble has been used for an array of research projects including probing the atmosphere of an extrasolar planet, pinning down the age of the universe and, more recently, providing evidence of so-called dark energy, a mysterious force that seems to be powering the universe's expansion.

With three flights completed since the Columbia disaster and the shuttle program recovered to the point of resuming construction of the half-built International Space Station, NASA will announce its decision on Hubble's fate on Tuesday.

The shuttle fleet will be retired in 2010 and NASA needs at least 14 more flights to finish building the $100 billion space station, a project of the United States, Russia, Europe, Japan and Canada.