If none of these options are pursued, the Hubble could crash to earth on its own as early as 2013, the astronomers said on Thursday in a report on easing the transition from Hubble to its planned successor, the James Webb Space Telescope.
The Webb telescope is not scheduled for launch until 2011, and scientists want to maintain an orbiting vantage point.
Launched in 1990, Hubble has proved a breakthrough astronomical observatory, looking out at the cosmos without the blurring effects of the Earth's atmosphere.
The four-storey, 13-tonne telescope has captured the births and deaths of stars, taken detailed pictures of objects as close as the moon 250,000 miles away, and as far as galaxies more than 12 billion light years away. It has also proved the existence of cosmic phenomena such as black holes - collapsed stars so dense that not even light can escape their gravitational pull.
series of missions by space shuttles have repaired Hubble's initially flawed main mirror and added updated instruments, most recently the Advanced Camera for Surveys installed during a shuttle Columbia mission in 2002.
"It is prudent for NASA to be prepared for a range of possibilities."
Independent astronomers panel
But with the shuttle fleet grounded since Columbia's fatal 1 February break-up over Texas, future servicing missions to Hubble have been thrown into question, the independent astronomers panel said in a report.
Originally, the National Aeronautics and Space Administration planned a shuttle mission to collect Hubble and bring it down to Earth intact. But since the Columbia accident, this is no longer considered an option.
"We conclude that it is prudent for NASA to be prepared for a range of possibilities, from the most pessimistic (no future shuttle servicing missions) to the most optimistic (two future servicing missions)," the panel of astronomers wrote.
Their three options are:
Send shuttles to service Hubble in 2005 and 2010 to maximize its scientific productivity.
Send one shuttle mission to Hubble before the end of 2006 to install improved instruments and replace the craft's gyros.
If no shuttles are available, send a robotic mission to install a propulsion module to bring Hubble down into the Pacific Ocean when science is no longer possible.
Hubble was originally projected to have a 15-year lifespan, but NASA now estimates it could last until 2010 with one shuttle servicing mission. That mission is tentative pending NASA's return to shuttle flight, which is unlikely before next April.