A report was released Thursday from the independent panel charged with evaluating options for the Hubble Space Telescope's twilight years of life and how to transition scientific research to the next-generation James Webb Space Telescope during the next decade.

Credit: NASA

Three options were conceived by the group -- including a concept to keep the observatory's window on the Universe open for many years to come by launching two space shuttle servicing missions and a less-desirable scenario that involves scrubbing all future human visits to the telescope.

The six-member panel officially began its work in early July and had until October 1 to submit a final report, but it was in fact completed and released soon after the board's public meeting July 31.

"A lot of astronomers and NASA officials were astonished when we said our report was ready just one week after our public meeting. This was possible because we reached unanimous agreement on our conclusions very quickly; remarkable when you consider there were six independent-minded scientists on the panel," said panel chairman Professor John Bahcall of the Institute for Advanced Study.

"Our secret is we did our homework very thoroughly. Many people helped to educate us."

Launched in 1990, Hubble has undergone four space shuttle servicing missions that have repaired, maintained, and expanded the scientific capability of the telescope. NASA officially plans one more shuttle-based service call to Hubble in 2005 or 2006, depending on when the three-shuttle fleet returns to flight following the Columbia accident.

The 10-page report released Thursday outlined three options for NASA to choose from to achieve a transition from Hubble to the almost $1 billion James Webb Space Telescope, the planned successor to Hubble currently scheduled to launch in about 2011, aboard a European Ariane 5 booster.

It also shot down plans to bring the observatory back to Earth via the space shuttle at some point in the future, saying the operation poses a high risk to astronauts especially in light of the Columbia tragedy.

Astronaut John Grunsfeld provided his thoughts to the panel last month on the possibility to return Hubble to Earth using the space shuttle. "If there were to be a mission after the SM-4 for the purpose of returning Hubble to Earth in the shuttle payload bay, the Astronaut Office would have reservations supporting the mission," Grunsfeld said.

Grunsfeld conducted spacewalks on two previous Hubble service calls in 1999 and 2002 to replace hardware and add new science instruments to the orbiting platform.

He concluded that "if it is determined that the science case drives an additional Hubble servicing mission, the astronauts will be there to help enable the scientific exploration of the Cosmos."

Discussions of sending Hubble into a much higher orbit are also off the table, NASA said in a report submitted to the panel on July 21. Mass, size, and cost were all cited as issues standing in the way of adding a propulsion system to do such a job.

Scientists want to keep the availability of valuable science data like that produced by Hubble unfettered and continuous throughout the transition from Hubble operations to the operations of JWST when it comes online in the next decade.

The panel concluded the most preferred option would have two more shuttle servicing missions take place, the first -- Servicing Mission 4 -- in 2005 that is already on the books, and another -- Servicing Mission 5 -- in about 2010 to keep Hubble going for even longer. Servicing Mission 3 was split between two flights in 1999 and 2002, so SM-5 would in fact be the sixth such mission to Hubble.

Under that option, an extra shuttle flight to Hubble would only take place if science data from the craft's instruments was successful when competed against similar data from other space missions.

The second option presented in the report has only one servicing mission to install improved scientific instruments and replacements for aging gyroscopes and other systems required to keep the spacecraft under control and in operation.

"We urge continuing efforts to maximize the productive scientific lifetime of the HST under the assumption SM-4 will be the last HST servicing mission," the board concluded.

The third option the board considered was one in which no shuttle flights to Hubble were made in the future. This scenario is considered remote, but is a possibility because shuttles going to the Hubble telescope are launched into a 28.5-degree inclination orbit, from which it is not possible to reach the international space station should there be a problem discovered.

Recommendations from the board investigating the Columbia accident require the capability to inspect and repair the thermal protection system of space shuttle orbiters when they are on-orbit. Options for such activities when at the space station are easier to implement because of the additional infrastructure available than when an orbiter is on a standalone flight.

Cancellation of an already-manifested flight would result in premature termination of the Hubble program and "a major loss of scientific opportunity," the panel said. "Outside support for NASA could be weakened." Two new scientific instruments slated to be installed into Hubble in the upcoming servicing mission would be left grounded as well. Under this scenario, the report mentioned that NASA could look at flying the Wide Field Planetary Camera-3 and the Cosmic Origins Spectrograph on a smaller, less expensive spacecraft to fill the gap between Hubble operations and the introduction of JWST.

Whichever scenario comes to fruition, the panel recommended attaching a propulsion module either during a manned servicing flight or through the use of a robotic mission. The telescope would then be nudged from orbit and into the Pacific Ocean at an indefinite time still to be determined. Initial talk of bringing the observatory back to Earth aboard a space shuttle was formally put to rest following the Columbia accident in February, the report said.

NASA's own requirement dictate that large objects in orbit like Hubble must be safely disposed of in a controlled manner. An uncontrolled re-entry for the observatory could put the public at risk due to falling debris. Analysis indicates an uncontrolled re-entry for Hubble would result in a 1 in 700 chance of a human casualty, according to a report from the Hubble program office.

"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 said in its final report.

"The continued vitality of the HST science program requires shuttle service missions. Full exploitation of the HST would require two service missions during the period before the end of mission."

Should there be no more orbital reboosts from visiting space shuttles, Hubble is predicted to fall from orbit due to natural decay as early as 2013. Continued reboosts up to 2010 could keep the observatory aloft until past 2020.

The other wildcard in the planning for retirement and disposal of the Hubble telescope is the launch date of JWST. The science community prefers an overlap period during which both generations of observatories can complement one another using a variety of observing methods and instrument wavelengths provided by both spacecraft.

Despite this feeling that permeates throughout the field of astronomy, the current plan NASA has calls for the shutdown of Hubble in 2010, followed a year later by the launch of JWST.

"A substantial gap in which no orbiting space telescope is available could erode scientific teams and decrease interest in the design of equipment that is essential for progress in the field," the board observed.

All is on schedule for a 2011 launch date for JWST, but the panel noted that any number of issues could arise to delay the launch. "We conclude that the JWST launch date might be delayed substantially beyond 2011. Appropriate planning should take into account the possibility that the JWST launch could be later than indicated by the current schedule."

JWST will extend discoveries made by Hubble into the infrared spectrum, the project team said in an overview submitted to the transition review board. It will be launched into orbit about the L2 Lagrange point about a million miles from Earth in the opposite direction of the Sun.

"NASA is deeply appreciative to Prof. Bahcall and the panel for getting this thoughtful report to us ahead of schedule," said Dr. Ed Weiler, NASA's Associate Administrator for Space Science. "We have a big job to do to study the panel's findings and consider our options, and we will respond as soon as we have time to evaluate their report."

The transition panel received 55 "thoughtful" e-mails from members of the scientific community and 40 "well-reasoned and fact-filled documents from NASA sources and other informed groups". About 200 e-mails from the general public were also sent via the panel's website. This input was in addition to testimony and presentations from a number of officials in a public meeting held July 31.