In a cost-cutting move prompted by President Bush's moon-Mars initiative, NASA could summarily put an end to Voyager, the legendary 28-year mission that has sent a spacecraft farther from Earth than any object ever made by humans.
The probable October shutdown of a program that currently costs $4.2 million a year has caused consternation among scientists who have shepherded the twin Voyager probes on flybys of four planets and an epic journey to the frontier of interstellar space.
Two Voyager spacecraft are still transmitting as they near the outer limits of the sun's influence.
"There are no other plans to reach the edge of the solar system," said Stamatios Krimigis, a lead investigator for the project since before its launch in 1977. "Now we're getting all this new information, and here comes NASA saying, 'We want to pull the plug.' "
NASA officials said the possibility of cutting Voyager and several other long-running missions in the Earth-Sun Exploration Division arose in February, when the Bush administration proposed slashing the division's 2006 budget by nearly one-third -- from $75 million to $53 million.
The administration is rearranging NASA's finances to fund Bush's "Vision for Space Exploration" to the moon and eventually Mars. Cuts in aeronautics funding prompted by the initiative have already provoked an uproar at some NASA centers.
Some members of Congress have also criticized the aeronautics cuts, and last year several joined the public outcry over NASA's decision to cancel a servicing mission to the Hubble Space Telescope, a move apparently unconnected -- at least initially -- to the moon-Mars proposal.
"Voyager is the same [as Hubble] -- one of the classic American contributions to space," said research physicist Louis J. Lanzerotti, who last year led a Hubble study for the National Academies of Science. "Voyager's photographs are all over astronomy textbooks."
Dick Fisher, NASA's deputy director for the Earth-sun division, acknowledged that Voyager's looming demise is a direct result of the new budget. He said the agency based its proposed cuts on a "senior review" by outside experts who in 2003 gave Voyager a low priority among the division's 13 "extended" missions.
"If we use that set of goals, we would be looking at certain missions that would have to be terminated," Fisher said in a telephone interview. "We have to [decide] whether to sweat the rest of the budget to pay for this."
An extended mission begins when a spacecraft has finished its original task but is still able to contribute new science. The best known one underway is that of the Mars rovers Spirit and Opportunity, which are exploring the Martian desert a year after the end of their 90-day "design" mission.
Voyager 1 and Voyager 2, destined originally for a five-year journey to Jupiter and Saturn, have been extended repeatedly ever since. Most systems are functioning well, and both spacecraft are expected to provide usable data until their plutonium power sources are used up -- probably in 2020.
Fisher said NASA has made no final decision on the cuts but has notified project scientists of its intentions and asked for cost-trimming proposals. He said the agency will make final decisions this month, perhaps by April 15.
The other programs on the block are Ulysses, launched in 1990 to study the sun; Geotail (1992), Wind (1994) and Polar (1996), to trace the interaction between solar events and their effects on Earth; FAST (1996), to study Earth's aurora; and TRACE (1998), to investigate the solar atmosphere and magnetic fields.
The impulse for Voyager arose in the early 1970s because of space geometry, Krimigis said in a telephone interview. Every 175 years, the solar system's four major outer planets -- Jupiter, Saturn, Uranus and Neptune -- are aligned in a way that one spacecraft can pass close to all four without carrying extra propellant.