WASHINGTON -- Scientists are launching a last-ditch effort to block NASA's plans to send the Compton Gamma Ray Observatory on a fiery suicide plunge into Earth's atmosphere.

Their goal: keep the spacecraft in orbit for another several years.

But time to save the still-functioning Compton is short.

Spacecraft engineers on May 28 will shut down all science operations on the $670 million satellite, in orbit 317 miles above Earth.

Then over the next few days, they'll command the spacecraft to fire its thruster rockets to lower its orbit and allow gravity to pull it closer to Earth.

The 17-ton (15,422-kilogram) observatory is slated to end its operational life in a blaze of glory at 1:21 a.m. ET on June 4 over an uninhabited part of the eastern Pacific Ocean.

NASA's decision in late March to purposely destroy Compton was ordered because the spacecraft is just one gyroscope failure away from loss of control.

Gyroscopes are used to keep spacecraft steady in orbit. Two of Compton's three gyroscopes are still working. But the loss of another would complicate the ability of controllers to accurately steer the spacecraft -- an outcome NASA wants to avoid.

When Compton finally falls from space, great chunks of the observatory are expected to survive the nosedive through Earth's atmosphere.

NASA is hoping to reduce the threat of damage to people or property by maneuvering the spacecraft to a watery demise.

Still, it is expected to scatter debris over a long, thin area about 2,547 mile4s long and 16 miles wide across the equator west of Mexico and northern South America.

A NASA policy written nine years ago said that if the Compton were down to only two working gyroscopes, then engineers must begin planning to bring the spacecraft out of orbit.

Those plans now are being called into question.

The American Astronomical Society's High-Energy Astrophysics Division (HEAD) has launched a letter-writing campaign to Congress.

The group's letter to lawmakers questions NASA's risk assessment calculations. The space agency has estimated that, if another Compton gyro were to fail, the chances of a human fatality caused by falling debris would be one in 1,000.

However, the letter points out that NASA also has said that a controlled reentry could be made with no gyros with a casualty probability of one in 4 million.

The letter calls for an independent review of the true risks and benefits, adding that Compton can still perform useful science.

"We've been wrangling with this issue for a couple of weeks," said Alice Harding, chair of the executive committee for the HEAD. "We are distributing the letter to our members but we are not endorsing it."

Harding said there's another issue being expressed by those wanting to save Compton.

Some scientists contend that solar flares may foul-up the attempt to safely bring down the observatory.

Flares blooming from the sun could strike the Earth just as engineers prepare to bring Compton from orbit.

If so, such bursts might change the density of the atmosphere, affecting the ability of ground controllers to precisely guide Compton's crash landing, she said.

"There's kind of a ground-swell out there" of scientists concerned about killing off Compton, Harding said. "But it's up to our members to make their own decision about taking action. We're not pushing them one way or another."

James Ryan, a physicist at the University of New Hampshire in Durham, said NASA has "inconsistently" described the risk of continuing to fly the Compton observatory.

"The scientific case for continuation of the mission is beyond question," he said, arguing that "virtually every sub-discipline of astrophysics" would suffer without Compton.

Ryan said Compton's demise should be delayed as the observatory is currently operating in a safe, long-lived orbit with a lifetime of greater than four years. To destroy it now, he said, would amount to a "national tragedy."

While he's sympathetic to the issues Ryan and other space scientists are raising, Alan Bunner, NASA science program director, said the only safe step is to take Compton out of orbit.

"We need to do it as soon and as safely as practical," he said.

Bunner agreed that there was a science case for keeping Compton aloft.

Valuable observations can still be made by the spacecraft, he said, although future spacecraft are being readied that can pick up where Compton left off.

"So we're talking about two years at most before we have some replacement for Compton in orbit. And that's a shame, but it's not fatal," Bunner said.

As far as the threat from solar flares, he felt that was exaggerated and not the big problem some scientists claimed.

The pleas to keep Compton alive are "understandable," Bunner said. "It's been a valuable mission and a lot of people hate to see it go."