Shortly after noon today, the Magellan spacecraft, which left Earth in May of 1989, is scheduled to fire its rocket for 83 seconds, slide into orbit around the cloud-shrouded planet Venus, and provide the problem-plagued U.S. space agency with a bit of good news.

The robot craft will disappear behind Earth's sister planet during the crucial moments of the maneuver into orbit, a period scientists said would be nerve-wracking. If the rocket fails to fire, NASA project manager Anthony J. Spear noted, "we go whizzing by Venus" into an orbit around the sun and "it would take 100 years before things line up for another try" at the planet.

Scientists at the Jet Propulsion Laboratory in California, where the Magellan project is managed, offered a bit of NASA black humor about the fear of another failure. They said NASA's chief scientist, Lennard Fisk, jokingly told them he had bought two airline tickets for today -- "one to Los Angeles and one to Tahiti."

"The major pressure we feel is the pressure to succeed," NASA solar system exploration official Wesley T. Huntress said when asked about the impact of a series of setbacks in the Hubble Space Telescope and space shuttle programs.

But as he and others pointed out, "the planetary program is doing extremely well" and Magellan seems to be performing as ordered.

The 3.7-ton spacecraft was launched on its 948 million-mile trip 15 months ago by the shuttle Atlantis. Its mission, at a cost of $744 million including the launch, is to spend one Venus day -- 243 Earth days -- using advanced radar to make the most complete and detailed radar pictures ever of the cloud-veiled surface.

Scientists want to study Venus, which was formed at about the same time as Earth, as a model of an Earthlike planet gone wrong, with a poisonous atmosphere 900 degrees Fahrenheit at the surface, the result of a runaway "greenhouse effect" that they hope Earth can avoid.

Engineers at JPL received a signal from Magellan Wednesday indicating that its solid-fuel rocket had been armed.

At 12:32 p.m. EDT today, onboard computers will order the rocket to fire for 83 seconds, slowing Magellan's velocity from about 25,000 mph to about 18,600 mph. This will allow it to be captured by Venus's gravity in a near-polar orbit ranging from 171 miles to almost 5,000 miles above the planet.

If all goes well, the spacecraft will emerge from Venus's shadow about 12:54 p.m. Since radio signals currently take almost 13 minutes to travel from Venus to Earth, a signal indicating the $287 million craft is safe in orbit will reach engineers at 1:07 p.m..

If a signal arrives at 1:02 p.m., five minutes early, this will indicate the rocket failed, the craft is speeding past the planet and the mission is a "no go," at least for this century.