For the first time in nearly five years, the Cassini spacecraft's main engine system ignited this evening for a critical course adjustment that will serve as a dress rehearsal of sorts for Saturn orbit insertion July 1.

An artist's concept of Cassini. Credit: NASA/JPL

The computer-initiated rocket firing, known as trajectory correction maneuver 20, began on time at 7:47:31 p.m. EDT and lasted some five minutes and 56 seconds. Assessment of Cassini's slightly altered trajectory, along with analysis of spacecraft telemetry, will not be available until later. But a spokeswoman for the Jet Propulsion Laboratory said "everything appeared to have gone well," based on subtle changes in the radio signal coming from the spacecraft.

TCM-20 was designed to change Cassini's velocity by about 78 mph, setting up a flyby of the moon Phoebe June 11. It also marked one of the final corrections before a 96-minute-long July 1 rocket firing required to put the spacecraft in orbit around Saturn.

TCM-20 was relatively minor as such things go, but it marked the first time the craft's propulsion system was fully exercised since a major 87-minute "deep space maneuver" in December 1998.

No one at NASA's Jet Propulsion Laboratory in Pasadena, Calif., expected any problems and a six-minute burn by the 100-pound-thrust main engine is "pretty small by SOI or DSM standards," Cassini propulsion engineer Todd Barber, a veteran of the Galileo and Mars Exploration Rover missions, said last week. "That's the bad news. The good news is it's totally regulated, so we'll be able to verify that all the valves work and that the helium's flowing properly."

Helium provides the pressure needed to push propellant and oxidizer into the engine's combustion chamber. Cassini's main helium regulator began leaking shortly after launch in 1997, but by using a downstream valve to isolate the system between firings, engineers are able to manage the leakage. Those same procedures were used for TCM-20 and will be used again for Saturn orbit insertion and a final major orbit adjustment burn in late August.

Unlike the lengthy, make-or-break SOI burn, TCM-20 could have been carried out in "blow-down" mode using residual tank pressure, Barber said, which would not have required use of the helium regulator.

"But it's ostensibly there not only to target Phoebe for our June 11 flyby, the major purpose of this burn is a propulsion system checkout," he said. "It gives us a real warm fuzzy going into SOI."

It currently takes radio signals from Earth, traveling through space at 186,000 miles per second, more than one hour and 20 minutes to reach Cassini. As a result, tonight's rocket firing, like all such burns, was initiated, controlled and shut down by the spacecraft's on-board computer.

The burn "is a good practice run, no question about that," said Cassini project manager Bob Mitchell. "But the science from this is kind of a big deal, too."

Discovered in 1898, Phoebe measures just 137 miles across, orbits Saturn at a distance of nine million miles and circles the planet in the opposite direction from its other moons. Because of that, and the tilt of its orbit, scientists believe Phoebe may be a captured asteroid or a Kuiper belt object left over from the birth of the solar system.

Voyager 2 imaged Phoebe in 1981 from a distance of about 1.4 million miles. Tonight's rocket firing will allow Cassini to pass within just 1,250 miles of the enigmatic moon.

"Voyager imaged Phoebe a long time ago and got some images that were quite distant, quite blurred and didn't really provide a whole lot of information," Mitchell said. "And now, we're going to get down to, I think, something like 18-meters-per-pixel resolution. And Phoebe has enough gravity that while it really doesn't do anything of any significance to the trajectory, it does tweak it enough that you can feel it.

"So the radio science guys ... will get a mass estimate from the flyby. From that and images that tell you the size, they get the density. So there's quite a bit of science coming back from this."

Cassini was launched Oct. 15, 1997, from the Cape Canaveral Air Force Station. Using two gravity assist flybys of Venus, one of Earth and a final boost from Jupiter, Cassini has taken seven years to reach its target. Carrying 12 sophisticated instruments and a European probe with a half-dozen instruments of its own that will descend through the atmosphere of the moon Titan, the $3.4 billion Cassini is the most complex deep space mission ever attempted.

The 96.4-minute Saturn orbit insertion rocket firing, which will change Cassini's velocity by slightly more than 1,400 mph, is scheduled to begin at 10:35:42 p.m. EDT on June 30. It should end around 12:11 a.m. July 1, kicking off a planned four-year orbital tour of the ringed planet and its many moons.

A detailed Cassini mission overview, based on interviews with senior scientists, managers and spacecraft engineers, will be posted here next week.