Cassini 'go' for Saturn orbit insertion burn
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 30, 2004
Executing stored instructions, the electronic brain of NASA's Cassini probe made final preparations for a critical 96-minute rocket firing tonight that will slow the craft by about 1,400 mph and allow Saturn's gravity to pull it into orbit.
The make-or-break Saturn Orbit Insertion - SOI - maneuver was scheduled to begin at 10:36 p.m. EDT and end around 12:12 a.m. Thursday. If successful, the burn will put Cassini in a long orbit around Saturn, kicking off a four-year tour of the ringed planet, its magnetosphere, its largest moon, Titan, and a retinue of smaller, icy satellites.
If the rocket firing fails or falls short of its 96-minute target duration, the $3.3 billion spacecraft will sail past Saturn and into a useless orbit around the sun.
"Unlike the two Voyagers that flew by Saturn in the early '80s and obtained just days worth of Saturn close-in science, Cassini-Huygens will be for Saturn what the Galileo mission was for Jupiter: a long-term science observatory," said Ed Weiler, NASA's associate administrator for space science.
But, he cautioned, "we are not there yet. Although things have gone very, very well so far, Saturn orbit injection will be the most critical event in the mission's life since launch. This main engine burn must be performed as planned or the mission will be lost.
"Unlike the Mars (rover) landings, where we had the 'six minutes from hell,' so to speak, in this case it'll be 96 minutes in purgatory. I hope the outcome will be as successful as our experiences with the Mars missions last January."
Engineers and managers at the Jet Propulsion Laboratory in Pasadena, Calif., believe it will. They said today the spacecraft has performed virtually flawlessly during the seven years it's taken Cassini to reach Saturn and they are confident it pull off the SOI burn, as Weiler said the most important single maneuver since launch from Earth on Oct. 15, 1997.
"Today, the Cassini spacecraft is in an excellent state of health and it is ready to perform the main engine burn tonight," said Julie Webster, lead vehicle engineer at JPL. "The spacecraft, the flight software and the on-board (computer command) sequences are now completely self contained and need no ground interaction from us to complete this burn. If a fault occurs on the spacecraft, the software will isolate the fault, identify the cause, fix it and continue on with the burn with no ground intervention."
The only issue of any significance is the possibility of high winds at a tracking station in Canberra, Australia. If the winds exceed about 47 mph, a 230-foot-wide dish antenna needed to pick up Cassini's radio signal will have to be stowed, delaying confirmation of a successful rocket firing.
"This has no bearing on the performance of the spacecraft," said Cassini program manager Bob Mitchell. "The spacecraft is perfectly capable of doing what it needs to do and weather on Earth is just not a factor.
"The thing that would be influenced is our blood pressure throughout the course of the evening," he joked. "However, a recent weather report from Canberra was favorable. It didn't eliminate the threat entirely, but it is favorable. So we don't believe it's going to be a problem."
Because of Cassini's enormous distance from Earth - some 934 million miles - radio signals from the spacecraft take an hour and 23 minutes to reach flight controllers at JPL. As a result, confirmation of events during SOI will be delayed by the same amount. In the timeline discussion below, "Earth-receive time," converted to Eastern Daylight Time, is used throughout.
"After 6.7 years we're at Saturn," said lead navigator, Jeremy Jones. "We're feeling the gravity well, falling into Saturn quite rapidly. ... This is the E-ticket ride, I'll tell you."
The numbers prove his point. At noon today, Cassini was 403,891 miles from Saturn and moving through space at 26,846 mph. By 6 p.m., the craft will be roughly half that far out - 233,014 miles - and its velocity will be up to 33,557 mph. Three hours later, the distance will be just 155,343 miles and its speed will be up to 42,506 mph.
The first critical milestone in the SOI sequence is the so-called ascending ring plane crossing. Approaching Saturn from below the plane of its rings, Cassini will cross that plane, in a broad gap between the F and G rings, at 10:11 p.m. That region is thought to be clear of any major pieces of ring debris, but no one knows for sure. At Cassini's velocity, an impact with anything larger than a very small pebble could cause major damage.
To be on the safe side, the spacecraft will re-orient itself an hour ahead of time, pointing its high-gain dish antenna in the direction of travel to act as a shield, cutting off telemetry from the spacecraft for the duration of the SOI maneuver. At the moment of ring plane crossing, Cassini will be just under 100,000 miles from Saturn - less than half the distance between the Earth and moon - and streaking through space at 50,335 mph.
"The antenna is a graphite-epoxy structure, so it's quite rugged and very capable of withstanding the kinds of small dust grains that we believe might be in this region," Mitchell said.
Once above the ring plane, Cassini will re-orient itself for the Saturn Orbit Insertion burn, pointing its main engine in the direction of travel. It also will begin transmitting a radio carrier signal using a low-gain antenna. The signal will not carry any data. But by monitoring how its frequency changes due to the SOI rocket firing's effect on Cassini's velocity, engineers will be able to confirm the start of the burn and the engine's overall performance through engine cutoff.
The moment of truth arrives at 10:35:42 p.m. when Rocket Engine Assembly A - REA-A - flashes to life, pushing against Cassini's 53,691-mph forward motion with a mere 100 pounds of thrust. Over the course of 96.4 minutes, REA-A will consume 1,874 pounds of rocket fuel, roughly one third of the 6,600 pounds Cassini was launched with, slowing the spacecraft by 1,396 mph and ensuring Saturn's gravity pulls it into the desired orbit.
Thirty minutes into the burn, at 11:06 a.m., Cassini will move behind the A ring as viewed from Earth and the carrier signal may be lost for up to 25 minutes or so. The signal should show back up for six minutes when Cassini "sees" Earth through a gap in the rings known as the Cassini division. Then communications may be lost again for 28 minutes or so as the spacecraft moves behind the B ring.
Saturn closest approach, or periapsis, will occur 10 minutes before the end of the burn, at 12:03 a.m. By this point, Saturn's gravity will have boosted Cassini's velocity to 69,350 mph. But by the end of the burn at 12;12 a.m., the velocity will have dropped to 68,000 mph and Cassini will be safely in orbit.
Weather in Australia permitting, engineers at JPL should be able to confirm the start of the rocket firing monitor its performance between ring blackouts and confirm the end of the burn. A few minutes later, Cassini is programmed to turn back toward Earth and transmit 20 seconds of engineering data starting around 12:30 a.m. The craft then will re-orient itself yet again to aim its cameras at Saturn's rings for 75 minutes of up-close science observations.
It then will re-orient itself antenna forward for the descending ring plane crossing and then point back toward Earth to begin downlinking its treasure-trove of science data. The first pictures are expected Thursday morning.
"Once we're done with about a 75-minute data observation period, we turn again for the descending ring plane crossing, again point the high-gain antenna as a protective shield," Mitchell said. "Once that's complete, we're going to turn and look back up at the rings, now on the sunlit side, take another set of images, mostly around the outer extent of the visible part of the rings. We then turn back to Earth and play this data back over about an 18, 19-hour period of time."
While confident, Mitchell cautioned "this thing is not a slam dunk by any means."
"There are two general concerns," he said. "One is the environment, primarily the environment of the rings. We have studied this very carefully, we believe we've taken prudent actions, we believe we've got the safest possible course of action, but it's not a guarantee.
"And then for the spacecraft itself, the spacecraft isn't going to do anything tonight that it hasn't already done in flight. But a 96-miute burn, where all of the systems have to work right for this amount of time, is a concern. Our confidence in the spacecraft is high, we have no specific reason to be concerned, but this is not a assured, either."
In preparation for the rocket firing, Cassini's on-board software opened low-pressure helium latch valves Tuesday. A backup inertial reference gyroscope was turned on and warmed up for use as needed during the burn. This morning, the main engine gimbal system was activated so the engine nozzle can be pre-positioned and the spacecraft's accelerometer was calibrated.
In short, Webster said, all systems were "go" for SOI.
"If the burn goes fine and we just don't get the signal back on the ground for any reason, we will get about 20 seconds of the high-gain coming in right before we turn go off and do the images," Webster said. "So if everything's perfect through the burn, we should get that little 20-second blip. That's very good because that tells us that the spacecraft didn't do any safing event or didn't stop the burn for any reason because the background sequences are still going. That's the good indication. That'll come in about 12:30 a.m. (EDT)
"I'm an old telecom person, so I actually wouldn't be concerned about this until past 3 a.m. ... I would not get any panic level until 6 or 7 in the morning. That's not unusual in low gain safing to have a little bit of trouble, especially with the Doppler changes that are going on with the spacecraft. Sometimes it just takes a while to find the signal. But I have full faith in this spacecraft, that it's going to make this burn one way or the other."