Cassini successfully arrives at Saturn
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 1, 2004; Updated at 2:10 a.m. EDT
NASA's $3.3 billion Cassini probe completed a seven-year, 2.2-billion mile voyage tonight, firing its main engine for a nerve-wracking 96 minutes to successfully brake into orbit around the ringed planet Saturn.
At that distance, radio signals, moving at 186,000 miles per second, needed an hour and 24 minutes to complete a one-way trip between Saturn and Earth. As a result, Cassini's on-board computer was responsible for carrying out the most critical maneuver since launch Oct. 15, 1997, a maneuver that simply had to work or the mission would end in failure.
To everyone's relief, Cassini's main engine fired up on time at 10:36 p.m. EDT and shut down at 12:12 a.m., putting the craft in its planned initial orbit around Saturn.
"Flight, telecom," the Cassini communications officer called out. "The Doppler has flattened out."
Translation: Cassini's engine had shut down and Cassini was in orbit. Flight controllers burst into cheers, sharing hugs and high fives as Cassini lived up to its reputation for near flawless operation.
"It feels awfully good to be in orbit around the lord of the rings," said Charles Elachi, director of the Jet Propulsion Laboratory. "It's going to be a huge leap in our understanding of the Saturnian system."
Ed Weiler, NASA's associate administrator for space science, described the rocket firing as 96 minutes of purgatory during a news briefing Wednesday. Halfway through the burn, "I started to think gee, here we are sitting on this little pale blue dot, third rock from the sun. We just landed on Mars twice. We flew by a comet and picked up some comet dust (with the Stardust mission) and all within six months, we're about to go into orbit around a planet a billion miles away. How do we get away with having so much fun?
"This has just been an incredible ride," he said. "This wasn't NASA going into orbit around Saturn, it's the Earth going into orbit around Saturn because 17 countries made this happen. This is the way exploration should be done: by the Earth."
David Southwood, director of science for the European Space Agency agreed Cassini is a "world mission."
"But this evening I have to say, it's been the Americans' evening," he said. "This was America doing it right. ... There are Europeans involved in just about everything in the instrumentation, the science on Cassini and Huygens. It really is a mission where everybody is working together.
"But this evening, you guys did it right," he said. "Thank you JPL, thank you USA, thank you NASA."
Referring to ESA's Huygens probe, which will make a parachute descent into the atmosphere of Saturn's moon Titan in January, Southwood said the Saturn Orbit Insertion rocket firing would be a tough act to follow and "we have now to get it right, too."
"You really showed us how it's done. It was a very professional show and frighteningly on the nail. We've got a lot to live up to. Thank you everybody. It's been a great eveing."
The most sophisticated - and expensive - robotic spacecraft ever built, Cassini approached Saturn from below the plane of its rings. Using its high-gain antenna as a shield, the spacecraft sailed through the ring plane at 10:11 p.m., passing through a broad gap between Saturn's F and G rings. The region was thought to be empty of any debris larger than dust grains, but at Cassini's enormous approach velocity - more than 53,000 mph at that point - impacts posed a major concern.
But right on schedule, after Cassini re-oriented itself for the Saturn Orbit Insertion rocket firing, ground stations in Australia and California picked up Cassini's radio carrier signal at 10:27 p.m. EDT, confirming the spacecraft had survived the ascending ring plane crossing.
"One hurdle down, one to go," said Todd Barber, lead propulsion officer. "We're approaching two minutes before the SOI burn. The hopes and dreams of thousands of scientists and engineers are resting on the next few moments. So Godspeed, Cassini-Huygens. May we see you in orbit."
And with that, the moment of truth was finally at hand.
As timers counted down to the start of the Saturn Orbit Insertion rocket firing, engineers at JPL monitored computer screens showing a graphical representation of the carrier signal from Cassini. They were looking for a very precise, predicted change in the frequency of the signal due to the effects of the rocket firing, much like a siren changes pitch as a police car races past.
And right on schedule, at 10:36 p.m., the signal changed exactly as predicted. On computer screens, a horizontal line representing the carrier frequency suddenly bent sharply downward, matching the slope predicted for a normal rocket firing. Flight controllers burst into applause, relieved to know Rocket Engine Assembly A had fired on time to begin slowing Cassini's ever-increasing velocity.
Producing just 100 pounds of push against the enormous 54,000-mph velocity of the 9,970-pound Cassini, the main engine had to fire 96.4 minutes to produce the required deceleration and to ensure Saturn's gravity could capture the spacecraft and warp its trajectory into the planned orbit.
Thirty minutes into the burn, at 11:06 p.m., Cassini moved behind Saturn's A ring as viewed from Earth, dimming the carrier signal for about 25 minutes. After fading in and out as it was blocked by ring debris, relatively clear reception was established at 10:31 p.m. when Cassini had a brief, clear view of Earth again through a gap in the rings known as the Cassini division. Six minutes later, exactly as predicted, communications dropped out again for 28 minutes or so as the spacecraft moved behind the thicker B ring.
Still picking up speed from Saturn's gravitational attraction, Cassini reached periapsis, the closest it will ever be to Saturn - 12,400 miles from the cloud tops - at 12:03 a.m., just nine minutes before the end of the SOI burn. By that point, Saturn's gravity had boosted Cassini's velocity to a blistering 69,350 mph, four times faster than a space shuttle in Earth orbit and 32 times faster than the bullet from an assault rifle.
Waiting for the carrier signal to reappear from behind the B ring, Barber provided an impromptu Saturn weather report, predicting temperatures of "minus 226 degrees Fahrenheit, winds of 1,100 miles per hour or so, pressure highly variable depending on where you are in the atmosphere. At the top of the atmosphere, better than the best vacuum on Earth. Down in the depths, millions of atmospheres of pressure. Chance of helium rain inside the interior: 100 percent. Hurricanes the size of the Earth."
By the end of the SOI burn at 12:12 a.m., the velocity had dropped to around 68,000 mph as Cassini streaked away from the planet after close approach. While most reporters (including this writer) were not aware of it, navigators changed their prediction for the burn duration Wednesday, expecting 97 minutes instead of 96. Analysis of the carrier signal's frequency showed the rocket engine actually generated about 1 percent more thrust than expected. Cassini's flight computer compensated by shutting the engine down one minute early to achieve the planned deceleration of 1,400 mph. That translated into a 96-minute burn as originally expected.
With the conclusion of the SOI rocket firing, Cassini was finally in its planned initial orbit around Saturn. Over the next four years, the spacecraft will study Saturn's windy atmosphere, its complex ring system, several of its icy moons and how the planet's magnetic field interacts with the space environment. In what promises to be one of the most exciting phases of the mission, a European-built probe called Huygens will be released from Cassini on Christmas Eve for a parachute descent into the thick nitrogen atmosphere of Saturn's moon, Titan, on Jan. 14.
In all, Cassini is expected to complete 77 orbits of Saturn over the next four years, requiring 157 trajectory-nudging rocket firings. The gravity of Titan will be used for major course changes, with 45 planned flybys. Seven close flybys of smaller, icy moons also are planned.
Safely in orbit, Cassini turned so that its high-gain antenna was aimed back toward Earth for a brief, 20-second burst of carrier signal at 12:30 a.m. That switch from the low-gain to the high-gain antenna confirmed the spacecraft was operating normally and had not suffered any "safing" events during the burn that could have shut down science operations during Saturn close approach.
"We've got it!" Barber reported as yet another round of cheers and applause burst out. After sending the brief call home, Cassini turned away to begin a 75-minute sequence of ring observations.
"I feel great!" said program manager Bob Mitchell. "It was kind of a nail biter throughout."
One hour and 46 minutes after the end of the SOI burn, Cassini was expected to turn once again, orienting itself so the high-gain antenna could act as a shield during a descending ring plane crossing. Once safely through the ring plane, Cassini was expected to begin transmitting science and engineering data back to Earth. The first pictures were expected around 8:40 a.m. Thursday.
On July 2, Cassini will make its first official flyby of Titan, passing the cloud-shrouded world at a distance of 205,000 miles.
Larger than Pluto and Mercury, Titan's thick nitrogen atmosphere is thought to mirror Earth's shortly after the planet's formation. Based on approach photos, Cassini's cameras should be able to "see" the surface through specific spectral "windows." But just how well the cameras will be able to image the surface won't be known until after the Friday flyby.
Data playback from the Titan flyby is expected to begin around 6:15 p.m. Friday. If all goes well, a minor trajectory correction maneuver is scheduled Saturday at 8:30 p.m. to fine tune the orbit with a predicted velocity change of just 11 mph. Starting July 6, Cassini will be out of contact while Saturn passes behind the sun as viewed from Earth, completing the initial phase of Cassini's orbital mission.
In late August, a major rocket firing is planned to raise the low point of Cassini's orbit well beyond the rings and to set up the second Titan flyby Oct. 26. After another Titan flyby Dec. 13, the European Space Agency's Huygens probe will be released from Cassini on Christmas Eve for the three-week trip to Titan.
Huygens will slam into Titan's atmosphere on Jan. 14 for a two-and-a-half-hour parachute descent to the surface. Data from Huygens, including panoramic pictures of its enigmatic surface, will be beamed back to Earth through Cassini's radio system.
After that, Cassini will continue on its own, flying through a ballet of ever-changing orbits and beaming down up to four gigabytes of data per day.