Cassini represents the end of an era in U.S. spacecraft and mission design, a multi-billion-dollar, multi-instrument robot with a broad science agenda requiring the efforts of more than 5,000 scientists and engineers from 17 nations.

The Cassini-Huygens mission emblem. Credit: NASA/JPL

Precursor missions included the Pioneer and Voyager deep space probes, which carried out the initial reconnaissance of the outer solar system with flybys of Jupiter, Saturn, Uranus and Neptune in the 1970s and 1980s. Venus, Mars and Jupiter were the focus of what amounted to NASA's second wave of exploration with orbiters sent to each planet for long-term observations: the radar-mapping Magellan to Venus, the recently completed Galileo mission to Jupiter and multiple orbiters and landers to Mars.

As initially envisioned, Cassini was part of a dual mission known as CRAF-Cassini. Both programs were to be built around the same Mariner Mark II spacecraft design to minimize costs. CRAF, or the Comet Rendezvous and Flyby Mission, was to do what its name suggested. Cassini was to explore Saturn.

But the CRAF mission was cancelled in 1992 and a few months later, new NASA Administrator Daniel Goldin, author of what would become known as the "faster, better, cheaper" mission design philosophy, threatened to cancel Cassini as well, deriding the expensive spacecraft as a "Battlestar Galactica."

Cassini lifts off from Cape Canaveral atop a Titan 4B rocket. Credit: NASA-KSC

"With many countries participating in this great mission, none of us could imagine Goldin taking this drastic step and damaging our country's relationship with the Europeans," wrote former Cassini mission manager Charles Kohlhase in an article for The Planetary Society. "To our joy, on June 14, 1994, Jean-Marie Luton (then director general of ESA) sent a powerful letter to Vice President Al Gore, with copies to the U.S. secretary of state, key office directors, and of course Dan Goldin."

The letter concluded with a paragraph that Kohlhase said saved Cassini from the budget axe: "Europe therefore views any prospect of a unilateral withdrawal from the cooperation on the part of the United States as totally unacceptable. Such an action would call into question the reliability of the U.S. as a partner in any future major scientific and technological cooperation."

European participation in upcoming NASA projects, including the agency's flagship international space station program, was considered crucial and in the end, the Cassini project survived. When all was said and done, NASA spent $2.6 billion on the project, the European Space Agency spent $500 million and the Italian Space Agency chipped in $160 million for a total cost of about $3.26 billion.

Getting to Saturn took almost as much time as it took to get the project approved and launched: 6.7 years. Because NASA did not have a rocket capable of boosting the heavyweight Cassini directly to Saturn, the spacecraft was forced to take a convoluted route, using the gravity of Venus, Earth and Jupiter to build up the velocity necessary to reach its target.

The Centaur upper stage of the U.S. Air Force Titan 4B rocket that launched Cassini on Oct. 15, 1997, was used to slow the craft down so it could fall into the inner solar system. After flying by Venus on April 26, 1998, and again on June 24, 1999, Cassini streaked past Earth at an altitude of just 727 miles or so on Aug. 18, 1999. From there, the spacecraft headed for Jupiter and a final velocity boosting flyby on Dec. 30, 2000.

This graphic illustrates Cassini's long trek to Saturn. Credit: NASA/JPL

While Cassini's primary mission is to last four years, it has enough power and propellant to last many years beyond, should NASA choose to fund additional mission support.

"We're long overdue for a visit to the lord of the rings," said Orlando Figueroa, director of solar system exploration at NASA headquarters. "We're looking forward to significant science coming out of the mission and we intend to keep it alive for as long as we can. This is a flagship mission of incredible importance to solar system exploration."

Throughout its seven-year voyage to Saturn, Cassini has operated in near flawless fashion. Aside from the issue with the receiver needed to collect data from Huygens, engineers have encountered only four other problems of any significance.

For reasons still not fully understood, the optics in Cassini's narrow-angle camera clouded up after launch, producing blurry images. But by warming the optics in a carefully controlled fashion, engineers were able to clear up the problem "to the point where we think it is at least as good as it ever was before," said program manager Bob Mitchell.

Another problem involved a radio science experiment, one unrelated to the issue with the Huygens probe. A component in a Ka-band radio failed well into the mission, resulting in a loss of sensitivity for certain gravitational wave studies. Part of that research was completed before the failure and other aspects of the experiment can be conducted as originally planned. The overall impact, Mitchell said, was minimal.

A third problem is a large leak in the main helium regulator used to pressurize Cassini's propulsion system for major rocket firings. Engineers believe foreign object debris, possibly from an explosive pyrotechnic device, which was fired shortly after launch to activate the system, lodged in the regulator, preventing a valve from seating fully.

Helium is used to push propellants through Cassini's plumbing and into the main engine's combustion chamber at a constant pressure. The regulator controls that pressurization, which is needed for long firings like the upcoming Saturn Orbit Insertion burn.

In this case, Cassini's complexity and built-in redundancy came to the rescue. By delaying the opening of a downstream latch valve to just 70 seconds or so before main engine ignition, engineers were able to work around the regulator issue with no impact to mission operations. The procedure was used for a major 88-minute Deep Space Maneuver rocket firing back in 1998 and again in late May for a six-minute burn to fine-tune the Phoebe flyby trajectory. No one expects any problems for the Saturn Orbit Insertion burn.

An artist's concept of Cassini performing an engine firing. Credit: NASA/JPL

The fourth problem, and one that has at least some potential for downstream impact to normal operations, involves Cassini's stabilizing reaction wheels. By controlling the spin of three such wheels, oriented 120 degrees apart, flight engineers can re-orient Cassini or keep it rigidly locked on target, reducing the spacecraft's motion to 10 times less than the movement of a clock's hour hand. A fourth wheel can be moved into position to take over in case of problems with any one of the other three.

Two years ago, engineers noticed "some chatter, some vibration" in the bearings of reaction wheel No. 3. It's a phenomenon known as cage instability, "kind of like a rattling ball bearing in your axle bearings of your car," said Julie Webster, lead spacecraft engineer. To be on the safe side, wheel No. 4, the spare, was spun up and No. 3 was moved into the backup position.

"So now we're operating on wheels 1, 2 and 4 and they're operating just fine," Mitchell said.

But friction levels also are increasing, albeit very slowly, in the other wheels.

"We think it's probably pretty normal, standard behavior," Mitchell said. "We have talked to some projects ... that have used wheels like this, or similar to this, and none of them has seen this kind of a problem. But none of them had the level of instrumentation that we've got, either. So we suspect they probably had exactly the same phenomena occurring and they just didn't see it."

Engineers have analyzed the telemetry from the three operating wheels and "we have extrapolated it out to well beyond end of mission and it doesn't get even close to what the torque threshold is for our ability to spin these things," Mitchell said. "But it's something that we're keeping an eye on."

MISSION PREVIEW
   PART 1: FLAGSHIP MISSION OF OUR TIME
   PART 2: CRAFT WELL EQUIPPED FOR SCIENCE
   PART 3: OVERCOMING HURDLES AND CHALLENGES
   PART 4: PREVIEW OF SATURN ORBIT INSERTION