Cassini Sails Flawlessly into Saturn’s Orbit


Cassini mission scientist, Don Matson (lower right); Charles Elachi and Don Mitchell (upper left); navigation team lead, Jeremy Jones (upper right); JPL mission control and program manager Robert Mitchell (lower left). Image Credit: JPL/NASA

The ringed planet has a new satellite: the Cassini-Huygens spacecraft.

At 8:56 p.m. PDT, Todd Barber, a Cassini team member, announced that a planned engine burn had slowed down the spacecraft sufficiently to enable Saturn’s gravity to capture it into orbit.

Fifteen minutes later, the mission control room at NASA’s Jet Propulsion Lab (JPL) in Pasadena, California, erupted in cheers as a second announcement was made, that the engine burn had also ended successfully. Had the burn not cut off as it did, the spacecraft would have used up all of its propellant and had none left for the orbital adjustment maneuvers that will enable it to conduct its planned scientific experiments.

“It feels awfully good to be in orbit around the lord of the rings,” said JPL Director Dr. Charles Elachi. “This is another small step in our quest of discovery that NASA and Europe are committed to do on behalf of all humanity, but it’s going to be a huge leap in our understanding of the Saturnian system.”

“There wasn’t a single thing that we might have asked to have done differently that would have made anything any better,” added Robert Mitchell, Cassini program manager. “We’ve got a very exciting four-year mission in front of us.”

Saturn rings A-G. Cassini passed through the F and G ring gaps. Image Credit: JPL/NASA

The SOI was a make-or-break maneuver for Cassini-Huygens. If it had failed to execute properly, the spacecraft would not have slowed down enough for Saturn to capture it into orbit. Instead, Cassini-Huygens would have sailed past Saturn, veering ever-farther into the outer solar system – and the $3 billion spacecraft and its four-year orbital mission would have been literally lost in space.

Although Cassini-Huygens’ main, high-gain antenna was turned away from Earth during the SOI, flight controllers were able to track the success of the engine burn by monitoring a signal sent by the spacecraft’s low-gain antenna. The signal contained no telemetry data about the state of the spacecraft’s systems or instruments. Instead flight engineers used only the signal’s carrier wave to tease out information about the progress of the SOI.

The carrier wave provided two types of critical information. First, as the engine burn proceeded, the acceleration of the spacecraft changed, and the frequency of the carrier wave changed along with it. This effect is known as a Doppler shift. Mission planners had developed a plot of the Doppler shift they expected to see during the burn. By tracking the actual Doppler shift in the signal from the spacecraft, they were able to tell thoughout the burn that it was proceeding as planned.

The strength of the Doppler signal also enabled flight controllers to track the spacecraft’s progress. During the burn, Cassini-Huygens was above the rings of Saturn while, from the spacecraft’s perspective, Earth was below the rings. As the craft proceeded along its trajectory, its line of sight was alternately blocked (occulted) by ring material, and then opened up through gaps in the rings.

Titan descent by Huygens probe around Christmas 2004. Image Credit: JPL/Space Science Institute

When the spacecraft’s line of sight to Earth was blocked by ring material, the strength of its signal dropped to a low level or temporarily was lost entirely. When the spacecraft had a clear line of sight to Earth through a gap in the rings, the signal got stronger. By tracking these changes in signal strength, flight controllers could verify that Cassini-Huygens was proceeding as planned.

Shortly after the SOI completed, the spacecraft turned to point its cameras at Saturn’s rings to capture a set of about 80 close-up images. Those images are scheduled to be sent back to Earth beginning at about 5:30 a.m. Thursday PDT. Flight engineers will now turn their attention to adjusting the spacecraft’s orbit to prepare it to release the Huygens probe toward Saturn’s giant moon Titan.

Huygens, designed and built by the European Space Agency (ESA), will separate from Cassini on December 24 PDT (December 25 in Europe). Three weeks later, on January 14, it will reach its target and for 2.5 hours will drift down through the moon’s thick atmosphere, collecting scientific data along the way.

Titan is the only moon in the solar system with an atmosphere. Like Earth’s atmosphere, Titan’s is rich in nitrogen. It also contains methane and other, more-complex organic hydrocarbons, believed to be precursors to life. Scientists are hopeful that by studying Titan they will be able to learn about the pre-biotic chemistry of early Earth.

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