Until now, Saturn had 31 known moons. Today, it has one more.
It's not a traditional moon, but a spacecraft that's now circling the planet. After a seven-year, 2-billion-mile trip from Earth, the Cassini spacecraft fired its engine right on schedule overnight, gently slowing itself into orbit around the giant ringed planet. And today, NASA scientists received images from the spacecraft that represent the best view the spacecraft will ever have of the planet's mysterious rings.
"Absolutely mind-blowing," imaging team leader Carolyn Porco said as a dark, streaked picture of the rings flashed on NASA's large screen. "Look at that sharp edge. That brings tears to my eyes."
Porco explained that scientists like herself can glean the most information from such sharp images in pictures sent from space.
The mood was no less jubilant several hours earlier when engineers at the Jet Propulsion Laboratory in Pasadena, Calif., received the signal that the ship's engine was working.
"Lots of hugs and high-fives in mission control," a NASA engineer said as the team learned all was going to plan.
From that point on, the mood was upbeat but mostly quiet. Cassini — so far from Earth that its radio signals took an hour and 24 minutes to reach home — was on autopilot. All controllers could do was listen, and hope they had not missed anything when they planned the mission.
So far, it appears they succeeded.
"This whole mission has been an incredibly smooth one to fly," said Julie Webster, the flight director for the orbital insertion maneuver.
The engine burn, which lasted more than an hour and a half, apparently ran within a second of what engineers expected.
Largest Probe in U.S. History
The Cassini probe would weigh more than 12,000 pounds on Earth, and it cost $3.3 billion to build and fly. It is the largest unmanned probe the United States has ever launched, and it was decades in the making.
A scientific panel first proposed it in 1980, and Congress first allocated money for it in 1989. As it fell behind schedule and its budget grew, NASA realized it would be the last ship of its kind. Newer probes — including the two rovers now exploring Mars — were designed to be less ambitious and less expensive.
Over the next four years, if all goes as planned, Cassini will circle Saturn 76 times, taking pictures, measuring the temperature and magnetic field of Saturn, and looking for clues to explain how its rings formed. On Christmas Eve, it is to release a smaller probe, called Huygens, to try to land on Saturn's largest moon, Titan.
Titan is an oddity in the solar system; it is the only moon known to have a substantial atmosphere. Its air is 50 percent thicker than Earth's at sea level, and the chemicals in it — organic compounds such as methane and ethane — closely match what scientists believe one would have found in our atmosphere 4 billion years ago, just before life formed.
It is extremely unlikely that Titan holds any hope of life; the temperatures there are several hundred degrees below zero. But it may offer hints of how life forms. The European engineers who designed Huygens have no idea what the probe will find if it survives entry into Titan's atmosphere, so they planned for every possibility they could think of. The probe can float, if it happens to land in a sea of liquid ethane.
"We are working on something that I can easily convince myself is one of the greatest achievements mankind's ever done," said Kevin Grazier, a physicist who has been on the Cassini project since 1998. "We refer to Saturn as the gem of the solar system. I don't think any other planet is as beautiful."