Cassini’s Last Dance

NASA – Public Domain

The Cassini space probe is nearing the end of its life. Cassini is the spacecraft that NASA sent to explore Saturn. It arrived at the ringed planet in 2004, and here are a couple of pieces I wrote about it then:

NASA -Public Domain

Cassini Goes to Saturn

Saturn is the planet that’s always depicted with a big, wide ring around it. The ring is like the brim of Saturn’s hat or the skirt of its tutu. The ring looks big but an average moon of Saturn contains as much material as the whole ring system.

The ring around Saturn, when you get a closer look at it, is really made of many smaller rings, each in an orbit of its own. We got that closer look when the Voyager probes 1 and 2 went by Saturn in 1980 and 1981 on their way out of the Solar System.

Saturn’s rings played a role in the arrival of the space probe Cassini, which finished its seven year transit from Earth this summer with a perfect insertion into its pre-planned orbit. The Cassini probe is named after the 17th century Italian astronomer, Giovanni Cassini, who discovered the big gap in the rings, now called the Cassini division. The spacecraft approached Saturn’s rings from “below,” slipped through a gap between two rings, did a 95 minute engine burn to slow down enough to stay in orbit and popped back down through the rings. Now it’s going to settle in for a four year mission, collecting data on the Saturnian system.

One of Saturn’s 31 known moons, Titan, is of particular interest and will be approached 45 times on the 76 scheduled orbits of the planet. Titan is a big moon, even larger than the planet Mercury, and is known to have a thick atmosphere. On one of its close approaches, Cassini will be releasing its payload, the Huygens probe, which will drop into Titan’s atmosphere and, hopefully, safely land on the surface.

Cassini is the size and mass of a 30 passenger bus and is the largest interplanetary spacecraft ever made. It contains 12 scientific instruments in addition to the Huygens probe, and will be measuring everything from regular visible light images to radio waves to magnetic fields. By the time the four year mission is completed we will have learned more about Saturn than we have since Galileo puzzled over the planet with the “ears” in his telescope 400 years ago.

If all continues to go as well as it has so far, we can look forward to at least four years of new images and new knowledge about the beauty of the Solar System, Saturn.

NASA – Public Domain

Cassini-Huygens

After a flight of seven years and over three billion kilometers, the spacecraft Cassini successfully entered orbit around the planet Saturn. It then reconnoitered the ring system and the many moons there for six months. The data it’s sending home will keep planetologists busy for years.On Christmas eve Cassini released the Huygens probe. On January 14th the probe will plunge at 19,000 kilometers per hour into the thick atmosphere of Saturn’s moon, Titan. It will make a two hour descent to the surface, deploying parachutes three times to slow down.

After the ablative heat shield has slowed Huygens down most of the way, the first parachute opens. Huygens jettisons the heat shield at about 150 kilometers up, and starts collecting data. Then the other parachutes reduce the rate of descent as much as possible, both to prolong the data-collection phase and to try to ensure the probe’s survival on landing.

The scientists aren’t relying on a safe landing. Their mission will be a success if they get good data during descent. The probe will take more than a thousand pictures in addition to analyzing the atmosphere on the way down. Even if it dies on impact or gets glommed in a sea of goop, Huygens will be able to send back a very deep sample of Titan’s thick air first.

The three weeks between separation from Cassini and contact with the atmosphere have been tense. Huygens was turned off to save power and won’t be turned on until the last minute. Full power has to be available for the very busy descent and any surplus will be welcome if it survives to study the surface.

Scientists are interested in Titan because of the unusually thick, rich atmosphere it has. It’s about 50% thicker than Earth’s atmosphere and, like ours, is mostly nitrogen. The other notable constituents are methane and hydrocarbons. Titan’s smog problem makes ours pale by comparison.

Meanwhile, having dropped off its passenger, Cassini had to adjust course to not follow it into Titan. Once Huygens goes quiet and there are no more data to relay to Earth, Cassini will get back to the business of exploring. There are over sixty more orbits to do around Saturn. Sixty more chances to get a look at the moons, the rings and the storms of Saturn. So much to see and only four more years to see it.

Both parts of the Cassini-Huygens mission were successful, to put it mildly. Huygens, the probe released by Cassini, landed on Saturn’s moon, Titan, and sent up a lot of data, relayed to Earth by the mother ship. That happened in 2004-2005, but launch was in 1997. It took seven years and much fancy maneuvering to get there. Planning and design for the mission began in the 1980s, with a large team of people from seventeen countries. This has been a truly big international project.

After taking care of the Huygens-Titan phase, Cassini got on with the rest of its four year mission, then kept going for another seven or eight. It has far exceeded what was originally asked of it, but now it’s running down and the team is planning a spectacular finish for it. In this phase it’s going to trace some highly elliptical orbits of Saturn, ranging from a maximum distance many times the diameter of the planet to a minimum between the rings and the atmosphere. As a grand finale it will plunge into Saturn’s atmosphere, where it will be destroyed. But not before sending back even more data.