After twenty years of studying the planet Saturn, the working life of the Cassini spacecraft will come to an end in the next few hours. On Friday, September 15, 2017 at about 7:55 am EDT, NASA will crash the probe into the surface of Saturn and end its mission.

As far back as the 1980s astronomers from around the world were making plans to send a probe to the gas giant, Saturn. We had already explored both Mars and Venus, and the Voyager probes had made flybys of the outer planets, but Saturn was still not well explored. Eventually a cooperative project involving NASA in the United States, ESA in Europe, and ASI in Italy was developed to launch an orbiter to Saturn.

Finally on October 15, 1997 a Titan rocket lifted the Cassini probe into space. It spent the next seven years traveling through the solar system before entering a stable orbit on July 1, 2004. 

One amazing aspect of the Cassini mission has been its unexpected longevity. The initial mission was planned for 2004 to 2008, a meagre four years of orbiting Saturn and taking various measurements from orbit. At the end of its initial mission, it was still operating and so the mission was extended another two years. When the second mission ended, the Cassini probe was still in good shape and so a third mission was initiated which would last seven more years, and would include further exploration of the moons of Saturn.

Unfortunately the probe is now reaching the end of its usable life. Although it is in a stable orbit, crews on Earth still need to send commands for it to correct its position and adjust its orientation, and each such movement uses a small amount of fuel. And that fuel supply is now nearly exhausted. The crews could allow the probe to continue in orbit and continue collecting data, but that is a risky choice. At some point the probe will no longer be able to have its orbit adjusted, and will crash into either Saturn or one of its moons.

And that is the big concern that led to the decision to crash Cassini intentionally. Two of these moons, Titan and Enceladus, have interesting geographies and chemical compositions, and may even harbor some form of microbial life. If the Cassini probe collides with either of them, it could contaminate their surfaces or otherwise damage them. An uncontrolled crash by Cassini into one of the moons could cause far more scientific harm than any benefit that we would garner by allowing the mission to continue. 

And so for those reasons, the decision was made to order Cassini to fly into the surface of Saturn. Hopefully it will be transmitting data during its descent, and will provide more data on the atmosphere of the gas giant. Whatever happens, this little probe that has lasted three times its expected lifespan will have provided more data to astronomers and scientists than we would have ever thought possible when it was first launched. It has been a very successful mission, and will no doubt lead to more great discoveries in the future!

Those wishing to view the final descent of Cassini can do so here.