At precisely 4:55:46 a.m. PDT (7:55:46 a.m. EDT) on Sept. 15, NASA received the final transmission from the Cassini spacecraft. This meant only one thing: The groundbreaking Saturn mission had come to an end – the spacecraft had encountered the uppermost atmosphere of Saturn and, 45 seconds later, it burned up like an artificial meteor caused by the intense heat of hitting the atmosphere at 75,000 miles (120,700 kilometers) per hour.
"This is the final chapter of an amazing mission, but it's also a new beginning," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at NASA headquarters in Washington, in a Sept. 15 statement. "Cassini's discovery of ocean worlds at Titan and Enceladus changed everything, shaking our views to the core about surprising places to search for potential life beyond Earth."
Having completed 22 daring dives through the approximately 1,200-mile (1,931-kilometer) gap between the planet and its innermost ring, Cassini's Grand Finale was able to do groundbreaking science in a region of space no spacecraft has ever been before. But the Grand Finale was just that – the final act of its 13 years of exploration at the Saturn system.
Cassini was running out of fuel, and mission managers wanted to get as much science out of the veteran probe while protecting Saturn's moons. The only way to truly protect them was to dispose of Cassini by flying it into Saturn, giving the trailblazing mission a fiery sendoff.
"This has been in the plan all along," says Linda J. Spilker, Cassini project scientist who works at NASA's Jet Propulsion Laboratory in Pasadena, California, and has worked with Cassini since its beginning. "We knew the mission had to end, and we carefully budgeted our fuel to last until Saturn's northern summer solstice in May 2017."
But what are scientists trying to protect these moons from? Basically, germs; bacteria from Earth that might be hitching a ride on the Cassini spacecraft.
"Tenacious" Bacteria and the Fear of Contamination
Although Cassini was launched in 1997, spending nearly 20 years in the vacuous, freezing, radiation-washed depths of space, traveling to Saturn and spending 13 years in orbit around it, some bacteria "can be tenacious," says Larry Soderblom, an interdisciplinary scientist with the Cassini mission. "We have to make sure we don't leave any of these kinds of Earthly bacteria on these promising moons."
International agreements govern planetary protection measures that must be taken by space agencies. Although planetary protection may conjure up thoughts of protecting Earth from marauding alien bugs, there is great effort put into protecting other worlds from our bacteria. It just so happens that Enceladus and Titan are particularly interesting moons that could be habitable for extraterrestrial life, so it would be unfortunate if our bacteria invaded these environments.
During its mission at Saturn, Cassini discovered plumes of water being vented into space from the icy moon Enceladus. Closer investigation of these plumes, originating from geysers blasting from polar fissures in Enceladus' icy crust, revealed this water was coming from a warm subsurface salty ocean and the water was laced with hydrocarbons and ammonia, or "many of the ingredients that life would need if it were to start in an environment like that," Soderblom tells HowStuffWorks.
So, the nightmare scenario would be that, if Cassini was just left to run out of fuel and careen uncontrollably around the Saturn system, there would be a small chance that it could crash into Enceladus in the future and any hitchhiking microbes on board might set up home in this ocean that lies beneath the thin crust.
"We try to sterilize spacecraft so they don't carry germs from the Earth," says Soderblom. "But we can't do that thoroughly enough that we can safely dispose of the spacecraft in one of these alien worlds that we may one day want to someday explore for habitability and look for life – we don't want to discover the life that we brought there!"
Titan was another concern. Saturn's biggest moon is also the only moon in the solar system sporting its own atmosphere. But this atmosphere is like nothing we experience on Earth. Too cold for liquid water, the only liquid on Titan is in the form of methane – any water is frozen rock-solid. Although this world is very alien, it does have some similarities to Earth's atmospheric processes. Instead of water, liquid methane falls as rain, creates rivers that cut into the hydrocarbon-rich surface, which in turn make valleys, and flows into vast liquid methane lakes.
Titan also hides a secret below its surface. After its many flybys of the moon, Cassini discovered that it also hosts a subsurface ocean of liquid water, only much deeper than Enceladus' ocean. Astrobiologists are excited about the possibility of exotic forms of "methane life" on the surface of Titan, but it's yet another moon in the outer solar system with a liquid water ocean that might be habitable for life as we know it. Although Earthlike microbes "wouldn't be so happy" on Titan's surface, Spilker says it's the subsurface water that we'd want to avoid contaminating.
In 2005, however, the European Huygens probe landed on the moon's surface. Piggybacking on Cassini since launch as part of the joint NASA/ESA/Italian Space Agency Cassini-Huygens mission, Huygens detached itself, coasted to the moon and descended through its trademark thick yellow haze. But if we've already sent a probe there, why worry about contaminating it with Cassini?
"When we landed the Huygens probe on Titan, we had no idea what we'd find on the surface, so it was really the discoveries of Huygens and Cassini that led to these new [planetary protection] requirements," says Spilker. "Also, Titan is not in the same class as Enceladus for planetary protection."
To go to Enceladus, she added, any lander would need to be very clean as liquid water is in contact with the moon's icy surface, so contamination of the subsurface ocean would be a very real possibility. Titan's ocean, on the other hand, is deep and doesn't appear to be in contact with the surface, so "this puts [Titan] in a different category," Spilker says. "While care is still taken with Titan, it's really Enceladus that we're most worried about contaminating."
To Crash or Not to Crash
Previous space missions have employed a variety of measures to avoid risking contamination of promising moons and planets, says Soderblom. "The Voyagers we disposed of by ejecting them from the solar system, the Galileo spacecraft was burned up in the atmosphere of Jupiter. The Mars spacecraft were extra, extra carefully decontaminated, and that's a very expensive process to go through so you can safely leave the spacecraft [on the planet's surface]."
And with Cassini, there were a variety of options on the table to prevent a collision with Enceladus or Titan, particularly earlier on in the mission.
"Back in 2009 and 2010, we had enough fuel to perhaps leave the Saturn system and go to Jupiter. We could have even gone to Uranus," remembers Spilker. However, to get to Uranus, it might have taken 20-30 years and mission planners decided that Cassini was really designed for the Saturn system anyway, so they decided to stay.
To avoid the inner Saturn system, they also considered sending Cassini on very wide orbits, staying well clear of the moons. In this scenario, when the spacecraft ran out of fuel, it would forever remain in a holding orbit around Saturn as a new artificial satellite itself.
"We also looked at possibly crashing it into Mimas or one of the other moons," adds Spilker. "That would have been quite a way to end [the Cassini mission] by making a new crater!"
But when they realized they could send Cassini through Saturn's rings by using Titan's gravity to redirect the probe, that option became the clear front-runner. "It didn't take much discussion because flying through this region would give us brand-new science – it's like having a new mission," explains Spilker.
Although Cassini's ashes are now scattered through Saturn's atmosphere, becoming a part of the planet itself, its legacy will live on for decades to come, and Enceladus awaits humanity's next mission, whenever that may be. Cassini did the groundwork, discovering subsurface oceans and habitable environments on what was once thought to be a collection of dead and barren moons. Now, Cassini mission scientists want to go back and directly look for life, perhaps even returning samples of Enceladus to Earth.