Decommissioning the Cassini Probe: Science and Environment
Saturn, with its mesmerizing gaseous surface, its characteristic rings, and its mythological background, is arguably the most distinctive member of our solar system. Yet, until not long ago we had surprisingly little data about it.
The Cassini-Huygens mission was conceived specifically to cover this gap. Launched on October 15, 1997, this collaboration between NASA, the European Space Agency, and the Italian Space Agency included two distinct vehicles: the Cassini orbital probe, named after Italian astronomer Giovanni Domenico Cassini, and the Huygens lander, named after Dutch astronomer Christiaan Huygens.
A spectacular view of Saturn, captured by Cassini in 2016.
Credits: NASA
The six-year transfer included gravity-assist maneuvers (and photo shooting opportunities) around Venus, Jupiter, the Earth, the Moon, and asteroid 2685 Masursky. On July 1, 2004, Cassini became the first spacecraft to ever orbit Saturn.
On December 25, 2004, Cassini released the Huygens lander, which descended on Saturn’s moon Titan on January 14, 2005. Huygens holds the record of being the spacecraft that performed the furthest lading from Earth, and its 350 pictures and telemetric data have increased enormously our understanding of a planetary body that bears similarities with a primordial planet Earth.
This landing sequence has been rendered from Huygens’ Descent Imager/Spectral Radiometer (DISR) data.
Credits: ESA.
In the past 13 years, Cassini has produced an incredible array of data about Saturn, its rings, and its two major moons Titan and Enceladus. Thanks to Cassini, we learned about Saturn’s climate and atmospheric phenomenon, its rotational period, its magnetic field, and the effect of its rings on radio signals. We discovered seven new moons, observed the lakes of Titan, photographed a hurricane on the surface of Saturn, witnessed the aftermath of the massive Great White Spot storm that recurs roughly every 30 years on Saturn, and much, much more.
While the prime mission was planned to last only four years, it has been extended twice, first in 2008, then in 2010. Now, after nearly 20 years in space, the spacecraft is running out of fuel, so mission controller have come up with Cassini's Grand Finale, a decommissioning plan that will ensure both a clean disposal of the vehicle, and a final batch of scientific data.
Interplanetary missions are often run until the end of communications, or the end of mission resources, whatever comes first. What happens to the spacecraft afterwards is not a major concern, as the spacecraft moves past the solar system towards infinity.
Artist’s depiction of Cassini’s Gran Finale.
Credits: NASA.
In the case of Cassini, however, a careless approach would not be acceptable. Both Titan and Enceladus present an environment that could harbor microbial or probiotic life, and an impact of Cassini, however unlikely, could contaminate their environment and jeopardize future studies of habitability and potential life on those moons.
To prevent this scenario, NASA decided to safely dispose Cassini in the atmosphere of Saturn, burning it up on impact.
But Cassini’s Grand Finale is not a brutal suicide maneuver: it is an exploratory mission in itself, with its own ambitious goals. The particular choice of trajectory, and the fact that the spacecraft is going towards its demise enables the collection of information that was too risky to obtain earlier in the mission.
On April 22, Cassini was inserted into a highly elliptical orbit that passes between Saturn and its rings. Now, the spacecraft will perform 22 orbits, each one lasting six and a half days, capturing a series of images that will be combined into a high-resolution movie of Saturn’s north polar region.
Cassini's flight path during the final two phases of its mission. In blue, the 22 Grand Finale Orbits. The arc in orange represents the final partial orbit, ending up in atmospheric re-entry.
Credits: NASA.
In the 60 minutes before the closest approach to Saturn, Cassini will capture the closest view of Saturn ever taken, with a resolution up to 10 times higher than previous material. Besides taking pictures, the spacecraft’s sensors will collect detailed data of Saturn’s gravity and magnetic fields, and the particle composition of the upper layers of the atmosphere.
On September 15, 2017, Cassini will plunge into Saturn’s atmosphere. Like our D-Sat, Cassini will sample the upper layers of the atmosphere, and keep transmitting data until the last instant.
Concluding a space mission is an emotional moment, as you may remember if you followed Rosetta’s final touchdown with Comet 67P/C-G on September 30, 2016. Over time, a spacecraft gains a personality of its own, and to its creators and mission planners it becomes an adventurous explorer fully devoted to its mission. A proper and responsible disposal is more than a moral duty: it is an opportunity to provide closure, and to perform an ultimate scientific contribution.
These are the same principles that we adopted in designing the mission profile of D-Sat, a small satellite that will perform two of its three scientific experiments while performing a direct and controlled re-entry into Earth’s atmosphere.
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