Why is the Cassini mission ending?
Artist’s view of Cassini diving between Saturn and its rings on the way to the Grand Finale. Credits: NASA/JPL-Caltech
The main reason the mission is ending is that the hydrazine propellant needed to control the spacecraft is running out.
“Cassini has about 10 to 20 kilograms of fuel left out of the 3 tonnes it was initially carrying,” estimates Philippe Zarka. “Part of that is unusable for mechanical reasons, but the hydrazine has only been used for small trajectory adjustments and to compensate for the kinds of tiny errors that accumulate over a 13-year mission. The 3-tonne load of fuel wasn’t used to get to Saturn; that job was done by the initial thrust from the launcher and a series of planetary gravity assists.”
The risk now would be to lose control of Cassini. Left to its own devices, it would inevitably end up crashing somewhere, with the chance that it might contaminate one of Saturn’s icy moons. Although Cassini was subjected to dry heat microbial reduction procedures, it isn’t sterile. More-effective sterilization methods can’t be used on spacecraft because their electronic components are too fragile, so even after 20 years in the vacuum of space, bacteria or latent lifeforms like spores could have survived the journey.
“A lot is at stake here, because future missions to Saturn will be looking for signs of life. We must prevent contamination of the surfaces of Enceladus or Titan, otherwise we could find organisms without being able to tell if they hitched a ride from Earth or really are new lifeforms. At least when it enters Saturn’s atmosphere Cassini will burn up, so any micro-organisms it might have been carrying will be destroyed with it.”
Doing science to the very end
But Cassini’s programmed destruction is more than just that—its Grand Finale will also offer the opportunity to obtain fresh data even closer to Saturn, which would have been too risky in the mission’s early stages.
As Thierry Fouchet explains in the previous article on Cassini’s legacy, the spacecraft’s daring final dive will enable scientists to study the composition of Saturn’s atmosphere and its concentration in helium. For Sébastien Charnoz, the Grand Finale will also serve to determine the mass and age of the rings. For Philippe Zarka, another key goal is to study the radio waves emitted by Saturn’s polar aurorae.
“Radioastronomers have been studying radio emissions from aurorae, the Sun and planets for more than half a century, but our knowledge of these areas varies greatly. Flying Cassini through Saturn’s auroral sources will allow unique and really precise measurements to be acquired; it’ll be like having a fully-fledged plasma physics laboratory in situ.”
What’s more, while adding to our existing body of knowledge, whole new discoveries are also expected about Saturn’s quirky magnetic field. It’s the only planet in the solar system where the magnetic field is almost perfectly aligned with its rotation axis.
“The lack of tilt in Saturn’s magnetic field defies explanation for now. In comparison, Earth’s dipole is tilted 11° to its geographic pole. Current models are based on the premise that a magnetic field depends on the conductive materials in the planet’s interior, but it’s hard to explain how such a field can be well aligned. That said, Saturn’s magnetosphere exhibits rotational modulations. Particle fluxes are modulated by the planet’s rotation, as are the aurorae. But we shouldn’t be seeing such rotational phenomena with an aligned magnetic field, so we hope that by getting closer we might detect magnetic anomalies that would explain why the field isn’t tilted and help us to understand what’s going on inside Saturn.”
Alignment of Saturn’s magnetic field compared to other planets © adapted from Bagenal, 2002, Encyclopedia of Astronomy and Astrophysics, Edited by Paul Murdin, Bristol: IOP Publishing.
The Grand Finale
Cassini’s Grand Finale is in fact a series of 44 orbits. The first 22 of these were completed between end 2016 and April 2017. These were ring-grazing orbits that took Cassini close to the F ring that is 2.3 Saturn radii from the planet. Then, in April, it performed a final manoeuvre with one last flyby of Titan to put it on course to dive between Saturn and its icy rings. This is the series of 22 closer orbits that is about to end.
“The F ring orbits were highly inclined,” points out Philippe Zarka, “almost polar, in fact, as are the closer orbits. Since April, Cassini has been completing one orbit a week, skimming the ionosphere just 1,700 kilometres above Saturn’s cloud tops.”
Cassini began its fateful plunge towards Saturn on 12 September. On 15 September at 13:53 CET, it will enter the planet’s atmosphere. Cassini’s last remaining reserves of fuel will be used to keep its radio antenna locked onto Earth as its thrusters work hard to counter atmospheric drag. As Cassini’s speed and the atmosphere’s density increase, it will start tumbling and all contact will be lost. The spacecraft will then continue its descent with a final flaming flourish, bringing to an end a 30-year space odyssey.
Artist’s view of Cassini’s demise in Saturn’s atmosphere. © NASA/JPL
What’s next after Cassini?
Pursuing investigations of the gas giant planets of our solar system, NASA’s Juno spacecraft has taken over at Jupiter where Galileo left off in 2003, and will be joined by ESA’s JUICE mission in 2030. But no follow-on mission to Saturn is yet planned, and Uranus and Neptune, the planets farthest out in the solar system, have only been studied during flybys and have so far received less attention. So what lies ahead for planetary exploration?
“It’s hard to predict,” replies Philippe Zarka. “The problem with space science is that budgets have to be shared across missions, spacecraft and telescopes. It would be great to go to Uranus and Neptune. Before the first discoveries of exoplanets were made, astronomers had formulated a theory for how the planets of the outer solar system formed. The very first exoplanet to be identified was a giant planet orbiting closer to its star than Mercury is to the Sun, so the whole theory had to be revised to take into account inward migration of planets. To take an analogy, we couldn’t have reached a theory on the life cycle of stars by just basing our studies on the Sun. We were only able to do that by studying thousands, even millions of stars. The same approach is needed for comparative planetology (the physical study of planets): each mission tells us a great deal about the specific planet under study, but it’s by looking at the physics of many exoplanets that we can determine general and specific rules.”
We don’t yet have the capability to send missions beyond our solar system, as the distances are too great and spacecraft velocities too slow. We therefore have to observe exoplanets remotely. Certain optical and infrared telescopes are beginning to do this, and certain astronomers are working to detect exoplanets with radiotelescopes, paving the way for comparative study of their magnetospheres. Great hopes are also being pinned on the James Webb Space Telescope (JWST) set for launch next year. JWST will be much more powerful than the Hubble Space Telescope that it will be succeeding, with the ability to analyse the composition of exoplanets’ atmospheres.
- Francis Rocard, head of CNES’s solar system programme : francis.rocard at cnes.fr
- Philippe Zarka, astrophysics research director at the LESIA space and astrophysics instrumentation research laboratory at the Paris Observatory : philippe.zarka at obspm.fr