Cassini's final orbits

The Cassini spacecraft, which has been exploring various unique features of Saturn for the past 13 years, is now due to end its mission. Cassini is considered to be one of the great space missions of all time for its groundbreaking discoveries. A gravitational slingshot manoeuvre around Titan (the largest satellite) has put it on its path towards inevitable destruction with the probe heading for a ‘grand finale’ on September 15 with its fiery plunge into the dense atmosphere of Saturn. 

Leaving Cassini in an uncontrolled orbit near Saturn was considered risky as it could crash sometime into some of its satellites like Enceladus or Titan (which could host some form of primitive alien life) contaminating these potential life supporting moons with some resilient residual terrestrial bacteria residing dormant in the spacecraft. This possibility was considered high. There were two options to avoid this. One was to put the probe into a very elongated orbit far from Saturn. But the science returns from this cannot be much.

Accordingly, as per NASA scientists’ decision, Cassini’s final flyby began on April 22, 2017. From that time, its orbit shrunk and took it between the planet’s rings and atmosphere. The remainder of the mission saw Cassini repeatedly dive between the atmosphere and the rings. These so called gap runs would repeat every six days till its death plunge on
September 15. It would fly less than 3,000 km above the cloud tops. Cassini’s swing-by on August 14 saw the spacecraft go closer than ever before to the cloud tops — skimming just 1,600 km above them.

This was designed to directly sample the gases of the extended upper atmosphere. The heavier helium (Saturn’s atmosphere is mostly hydrogen) is expected to be sinking down, enabling Saturn to radiate nearly three times as much energy as it is absorbing from the Sun (from the loss of gravitational potential energy). Thus, getting a precise estimate of the hydrogen and helium in the upper layers sets constraints on interior material distribution. Dipping down into the atmosphere could cause drag and buffeting effects and thrusters are expected to maintain stable flight.

Solving big puzzles

Apart from unique data on the atmospheric chemical composition, the Cassini mission is still expected to solve some big puzzles about Saturn. While it is generally said that the length of one day on Saturn is 10.5 hours, scientists are still looking for a more precise value that Cassini can provide. One solution is searching for an offset between the magnetic axis and the rotation axis but all observations reveal these two features are almost perfectly aligned. The theories require an offset. To generate the magnetic field currents, it is required to flow in the deep metallic hydrogen layer inside Saturn but without the offset,the field is expected to die out.

Cassini is currently flying a series of loops around Saturn threading the gap between its atmosphere and rings. It will run the gap between planet and rings 22 times and the close-in dives are expected to yield high quality data about the vast ring system apart from composition, structure, and dynamics of the atmosphere. There is a need to understand the ring composition. The age of the rings is uncertain. The data gathered by Cassini indicates that the spectacular rings of Saturn are relatively younger than thought. Are they a relatively recent phenomena, a result of breakup of a moon or moonlets? Moreover, the mass of the rings is uncertain by at least a 100%! The more massive they are, the older they are. A key property of these spectacular rings and bands of ice is their total mass.

Hitherto, the Cassini flyby outside the ring system was not able to determine the mass of the rings. Now that Cassini is edging closer to the inner D ring, measuring how the velocity of the probe is changing as it flies through the gravity field generated by the planet and the great encircling bands of ice and dust would determine the ring mass. It turns out that we can tell the velocity of Cassini to an accuracy of a few micron per second. This is incredible considering it is a billion kilometres away.

Initially, Cassini encountered very few particles as it dived between Saturn and its rings. At that velocity, the probe could be damaged by impact with fragments of ice or rock. So, this is again a puzzle, why the dust level is lower than expected. It has been suggested in some models that some of Saturn’s bigger moons could have been made in the rings. The wide band of ice and dust that surrounds Saturn has been likened to a miniature version of the kind of discs seen orbiting far off new stars.

In these discs, planets are expected to form. So, picturing the processes of forming moonlets could shed light on how planets can form from dust discs around young stars.

Exploring its moons

In this context, Cassini while observing moonlets present in A ring, the underlying mystery about the object dubbed Peggy was discovered accidentally by Carl Murray while using Cassini to image Prometheus, a bigger moon connected with the F ring. His discovery was made on April 15, 2013. He got the image but was drawn to a 2,000 km long smudge in the background. So, he had the resolved picture of an embedded object (in the rings) he knows exists, but cannot see. The effect of the ice and dust around it was tracked but there was no direct image of Peggy.

Some of Saturn’s moons are perhaps formed by the accumulating material surrounding them and this display of the early phase of this evolution can now be seen in Peggy. Did Peggy arise from collisions or accretion? If bigger moons form from this processes, the rings are very old. As Cassini orbits close to Saturn, inside the rings, resolutions could come down to two kilometres per pixel and this might be enough to see Peggy, whose orbit is evolving and changing in time. Its orbit is believed to be less than five kilometres across. The long smudge pictured earlier was possibly due to its collision with another object kicking up the icy dust. ­

Some of the earlier highlights of the Cassini mission involved the moon Titan and Enceladus. The Huygens probe, which detached from Cassini, descended on to the moon Titan. Although its batteries were nearly drained, it took fascinating pictures as it fell. After only a few hours on the surface, it succumbed to the intense cold. The probe had enough time to send back a single photo after landing, the only one we have from the surface of a body beyond Mars! The fly past Enceladus (in 2015) revealed icy geysers ejecting plumes of high velocity, suggesting an ocean beneath its icy shell. The extraordinary world of Titan contains lakes and seas of methane, ethane, etc. The north latitudes are dominated by great methane lakes, with nitrogen gas bubbles from below producing bumps on liquid surface.

Cassini took a picture of Earth, a bright speck more than a billion kilometres away.  Now in its final plunge phase, Cassini could continue to settle tricky questions about Saturn, like the precise length of its day, its magnetic field and especially by getting inside the rings, their mass. If they are a lot more massive, they are perhaps as old as Saturn. If they are less massive they could be very young, less than a hundred million years. Other puzzles that might get some insights include: are moonlets like Peggy
still forming? What exactly caused the rings?

In short, there is a lot of science waiting to be revealed till its final fiery plunge at hundred thousand kilometres per hour on September 15.

(The author is with Indian Institute of Astrophysics, Bengaluru)