— Leigh Fletcher (@LeighFletcher) September 8, 2017
After almost twenty years in space, the Cassini spacecraft is now just seven days away from its final encounter with the giant planet, ending humankind’s first detailed exploration of the ringed planet. Cassini’s Grand Finale is the ambitious culmination of a mission by a nuclear-powered robotic explorer that has travelled a total of 4.9 billion miles, completing 294 orbits of Saturn, with 360 engine burns, 2.5 million commands executed and 635GB of science data collected. That science data, which includes half a million images, has been used to generate more than 4000 papers in scientific journals, and has started the careers of many young scientists, myself included. But to me, the most important credential that this mission can be proud of is its international nature – 27 nations, including the UK, were involved in this fantastic mission. Cassini-Huygens will truly be the benchmark against which all future missions are compared.
|Cassini’s vital statistics.|
The Grand Finale started on April 22nd with the 126th close flyby of Titan, which initiated a series of 22 close polar orbits around the giant planet. The first ring plane crossing, bringing Cassini between Saturn and its innermost rings, occurred on April 26th. The spacecraft was travelling at roughly 70,000 mph relative to the cloud tops. Thankfully, the dust that we suspected might be present in this unexplored region was absent, so that spacecraft constraints could be relaxed just a little. As Cassini continued to loop around Saturn once every 6 days, the final five orbits actually dipped the spacecraft down into the tenuous upper atmosphere, allowing the mass spectrometer (INMS) to directly sample the composition of the atmosphere. Here too, Cassini was lucky – although contingency plans had been in place should anything go wrong, no rocket firings to maintain attitude control were actually required, and the spacecraft emerged from these encounters unscathed.
The Final Week
And so we arrive at Cassini’s final week before its plunge into the Saturnian clouds on September 15th. On Saturday September 9th, at 01:09BST (subtract one from all my times to get them in UT), Cassini will execute its final dive between Saturn and the rings, skimming just 1680 km above the clouds. Then on Monday September 11th at 20:04BST, the final distant encounter with Titan (the 127th flyby at 120,000 km distance, known as the “goodbye kiss”) will slow the spacecraft sufficiently that Isaac Newton and the force of gravity will never allow Cassini to escape its final orbit.
On Wednesday and Thursday September 13th-14th, Cassini will be assembling its final picture show, including colour mosaics of Saturn and its rings, a movie of Enceladus setting behind the northern limb of Saturn, and observations of tiny moonlets within the ring system. Linda Spilker, Cassini’s project scientist, described these bittersweet images beautifully as “like taking one last look around your home before you move out.” The final image will be taken at 20:58BST on Thursday September 14th, before the spacecraft is reconfigured for atmospheric entry on Thursday night.
The Final Moments
|Instruments working during Cassini’s final moments at Saturn.|
On Friday September 15th, at 06:08BST, Cassini will cross the orbit of Enceladus one final time. Two hours later, the spacecraft will roll to ensure that the mass spectrometer is pointed in the direction of travel, able to access the oncoming gases during the plunge. Rather than storing science data onboard, Cassini will begin transmitting in real time (3.4 kbps) for the last 3.5 hours, with its high-gain antenna pointed directly at Earth. Eight of Cassini’s 12 instruments will be on and taking data – the mass spectrometer (INMS), magnetosphere and plasma science instruments, the radio science subsystem, and the infrared and ultraviolet spectrometers (CIRS included).
At 11:31BST the fight begins, as the spacecraft begins to enter the atmosphere at about 1190 miles (1920 km) above the cloud tops and needs to use its thruster to battle against the torques on the spacecraft. The thrusters will ramp up to 100% of their capacity in a matter of seconds as the probe falls through 250 miles (to around 940 miles above the clouds), where the torques are expected to overcome Cassini’s ability to correct its attitude. At this point, we expect Cassini to begin to tumble around several axes, such that the high gain antenna is no longer locked on Earth. The final photons will have been transmitted back to the Deep Space Network of radio telescopes on Earth at 11:32BST. Cassini will continue to fight, its fault-protection systems trying in vain to stabilise the spacecraft, but within seconds the high loads on the spacecraft will start to destroy structural components. The spacecraft will break apart, burning up like a meteor and melting, the individual materials dissociating so that the debris forever becomes a part of Saturn.
Meanwhile, those last photons from a ghost spacecraft will take 83 minutes to cross the 1.5 billion kilometres to Earth, where the final loss of signal is expected at around 12:55BST. At that moment, I expect the silence at JPL, and all the other laboratories that have been part of this grand mission, to be deafening.
At a Cassini CIRS team meeting in June, I learned that the instrument operations team had uplinked a very special sequence of four tables to the spacecraft, to be uploaded to the instrument memory during that final plunge. The four tables contained a message from all of the 282 scientists and engineers that had been involved in CIRS between 1990 and 2017. It’s an incredible privilege to be on this list, and to know that Cassini will be thinking of home as it completes its final journey!
|The names in Cassini/CIRS memory in the final seconds.|
Finally, Ralph Lorenz has a nice article on arxiv about whether we’ll be able to see this event from Earth https://arxiv.org/abs/1708.05036