Cassini’s final orbit is underway. Yesterday, Cassini made its final (survivable) dip into Saturn’s atmosphere, completing its final periapse. Sixteen hours from now, it will encounter Titan for one last time. This will not be a close flyby, but it will be close enough to feel Titan’s gravity, which is the point of the exercise — Titan’s gravity will give the spacecraft a little nudge, transforming the next close approach into a kamikaze plunge. Here’s a brief timeline of Cassini’s last few days, from the mission website (times given in UTC, which is used for spacecraft time, and in PDT, since the mission is run out of JPL in California):
September 11, 12:04 PM PDT (19:04 UTC) – the “goodbye kiss”, as Cassini passes 119,049 km from Titan
September 11, 10:27 PM PDT (September 12, 05:27 UTC) – final apoapsis, 1.3 million km from Saturn
September 12, 4:56 PM PDT (23:56 UTC) – final Titan data downlink begins
September 12, 6:19 PM PDT (September 13, 01:19 UTC) – signal from Titan data downlink reaches Earth
September 14, 12:38 PM PDT (18:38 UTC) – Cassini takes its last picture
September 14, 1:22 PM PDT (20:22 UTC) – Cassini slews its main antenna to Earth to begin the final downlink of data from its recorder; this data stream will be continuous until the end of the mission
September 14, 2:45 PM PDT (21:45 UTC) – signal from the final transmission begins reaching Earth
September 14, 8:15 PM PDT (September 15, 03:15 UTC) – Canberra, one of the three Deep Space Network sites, begins receiving the signal; Canberra will be the last station to receive signals from Cassini
September 14, 10:08 PM PDT (September 15, 05:15 UTC) – Cassini passes the orbit of Enceladus for the last time
September 15, 12:14 AM PDT (07:15 UTC) – Cassini rolls to allow its instruments to allow its INMS instrument to sample particles. Spacecraft also configures itself for real-time data transmission; no more data will be stored in the recorder
September 15, 12:22 am PDT (07:22 UTC) – Cassini passes the distance of the F-ring for the last time
September 15, 1:37 AM PDT (08:37 UTC) – Cassini’s real-time data stream begins to reach Earth
September 15, 3:31 AM PDT (10:31 UTC) – Cassini begins to enter the atmosphere, with thrusters firing at 10% to maintain signal lock on Earth
September 15, 3:32 AM PDT (10:32 UTC) – Cassini will increase thruster firing to maintain lock until 100% capacity is reached; at that point, it will no longer be able to maintain the lock, and signal will be lost
September 15, 4:54 AM PDT (11:54 UTC) – Canberra receives the signals indicating that Cassini has reached the atmosphere and is firing its thrusters
September 15, 4:55 AM PDT (11:55 UTC) – Cassini’s signal stops being received; end of mission.
At this point, Cassini will have spent nearly 20 years in space, and conducted an impressive 293 orbits of Saturn. It’s one of the most impressive and capable space probes we’ve ever sent out into the cosmos. It’s bittersweet to think of its demise in this way, but nothing lasts forever, and at least Cassini is getting to go out with a bang, collecting some priceless data that could not be obtained any other way.
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