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Cassini’s end of mission

The international Cassini mission reaches its dramatic finale this Friday by plunging into Saturn’s atmosphere, concluding 13-years of exploration around the ringed planet.

End of Mission Timeline

On the final orbit, Cassini will plunge into Saturn fighting to keep its antenna pointed at Earth as it transmits its farewell.

In the skies of Saturn, the journey ends, as Cassini becomes part of the planet itself.

On Sept. 15, 2017, the Cassini spacecraft will make a fateful plunge into Saturn’s atmosphere, ending the mission just one month shy of its 20th launch anniversary.

Projected Times

Because Saturn is so far from Earth, Cassini will have been gone for about 83 minutes by the time its final signal reaches the Deep Space Network’s Canberra station in Australia on Sept. 15, 2017.

The current predicted time for loss of signal on Earth is 4:55 a.m. PDT (7:55 a.m. EDT) on Sept. 15, 2017.

This time may change as Saturn’s atmosphere slows Cassini during each of the final orbits.

Times are spacecraft event time, i.e., when the events happen at Saturn.

“ERT” refers to Earth received time, which is the time when the spacecraft’s signal relaying the event arrives on Earth.

After events happen at Saturn, it takes 83 minutes for Cassini’s radio signal to reach Earth.

All times are estimates and may change by a few minutes based on the density of Saturn’s atmosphere as encountered by the spacecraft in its final five orbits.

Sept 8

8:09 pm EDT (5:09 pm PDT) Final dive through the gap between Saturn and the rings (closest approach to Saturn is 1,044 miles, 1,680 kilometers above the cloud tops)

Sept 9

9:07 am EDT (6:07 am PDT)
Downlink of data from last Grand Finale dive begins
ERT 10:29 am EDT (7:29 am PDT)

Sept 11

3:04 pm EDT (12:04 pm PDT) Final, distant Titan flyby (aka, the “goodbye kiss”) closest approach (altitude 73,974 miles, 119,049 kilometers above Titan’s surface)

Sept 12

1:27 am EDT (10:27 pm PDT - Sept. 11) Apoapse, or farthest point from Saturn in the orbit (800,000 miles, 1.3 million kilometers from Saturn)
7:56 pm EDT (4:56 pm PDT) Downlink of final Titan data begins

ERT 9:19 pm EDT (6:19 pm PDT)

Sept 14

3:58 pm EDT (12:58 pm PDT) Scheduled time when the final image will be taken by Cassini’s cameras

4:22 pm EDT (1:22 pm PDT) Spacecraft turns antenna to Earth; communications pass begins for final playback from Cassini’s data recorder, including final images.

Communications link is continuous from now to end of mission (~14.5 hours) 5:45 pm EDT (2:45 pm PDT)

11:15 pm EDT (8:15 pm PDT)

Deep Space Network station in Canberra, Australia, takes over tracking Cassini to end of mission

Sept 15

1:08 am EDT (10:08 pm PDT - Sept. 14) High above Saturn, Cassini crosses the orbital distance of Enceladus for the last time
3:14 am EDT (12:14 am PDT)

Spacecraft begins a 5-minute roll to point instrument (INMS) that will sample Saturn’s atmosphere and reconfigures systems for real-time data transmission at 27 kilobits per second (3.4 kilobytes per second).

Final, real-time relay of data begins
ERT 4:37 am EDT 1:37 am PDT

3:22 am EDT (12:22 am PDT) High above Saturn, Cassini crosses the orbital distance of the F ring (outermost of the main rings) for the last time

6:31 am EDT (3:31 am PDT) Atmospheric entry begins; thrusters firing at 10% of capacity

ERT 7:54 am EDT (4:54 am PDT)

6:32 am EDT (3:32 am PDT) Thrusters at 100% of capacity; high-gain antenna begins to point away from Earth, leading to loss of signal

ERT 7:55 am EDT (4:55 am PDT)

Pinpointing a Moment

As Cassini heads for its Sept. 15 plunge into Saturn, the mission team will continue to update their predicted time for loss of signal. This is the predicted time during Cassini’s dive into Saturn when the spacecraft is expected to begin tumbling due to increasing atmospheric density, permanently severing the spacecraft’s radio link with Earth. At this point the spacecraft’s mission is over.

The predicted time for loss of signal changes because of effects from Saturn’s atmosphere on each of the spacecraft’s final five orbits. On these passes, Cassini dips briefly into Saturn’s upper atmosphere, which causes drag.

This drag alters Cassini’s velocity, which in turn affects when the spacecraft will reach Saturn’s atmosphere on the mission’s final day. More drag makes the spacecraft slow down in its orbit, which can move the end-of-mission time slightly earlier, by seconds or even minutes. The time could move forward slightly if the atmosphere turns out to be less dense than expected based on the previous passes.

Cassini’s flight team reviews the trajectory after each pass to see how the spacecraft’s course was affected by the atmosphere. They use the new information to update their prediction of how the remaining passes will further alter the trajectory, and from these predictions they generate an updated time for loss


Good to know:
• It takes about 83 minutes for radio signals to travel across the 1.4 billion km between Earth and Saturn.
• No images will be taken during the final plunge into Saturn, as the data transmission rate required to send images is too high and would prevent other high-value science data from being returned.
• The final images will be taken on 14 September and are planned to include images of Titan, Enceladus, moonlet ‘Peggy’, a propeller feature in the rings and a colour montage of Saturn and its rings, including the aurora at the north pole (examples of previously released images of these targets are shown here).
• Eight instruments (CDA, CIRS, INMS, MAG, MIMI, RPWS, RSS, UVIS) will collect data during the final plunge, transmitting it back to Earth in near-real time.
• The mission is ending because, after two decades in space, its fuel is running out. To ensure a safe disposal of the spacecraft, and to avoid an unplanned impact onto pristine icy satellites such as ocean-bearing moon Enceladus, Cassini is being directed into the gas planet itself, where it will burn up.
• Since April, Cassini has been making weekly dives through the 2000 km gap between Saturn and its rings. This ‘Grand Finale’ maximises the scientific return of the mission, giving close dives past the inner and outer edges of the rings and the planet’s small inner moons, as well as close encounters with the upper reaches of Saturn’s atmosphere.
• The 22 Grand Finale orbits were supported by ESA ground stations, which received signals from Cassini to gather crucial radio science and gravitational science data.
• A final distant flyby with Titan on 11 September gave the gravitational assist needed to put the spacecraft on an impact course with Saturn.

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