It's been hard to bring myself to write the following sentence: the Cassini mission ends soon. We're halfway through the "Grand Finale" orbits. Only eleven and a half orbits remain until Cassini meets its fate on September 15. JPL is planning a big press event for the final days of the mission. In the meantime, here's a look at the great mission's final science orbits, mostly gleaned from the as-always-excellent Cassini website.
Right now the mission is in an orbit that circles Saturn about every six and a half days. That is a very short orbit for Cassini. It reminds me of one of those coin-funnel "wishing well" devices, where the coins circle faster and faster as they approach the drain. Cassini is now the one circling the drain.
Near periapsis, Cassini passes between the rings and the planet. The Cassini team has fine-tuned the ring-plane crossings to be at slightly different distances from rings and Saturn each time in order to allow the particle-tasting instruments to check out how the environment varies with distance from Saturn. The final five orbits, beginning on August 14, will actually take Cassini through the uppermost atmosphere.
Most of the science during this period focuses on small-scale structure in Saturn's rings, and detailed features in Saturn's polar aurorae. There is one remaining gravity science orbit, which should help determine the mass of the B ring once and for all. There will be observations of Titan's clouds, and a last look in infrared wavelengths at the warm south polar tiger stripes of Enceladus. And there will be a few final "Kodak Moments" -- images taken purely for their aesthetic and emotional significance. One that I am particularly looking forward to is an inside-out view of Saturn's rings, taken from Cassini's unique new perspective; another is a very distant, wistful image of Neptune and Triton.
NASA / JPL-Caltech / SSI / Emily Lakdawalla
Fine-scale waves in Saturn's rings
Cassini took this photo on 4 June 2017, close to periapsis on its seventh "Grand Finale" orbit, when it passed between the planet and the rings. It has been cleaned of cosmic ray hits and detector noise.
Below is a timeline of what's to come in the final two and a half months. Cassini has two main kinds of instruments: optical remote sensing (the ISS cameras, UVIS ultraviolet imager, VIMS visible and infrared spectrometer, and CIRS thermal infrared spectrometer), and fields and particles (magnetometer, RPWS radio and plasma wave science, and INMS ion and neutral mass spectrometer). Optical remote sensing instruments study things from afar, while fields and particles instruments study things in situ. The RADAR instrument sometimes plays like an optical remote sensing instrument, sending radio waves toward a distant target and watching for their reflections.
I am super sad that this mission is ending. But throughout the next 10 weeks I'm planning to repeatedly make the point that in a super productive 13-plus years of science operations at Saturn, Cassini has returned a tremendous quantity of data. I could start sharing an archival Cassini picture every day, and continue for the rest of my life, and odds are good you wouldn't have seen that particular one before. Don't be sad that Cassini is ending; be mad that there is no new outer planet flagship mission on its way to exploring a giant planet, like Uranus or Neptune.
Rev 282 Apoapsis
3 Jul 03:54
On this orbit, optical remote sensing instruments (UVIS, CIRS, and ISS) focus on ring structure. They will watch the C and A rings occult Kappa Canis Majoris, look at propellers, and look at the border region of the F ring and A ring.
Grand Finale ring plane crossing #12
6 Jul 09:31
Spacecraft will be shielded by its High Gain Antenna. RPWS will listen for impacts of ring particles. Cassini gets within 3,730 kilometers of Saturn’s 1-bar level. Cassini also passes within 3,980 kilometers of the inner edge of the D ring.
Rev 283 apoapsis
9 Jul 16:13
CIRS will study the lit side of the middle A-ring to determine its composition and structure. The CIRS instrument then targets the point where Saturn’s shadow falls across the B-ring, studying how the ring’s temperature changes as it crosses into Saturn’s shadow. ISS will look at propellers, the C ring, and inner B ring.
Grand Finale ring plane crossing #13
12 Jul 20:44
As Cassini approaches to within 50,000 km of Saturn's 1-bar level, INMS will sample the exosphere and ionosphere. Cassini gets within 2,860 kilometers of Saturn’s 1-bar level. Cassini also passes within 4,850 kilometers of the inner edge of the D ring
Rev 284 Apoapsis
16 Jul 02:21
During this orbit, Cassini’s Radio Science Subsystem (RSS) performs its last science observations of the mission. The instrument conducts radio occultations of Saturn’s ring system and runs a gravity experiment to characterize Saturn's gravitational field with unprecedented detail. RSS determines Saturn's gravity by very precisely tracking the orbit of Cassini relative to the Earth as the spacecraft skims Saturn’s atmosphere. By measuring the Doppler shift in Cassini’s radio signal, scientists can learn how mass is distributed within Saturn. In addition, Cassini simultaneously feels the gravitational pull of the rings (the B ring, in particular) and determines their mass very accurately. The mass helps scientists determine the age of the ring system.
Grand Finale ring plane crossing #14
19 Jul 07:50
When Cassini is in the segment of this orbit in which it’s nearest to Saturn, the spacecraft rolls so that the Magnetometer (MAG) can collect unique measurements that will lead to a better understanding of Saturn’s magnetic field, the planet’s rotation rate, the size of the central core and other characteristics of Saturn. Cassini gets within 2,790 kilometers of Saturn’s 1-bar level. Cassini also passes within 4,910 kilometers of the inner edge of Saturn’s D ring. Afterward, VIMS will look at the nighttime southern hemisphere.
Rev 285 Apoapsis
22 Jul 13:27
The optical remote sensing instruments will study Titan's clouds, and UVIS will look at Saturn's aurora; RPWS will listen for "whistlers" caused by Saturn lightning.
Grand Finale ring plane crossing #15
25 Jul 18:55
Cassini gets within 2,810 kilometers of Saturn’s 1-bar level. Cassini also passes within 4,890 kilometers of the inner edge of Saturn’s D ring.
Rev 286 Apoapsis
29 Jul 00:33
UVIS and VIMS will study Saturn's aurora, capturing some of the highest-resolution auroral images of the entire mission. CIRS will take the temperatures of Enceladus' south polar tiger stripes for the last time.
Grand Finale ring plane crossing #16
1 Aug 06:05
Cassini gets within 2,920 kilometers of Saturn’s 1-bar level. Cassini also passes within 4,790 kilometers of the inner edge of Saturn’s D ring.
Rev 287 Apoapsis
4 Aug 11:46
More auroral observations for UVIS, while ISS studies outer moon Kiviuq, ring propellers, and Titan.
Grand Finale ring plane crossing #17
7 Aug 17:19
Cassini gets within 2,940 kilometers of Saturn’s 1-bar level. Cassini also passes within 4,760 kilometers of the inner edge of the D ring.
Rev 288 Apoapsis
10 Aug 22:56
CIRS studies Saturn's atmosphere; ISS studies "streaks" in the C ring. ISS will image Neptune and Triton on August 10 and 16.
"Final Five" Periapsis dip #1
14 Aug 04:22
This is the first of five orbits in which Cassini’s elliptical orbit carries it so low that the spacecraft passes briefly through Saturn’s outermost atmosphere. Cassini’s reaction control thrusters are at the ready to correct the spacecraft’s orientation in case Saturn’s atmosphere pushes on the spacecraft hard enough to cause any rotation. INMS performs the first ever direct sampling of Saturn’s atmosphere. The instrument measures densities of different species of molecular hydrogen, helium and a variety of ions in the immediate vicinity of the spacecraft. Cassini’s RADAR instrument operates at the same time, studying the small-scale structure and ammonia concentration of Saturn’s atmosphere. Cassini gets within 1,710 kilometers of Saturn’s 1-bar level. Cassini also passes within 5,990 kilometers of the inner edge of the D ring.
Rev 289 Apoapsis
17 Aug 09:53
More UVIS aurora; CIRS studies the temperature of the wintertime south polar vortex; VIMS creates a mosaic of the aurora.
"Final Five" Periapsis dip #2
20 Aug 15:23
ISS will get an "inside-out" view of the rings. Cassini gets within 1,660 kilometers of Saturn’s 1-bar level. Cassini also passes within 6,040 kilometers of the inner edge of Saturn’s D ring.
Rev 290 Apoapsis
23 Aug 20:51
CIRS studies Saturn's northern hemisphere upper troposphere; VIMS maps the equatorial region; CIRS performs its final Saturn limb observation; RADAR maps the atmosphere.
"Final Five" Periapsis dip #3
27 Aug 02:20
This is the deepest-dipping of the "final five" orbits. INMS performs its second session directly sampling Saturn’s upper atmosphere. The instrument measures densities of different species of molecular hydrogen, helium and a variety of ions in the immediate vicinity of the spacecraft. Cassini gets within 1,630 kilometers of Saturn’s 1-bar level. Cassini also passes within 6,070 kilometers of the inner edge of the D ring.
Rev 291 Apoapsis
30 Aug 07:48
ISS observes haze in Titan’s atmosphere and the distant outer moon Thyrmr, and UVIS and VIMS observe Saturn’s sunlit north polar auroral region. VIMS and CIRS work together to study Saturn’s atmosphere.
"Final Five" Periapsis dip #4
2 Sep 13:17
INMS directly samples Saturn’s upper atmosphere. Cassini gets within 1,640 kilometers of Saturn’s 1-bar level. Cassini also passes within 6,080 kilometers of the inner edge of the D ring.
Rev 292 Apoapsis
5 Sep 18:47
The last full orbit of Saturn. During this orbit, CIRS and VIMS work together to determine the abundance of helium in Saturn’s atmosphere.
"Final Five" Periapsis dip #5
9 Sep 00:18
INMS directly samples Saturn’s upper atmosphere. RADAR and ISS operate at the same time; RADAR is mapping ammonia, while ISS will capture an iconic image of the rings looking outward from Saturn. Downlink is expected at 07:29 PDT.
12 Sep 05:37
During this partial orbit, ISS will take some final "Kodak moment" images, keepsakes to remember Saturn by. When Cassini is three and half hours from its expected end of mission, CIRS, UVIS, and magnetospheric and plasma science instruments are transmitted to Earth in nearly real time, just seconds after each observation is made. Cassini usually holds onto those data for hours or days before turning its high-gain antenna toward Earth to transmit them.
15 Sep 10:44
Thrusters will maintain attitude control for about a minute. Earth Recieved Time (ERT) of the final data transmission is about 5:07 a.m. PDT.
Loss of signal
15 Sep 10:45
Estimated Earth Received Time (ERT) for the end of mission is 5:08 a.m. PDT on September 15.
Thanks to Jason Perry for his notes on imaging plans.