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Cassiniretirement

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Artist's concept ofCassini'scontrolled atmospheric entry into Saturn

TheCassinispace probewasdeliberately disposedof via a controlled fall intoSaturn's atmosphere on September 15, 2017, ending its nearly two-decade-longmission.[1][2][3][4]This method was chosen toprevent biological contaminationof any of the moons of Saturn now thought to offerpotentially habitableenvironments.[5]Factors that influenced the mission end method included the amount of rocket fuel left, the health of the spacecraft, and funding for operations on Earth.[6]

Some possibilities forCassini'slater stages were aerobraking into orbit around Titan,[6]leaving the Saturn system,[6]or making close approaches and/or changing its orbit.[6]For example, it could have collectedsolar winddata in aheliocentric orbit.[3]

End of mission options[edit]

Color Meaning[3]
Red Poor
Orange Fair
Yellow Good
Green Excellent

During planning for its extended missions, various future plans forCassiniwere evaluated on the basis of scientific value, cost, and time.[3][7]Some of the options examined included collision with Saturn atmosphere, an icy satellite, or rings; another was departure fromSaturnorbit toJupiter,Uranus,Neptune,or acentaur.[3][8]Other options included leaving it in certain stable orbits around Saturn, or departure to a heliocentric orbit.[3]Each plan required certain amounts of time and changes in velocity.[3]Another possibility wasaerobrakinginto orbit around Titan.[6]

This table is based on page 19 ofCassini Extended Missions(NASA), from 2008.[3]

Cassiniend of mission options with science evaluation circa 2008[3]
Type Option Set-up requirements Execution time Operability +
assurance ofEOL 		Velocity change
(Δv) required 		Science evaluation ca. 2008
Impact
with... 		Saturn's surface:
short-period orbits 		High inclination achievable via
any XXM design 		2–10 months total Short time between last encounter and impact 005–30 m/s D-ring option satisfies unachieved original mission goals;
cheap and easily achievable
Saturn's surface:
long-period orbits 		Specific orientation and inclination required 4–22 months to set up long period orbit +
3 years for final orbit 		3 years between last encounter and impact 005–35 m/s Operations costs required for 3 years
with no science could be applied elsewhere
icy satellite Can be implemented from any geometry 0.5–3 months total Short time between last encounter and impact 005–15 m/s Cheap and achievable anywhere/time
main rings Can be implemented from any geometry 0.5–3 months total Short time between last encounter and impact
but difficult to prove spacecraft destruction 		005–15 m/s Cheap and achievable anywhere/time;
close-in science before impact
Escape
to... 		giant planet Specific orbit period, orientation and
inclination required + specific departure dates 		1.4–2.4 years to escape + long transfer time:
Jupiter12,Uranus20,Neptune40 years 		Planetary impact can only be guaranteed
shortly after escape for Jupiter 		005–35 m/s Giant-planet science unlikely
heliocentric orbit Can be implemented from any geometry 9–18 months to escape, open-ended Solar orbit Last encounter goes to escape 005–30 m/s Solar wind data only
Centaur Large target set offers wide range
of departures 		1–2 years to escape + 3+ year transfer Last encounter goes to escape; must maintain
teams for 3+ years for centaur science 		005–30 m/s Multi-year lifetime and funding seems better spent in target-rich Saturnian environment
Stable
orbit
outside... 		Titan Specific orientation and orbit period required 13–24 months + open-ended time in stable orbit 200 days between last encounter and final orbit 050 m/s Limited Saturn / magnetospheric science, but for long period of time
Phoebe Specific orientation and orbit period required 8+ years + open-ended time in stable orbit Many months between last encounter and final orbit 120 m/s Solar winddata; very rare passages through magnetotail

Atmospheric entry and destruction[edit]

Animation of Cassini's Grand Finale
Cassini·Saturn
Last image taken byCassini,at 19:59 UTC (14 September 2017).

On July 4, 2014, theCassiniscience team announced that the proximal orbits of the probe would be named the "Grand Finale".[9]This would be immediately preceded by a gradual shift in inclination to better viewSaturn's polar hexagon,and a flyby ofEnceladusto more closely study itscryovolcanism.[10]This was followed by a dive into Saturn's atmosphere.[9]

There was budgetary drama in 2013–14 about NASA receiving U.S. government funding for the Grand Finale. The two phases of the Grand Finale ended up being the equivalent of having two separateDiscovery Program-class missions in that the Grand Finale was completely different from the mainCassiniregular mission. The U.S. government in late 2014 approved the Grand Finale at the cost of $200 million. This was far cheaper than building two new probes in separate Discovery-class missions.[11]

Scientific data was collected using eight of its twelve science instruments. All of the probe's magnetosphere and plasma science instruments, plus the spacecraft's radio science system, and its infrared and ultraviolet spectrometers collected data during the final plunge. The data rates flowing back from Saturn could not support imaging during the final plunge, so all pictures were downlinked (transmitted back to Earth) and cameras were switched off, before the final plunge began.[12]The predicted altitude for loss of signal was approximately 1,500 km (930 mi) above Saturn's cloud tops, when the spacecraft began to tumble and burn up like a meteor.[13]

Cassini's final transmissions were received by theCanberra Deep Space Communication Complex,located inAustraliaat 18:55:46AEST.In a bittersweet ending for the scientists involved, some of whom had been involved in the mission for decades,[14][15]data was received for 30 seconds longer than anticipated, and the spacecraft's ultimate demise was predicted to have occurred within 45 seconds after that.[16]Homages were paid on social media.[17]NASA's video won anEmmyfor "Outstanding Original Interactive Program".[18]

Cassiniimpact site onSaturn(visual/IRmapping spectrometer; September 15, 2017)

See also[edit]

References[edit]

  1. ^Brown, Dwayne; Cantillo, Laurie; Dyches, Preston (September 15, 2017)."NASA's Cassini Spacecraft Ends Its Historic Exploration of Saturn".NASA.RetrievedSeptember 15,2017.
  2. ^Chang, Kenneth (September 14, 2017)."Cassini Vanishes Into Saturn, Its Mission Celebrated and Mourned".The New York Times.RetrievedSeptember 15,2017.
  3. ^abcdefghiSpilker, Linda (April 1, 2008)."Cassini Extended Missions"(PDF).Lunar and Planetary Institute.Archived(PDF)from the original on April 23, 2008.
  4. ^Mason, Betsy (February 3, 2010)."Cassini Gets Life Extension to Explore Saturn Until 2017".Wired.Archivedfrom the original on February 9, 2010.
  5. ^Blabber, Phillipa; Verrecchia, Angélique (April 3, 2014)."Cassini-Huygens: Preventing Biological Contamination".Space Safety Magazine.RetrievedAugust 1,2015.
  6. ^abcdeBailey, Frederick; Rabstejnek, Paul."Cassini Mission and Results".Oglethorpe University. Archived fromthe originalon February 19, 2008.RetrievedDecember 4,2013.
  7. ^Pappalardo, Bob; Spilker, Linda (March 15, 2009)."Cassini Proposed Extended-Extended Mission (XXM)"(PDF).Lunar and Planetary Institute.Archived(PDF)from the original on July 18, 2012.
  8. ^Okutsu, Masataka, et al. "Cassini End-of-Life Escape Trajectories to the Outer Planets (AAS 07-258)." ADVANCES IN THE ASTRONAUTICAL SCIENCES 129.1 (2008): 117.
  9. ^ab"The Grand Finale Toolkit".Jet Propulsion Laboratory.RetrievedSeptember 19,2017.
  10. ^"Cassini Names Final Mission Phase Its 'Grand Finale'".Astrobiology Magazine.July 4, 2014. Archived from the original on August 22, 2019.RetrievedAugust 1,2015.{{cite news}}:CS1 maint: unfit URL (link)
  11. ^"Cassini's awesomeness fully funded through mission's dramatic end in…".planetary.org.RetrievedDecember 2,2020.
  12. ^Cassini conducts last picture show.Jonathan Amos.BBC News.14 September 2017.
  13. ^Cassini Spacecraft Makes Its Final Approach to Saturn.JPL, NASA News. September 13, 2017.
  14. ^Branson-Potts, Hailey (September 15, 2017)."Cassini scientists celebrate the mission's end with a few hundred of their closest friends".The Los Angeles Times.RetrievedSeptember 16,2017.
  15. ^Chang, Kenneth (September 15, 2017)."Cassini Vanishes Into Saturn, Its Mission Celebrated and Mourned".The New York Times.RetrievedSeptember 16,2017.
  16. ^Netburn, Deborah (September 15, 2017)."As NASA's Cassini mission flames out over Saturn, scientists mark bittersweet end of mission".The Los Angeles Times.RetrievedSeptember 16,2017.
  17. ^Kaplan, Karen (September 15, 2017)."Cassini plunges to its death at Saturn, and humanity says goodbye".The Los Angeles Times.RetrievedSeptember 16,2017.
  18. ^"Nominees/Winners | Television Academy".Academy of Television Arts & Sciences.RetrievedAugust 14,2019.

Further reading[edit]

  • Ralph Lorenz (2018).NASA/ESA/ASI Cassini-Huygens: 1997 onwards (Cassini orbiter, Huygens probe and future exploration concepts) (Owners' Workshop Manual).Haynes Manuals, UK.ISBN978-1-78521-111-9.

External links[edit]

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