APOD: Dione, Rings, Shadows, Saturn (2015 Aug 24)

[APOD Robot]

Dione, Rings, Shadows, Saturn

Explanation: What's happening in this strange juxtaposition of moon and planet? First and foremost, Saturn's moon Dione was captured here in a dramatic panorama by the robotic Cassini spacecraft currently orbiting the giant planet. The bright and cratered moon itself spans about 1100-km, with the large multi-ringed crater Evander visible on the lower right. Since the rings of Saturn are seen here nearly edge-on, they are directly visible only as a thin horizontal line that passes behind Dione. Arcing across the bottom of the image, however, are shadows of Saturn's rings, showing some of the rich texture that could not be seen directly. In the background, few cloud features are visible on Saturn. The featured image was taken during the last planned flyby of Dione by Cassini, as the spacecraft is scheduled to dive into Saturn's atmosphere during 2017.

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[hoohaw]

Why dive into Saturn? Why not, instead, synchronize with the rings and slowly approach to try to get a tight close-up before hitting something? Does anyone know why? Imagine a close-up picture of a chunk of a ring!

[NGC3314]

Cassini does not have enough fuel for such dramatic maneuvers - most of the velocity change for its endgame mission plan comes from carefully timed encounters with Titan. One of them has to be close enough to change its closest orbital point from outside to inside the main rings in one jump. The giant planets' gravity wells are so deep that traveling between their moons' orbits is energetically comparable to going between the inner planets (especially for Jupiter, which makes Europa probes such huge technological challenges).

[Brittanie]

I think it is interesting to see real images from NASA compared to the images we were shown as children. I will be interested to see the new images from the Cassini mission in 2017.

[Chris Peterson]

Why dive into Saturn? Why not, instead, synchronize with the rings and slowly approach to try to get a tight close-up before hitting something? Does anyone know why? Imagine a close-up picture of a chunk of a ring!

NGC3314 has given you the technical reasoning. There is also the mission plan, which calls for ending things in a way that doesn't leave a piece of scrap in the Saturnian system which will eventually collide with- and contaminate- something there. It does our future research goals no good to leave manmade, Earth-sourced debris around Saturn.

[neufer]

Why dive into Saturn? Why not, instead, synchronize with the rings and slowly approach to try to get a tight close-up before hitting something? Does anyone know why? Imagine a close-up picture of a chunk of a ring!

Cassini does not have enough fuel for such dramatic maneuvers - most of the velocity change for its endgame mission plan comes from carefully timed encounters with Titan. One of them has to be close enough to change its closest orbital point from outside to inside the main rings in one jump.

The giant planets' gravity wells are so deep that traveling between their moons' orbits is energetically comparable to going between the inner planets (especially for Jupiter, which makes Europa probes such huge technological challenges).

• The giant planets' gravity wells are so deep that
  traveling between their inner moons' orbits
  is energetically comparable to going between asteroids:
  
  • Europa average orbital speed: 13.740 km/s
    Mimas average orbital speed: 14.28 km/s
    Ceres average orbital speed: 17.91 km/s

<<The planned end of mission for Cassini is disposal by a controlled fall into Saturn's atmosphere in 2017. This method was chosen because it is imperative to ensure protection and prevent biological contamination to any of the moons of Saturn now thought to offer potential habitability environments. Some possibilities for Cassini's later stages were aerobraking into orbit around Titan, leaving the Saturn system, or making close approaches and/or changing its orbit. For example, it could collect solar wind data in a heliocentric orbit.>>

[Curious Jay]

Will Cassini be able to relay scientific data as it takes its swan dive or will it mostly just be lost in interference?

[Asterhole]

What's truly astounding is that the Cassini orbiter reached the Saturnian system in 2004. Eleven years later it is still blowing us away with stunning images of this strange and beautiful realm... and we still got another full year or more to go. Sending this trusty little probe to its fiery death may seem anticliamatic, but I'd like to think it'd go down a in blaze of glory with cameras, sensors and transmitters full-on sending us one last blast of data before the end. Go Cassini, go!

[neufer]

Will Cassini be able to relay scientific data as it takes its swan dive or will it mostly just be lost in interference?

Not really. It would require some special probe to enter first:

The Galileo spacecraft had just a special atmospheric-entry probe that directly entered and returned data:

[b][color=#0000FF]The Galileo Probe[/color][/b]

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<<The Galileo Probe was an atmospheric-entry probe carried by the main Galileo spacecraft to Jupiter, where it directly entered and returned data from the planet. The 339-kilogram measured about 1.3 meters across. Inside the probe's heat shield, the scientific instruments were protected from extreme heat and pressure during its high-speed journey into the Jovian atmosphere, travelling at 47.8 kilometers per second.

The probe was released from the main spacecraft in July 1995, five months before reaching Jupiter, and entered Jupiter's atmosphere with no braking beforehand. The probe was slowed from its arrival speed of about 47 kilometers per second to subsonic speed in less than two minutes.

At the time, this was by far the most difficult atmospheric entry ever attempted; the probe had to withstand 230 g and the probe's 152 kg heat shield, making up almost half of the probe's total mass, lost 80 kg during the entry. NASA built a special laboratory, the Giant Planet Facility, to simulate the heat load, which was similar to the convective heating experienced by an ICBM warhead reentering the atmosphere combined with the radiative heating of a thermonuclear fireball. It then deployed its 2.5-meter parachute, and dropped its heat shield, which fell into Jupiter's interior.

As the probe descended through 156 kilometres of the top layers of the Jovian atmosphere, it collected 58 minutes of data on the local weather. It only stopped transmitting when the ambient pressure exceeded 23 atmospheres and the temperature reached 153 °C. The data was sent to the spacecraft overhead, then transmitted back to Earth. All the probe's electronics were powered by lithium sulfur dioxide (LiSO2) batteries that provided a nominal power output of about 580 watts with an estimated capacity of about 21 ampere-hours on arrival at Jupiter.>>

Stuart Robbins

<<There are two main science goals for the Cassini death dive into Saturn that I know of. One of them is simply higher scale imagery of objects closer to Saturn as it journeys inwards. The other is that it will fly between the planet and the rings, and based upon the radio signals coming back to Earth, we should be able to, for the first time, directly measure the mass of the rings to within around 10¹⁹ kg. This may seem pretty coarse, but there are several specific prediction papers (including one of mine ...) that estimates the ring mass in multiples of the moon Mimas, which has a mass of 3.8⋅10¹⁹ kg.

A third that I thought of a few hours after writing this is that the spectrometers on board should be able to sample the saturnian atmosphere during entry and send back detailed composition information, just as it did when flying through the plumes of Enceladus.>>

Sept. 9, 2017 292 Distant flyby (altitude = 116,000 km; 72,000 mi) of moon Daphnis

Sept. 9, 2017 292 Distant flyby (altitude = 110,000 km; 68,000 mi) of moon Atlas

Sept. 9, 2017 292 Cassini passage through ring plane [North to South] (60,000 km; 40,000 mi wrt Saturn)

Sept. 9, 2017 292 Closest distance (62,000 km; 39,000 mi) to Saturn on rev number 292

Sept. 9, 2017 292 Distant flyby (altitude = 115,000 km; 72,000 mi) of moon Prometheus

Sept. 9, 2017 292 Cassini views the Earth as it passes behind Saturn's Rings

Sept. 11, 2017 292 Cassini passage through ring plane [South to North] (1,160,000 km; 720,000 mi wrt Saturn)

Sept. 11, 2017 292 Distant flyby (altitude = 84,000 km; 52,000 mi) of moon Titan

Sept. 12, 2017 293 Farthest distance (1,300,000 km; 800,000 mi) from Saturn. Start rev number 293.

Sept. 15, 2017 293 Distant flyby (altitude = 111,000 km; 69,000 mi) of moon Janus

Sept. 15, 2017 293 Distant flyby (altitude = 91,000 km; 57,000 mi) of moon Pan

Sept. 15, 2017 293 Distant flyby (altitude = 86,000 km; 53,000 mi) of moon Pandora

Sept. 15, 2017 293 Distant flyby (altitude = 92,000 km; 57,000 mi) of moon Epimetheus

Sept. 15, 2017 293 End of mission, atmospheric entry into Saturn

[Boomer12k]

Hoping to see many more interesting images before the end....thanks Cassini...God Speed...


:---[===] *

[MarkBour]

It would be interesting to see a fuller discussion of why dropping Cassini into Saturn's atmosphere (and dropping Galileo into Jupiter's) would be viewed as less contaminating than the alternatives.

[neufer]

It would be interesting to see a fuller discussion of why dropping Cassini into Saturn's atmosphere (and dropping Galileo into Jupiter's) would be viewed as less contaminating than the alternatives.

A crash landing on an icy moon would be relatively slow
and quite possibly leave a lot of cold preserved organics.

A crash landing on Saturn would be fast and totally incinerate all the organics.

[MarkBour]

Thanks for the reply. So, more than anything else, this is about microbes that may have hitchhiked on Cassini successfully to Saturn, correct? And as the spacecraft enters the atmosphere, it will not have any chunks separate from the main craft and slow down gently before they reach a high enough temperature to fry all bacteria? I'm guessing this kind of thing was discussed in great detail by NASA mission planners. I wonder if there is a way to find out more about it ... e.g. whether or not any simulations were done, or at least what kind of discussion may have occurred in the planning sessions.

The way we'll find out we were wrong about it would be when we notice that Saturn has grown a large red spot 20 years from now ...

[Chris Peterson]

Thanks for the reply. So, more than anything else, this is about microbes that may have hitchhiked on Cassini successfully to Saturn, correct?

No, that's just one concern. Also, not to have a cloud of debris orbiting Saturn creating a risk for future missions. And not having the debris contaminate (non-biologically) bodies in the system. That's not a concern for Saturn, which will certainly not be measurably contaminated in the way the surface of a moon could be.

[DavidLeodis]

In the explanation it states "In the background, few cloud features are visible on Saturn". That gives the impression that the clouds are clearly visible but I cannot definitely see any. I wonder therefore how obvious the clouds are to others and if there had been no mention in the explanation would they have readily seen them?

[geckzilla]

In the explanation it states "In the background, few cloud features are visible on Saturn". That gives the impression that the clouds are clearly visible but I cannot definitely see any. I wonder therefore how obvious the clouds are to others and if there had been no mention in the explanation would they have readily seen them?

There are some splotchy patches in the lower right corner.

[neufer]

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Thanks for the reply.

So, more than anything else, this is about microbes that may have hitchhiked on Cassini successfully to Saturn, correct?

That is what is claimed.

The fact that it brings a finality to the mission on the day after the anniversary of Giovanni Cassini's death (14 September 1712) may have something to do with it.

A perfect time for a big conference to point to the mission's successes!

[rdsman1@hotmail.com]

The shadow of the rings appears to be arcing in the wrong direction. Shouldn't they arc the other direction? Can you elaborate any on this?

[neufer]

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The shadow of the rings appears to be arcing in the wrong direction. Shouldn't they arc the other direction? Can you elaborate any on this?

Sun angle below = Ring shadow slice curving away above.

Sun angle above = Ring shadow slice curving away below.
. http://apod.nasa.gov/apod/ap120703.html