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Juno and Cassini Grand Finale Science Comparison
NMRguy
post Jul 15 2017, 04:26 PM
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Cassini is a flagship-class (“Battlestar Galactica”) spacecraft with thermoelectric power and a robust suite of remote sensing instruments. It has excelled at its mission over the past thirteen years. Juno is a more modest design with its New Frontiers budget, but is outfitted with instruments highly optimized to investigate Jupiter’s internal composition though measurements of gravity field, magnetic field, and microwave/IR remote sensing on the atmosphere.

Given Cassini’s imminent end of mission (sigh) and its transfer into the proximal orbits as part of its Grand Finale, we are fortunate to have concurrent, close-pass investigation of the two Gas Giants.

My question for the group—has anyone done a detailed comparison of the instrument sensitivities and orbit geometries of the two missions to determine what data quality we should expect? Is Cassini expected to provide equivalent resolution data to provide similar confidence limits on the internal composition of Saturn as Juno will return for Jupiter? What should we expect for gravity science and magnetometer data between the two spacecraft?


Some comparisons based on publicly available information (there may be errors):

1. Radio Science Subsystems (RSS)

Cassini has a very large high-gain antenna (4m diameter) and was designed to be able to simultaneously transmit three radio frequencies (X, Ka, and S-bands). The current best estimate for RSS peak operating power is 80.7W.

Juno has large high-gain antenna (2.5m diameter) and is dual band (X and Ka).


2. Magnetometer (MAG)

Juno has two sets of MAG sensors (specifically, two fluxgate magnetometers plus four advanced start tracker cameras), positioned on a boom at the end of one of the solar arrays 33ft and 39ft from the center of the spacecraft.

Cassini has two sets of MAG sensors, including a vector/scalar magnetometer positioned on the end of the 36ft mag boom and a flux gate magnetometer on the boom at ~18ft away from the spacecraft. The status of the vector/scalar magnetometer is unclear.


3. Microwave Radiometer (MWR)

Juno has an microwave radiometer instruments that can measure microwave brightness temperatures of Jupiter with six passive microwave antennas that are sensitive to wavelengths between 1.3cm and 50cm (frequencies between 0.6 GHz and 22 GHz).

I am not aware of an equivalent capability on Cassini.


4. Auroral Mapping and Cloud Composition

Juno has the Jovian IR Auroral Mapper (JIRAM), an imaging spectrometer that is sensitive to heat radiation with wavelengths of two to five microns. It also carries the Ultraviolet Spectrograph, which images and speaker the spectrum in the 70 to 205 nanometer range.

Cassini has a robust suite of IR spectrometers. Composite Infrared Spectrometer (CIRS) is a 2D imaging spectrometer with three focal planes sensitive in the Far-Infrared (17 to 1000 micron) and Mid-Infrared (7 to 9 micron and 9 to 17 micron). The Visible and Infrared Mapping Spectrometer (VIMS) Infrared Channels is sensitive between 0.85 and 5.1 micron. Cassini also flies with the Ultraviolet Imaging Spectrograph, which is sensitive in the 55.8 to 190 nanometer range.


5. Visual Imagers

We are fortunate that Juno is flying with JunoCam, a limited wide angle four channel push broom imager. Images recently returned are exceeding expectations.

Cassini’s Imaging Science Subsystem and VIMS-Visible are the best cameras that 1980s money could buy and continue to knock it out of the park.


6. Orbit Geometry

Juno remains in the initial long duration capture orbit (53.5 days per orbit). Closest approach is 4,100 km from the cloud tops, while apojove is ~8 million kilometers from the planet.

Cassini is flying its proximal orbit with closest approach between Saturn and its rings (6.5 days per orbit). Closest approach is 3,800 km, while apoapse is at Titan’s orbit at around 1.2 million kilometers from the planet.


7. Spacecraft Geometry at Closest Approach

Juno has two spacecraft orientations during close pass, including “Radiometer Pass” and “Gravity Pass”. With the former, the spacecraft solar arrays are orthogonal to the surface of the planet, aligning the microwave radiometer antennas with the nadir, while the latter orients the spacecraft high-gain antenna to Earth. Some sources claim that lower quality gravity science can be accomplished during the Radiometer Passes (X-band only telemetry), but the highest quality gravity data should be expected from the Gravity Pass orientation (X and Ka-band). In the original plan, Juno was to accomplish 36 orbits, 5 of which were MWR and the remaining 31 to be Gravity Science.

Cassini science teams compete for orientation priority among the 22 final orbits. Six will be dedicated to Gravity Science; the remainder will focus on remote sensing.



Summary and Observations:

• Cassini and Juno are robustly equipped to provide high quality data to investigate the inner structure of the gas giants.
• Juno and Cassini proximal orbits are optimized for close pass investigation of the Gas Giants and their internal structures.
• Cassini is flying with state-of-the-art 1980s technology (well, at least technology that was space-proven by the late '80s), while Juno has the benefit of a couple decades of technological advances.
• Juno will provide five times the number of Gravity Science orbits that Cassini can accomplish, so scientists will have a larger and richer data set for Jupiter relative to Saturn.
• It is possible that Juno’s RF frequency generation is more stable than Cassini given technology advances, which may improve data quality. Thoughts?
• Juno’s dual magnetometer with star trackers should provide higher quality magnetic field data relative to Cassini, especially if there are issues with Cassini’s vector/scalar magnetometer.
• Cassini’s imaging and optical spectrometers can collect all of the same measurements that Juno will provide.


Are there any experts that can weigh in on data expectations? Thanks.


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mcaplinger
post Jul 15 2017, 04:57 PM
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I'm not sure what else you're looking for here, you've captured the differences in the instrument suites and capabilities pretty accurately. The goals are very different between the two missions, and the targets are also very different. As you say, Juno has more capability for gravity science because of the Ka-band and will also do many more close passes than Cassini; Cassini was never intended to do detailed interior studies of Saturn and anything it gets will be a bonus.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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NMRguy
post Jul 15 2017, 05:08 PM
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I'm just a former chemist. I find RF and MAG technologies a little confusing. Any insider thoughts welcome!
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mcaplinger
post Jul 15 2017, 08:07 PM
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Gravity science is a key part of the Juno mission and AFAIK not really a key part of the Cassini mission. Radiometric tracking is a complex subject and I don't claim to know that much about it. https://arxiv.org/abs/1411.1613 is a paper by Juno team members from ASI, which supplied the radio science Ka-Band Translator for Juno, so you might start there.

[added] Actually, it turns out that Cassini has an earlier version of the Ka-band translator, see http://lasp.colorado.edu/~horanyi/graduate_seminar/Radio.pdf
(which also has a description of the Cassini radio science goals.)


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Disclaimer: This post is based on public information only. Any opinions are my own.
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