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Inspired by the comment by Nick S on this answer by Organic Marble, what made Cassini require so much power?

An excerpt from the aforementioned answer:

The flight units used by mission, with power levels at launch, were:

  • Galileo: Flight Units 1 (289 We) and 4 (288 We)

  • Ulysses: Flight Unit 3 (289 We)

  • Cassini: Flight Units 2 (296 We), 6 (294 We) and 7 (298 We)

  • New Horizons: Flight Unit 8 (245.7 We at bus instead of connector pins)

The listed power levels are electrical power levels in watts (We),

Sources

So Cassini (1997) had 1.5x the power budget of Galileo (1989), 3x that of Ulysses(1990), and a bit over 3x that of New Horizons (2006).

What made it have such a high power budget? Or did it have a similar power budget to other probes, and just was unable to use solar panels? I see here that NASA decided that they couldn't use solar panels, but it doesn't appear that the other missions use solar panels either. And it also doesn't seem like it's a matter of age, since there were much smaller power budgets for probes that came both before and after.

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2 Answers 2

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Cassini had a RADAR subsystem

  • To determine whether oceans exist on Titan, and, if so, to determine their distribution.
  • To investigate the geologic features and topography of the solid surface of Titan.

The primary goal of the RADAR was to pierce Titan's thick veil of smog and aerosols to map its surface.

The paper found by PearsonArtPhoto has a list of the instruments:

Cassini Radar (RADAR)                          108.4 W
Radio-Science (RSS)                             82.3
Imaging Science (Subsystem (ISS))               59.3
Composite Infrared Spectrometer (CIRS)          43.3
Ion and Neutral Mass Spectrometer (INMS)        26.6
Visible and Infrared Mapper Spectrometer (VIMS) 24.6
Magnetospheric Imaging Instrument (MIMI)        23.4
Cosmic Dust Analyser (CDA)                      19.3
Cassini Plasma Spectrometer (CAPS)              19.2
Radio and Plasma Wave Science (RPWS)            17.5
Ultraviolet Imaging Spectrometer (UVIS)         14.6
Dual Technique Magnetometer (MAG)               12.4
Sum                                            450.9 W

There were other systems of the space craft needing power, including the transmitter and receiver for communication, and the control computer.

There were 885 watts (633 watts at end of mission) from the RTGs, but simultaneous operation of all 12 instruments was not possible. So one of the three RTG was needed due to the very long mission time of nearly 20 years.

See:
https://pds-imaging.jpl.nasa.gov/documentation/RADARUsersGuide2ndEdV2.pdf
and
https://pds-imaging.jpl.nasa.gov/data/cassini/cassini_orbiter/CORADR_0284/DOCUMENT/BODPSIS.PDF

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  • $\begingroup$ To clarify: when you say “So one of the three RTG was needed due to the very long mission time of nearly 20 years.” Are you saying that 2 RTGs were needed for the usual power draw, and the third was required due to the lowered generation towards the end of the mission? $\endgroup$
    – fyrepenguin
    Commented Oct 28, 2021 at 23:36
  • $\begingroup$ Also: “but simultaneous operation of all 12 instruments was not possible.” Was this not something limited by the power budget, but rather some other constraint? $\endgroup$
    – fyrepenguin
    Commented Oct 28, 2021 at 23:37
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    $\begingroup$ @fyrepenguin Simultaneous operation of all capabilities is generally not possible because it would become too costly to power it. That's not just in space (where power is always at a premium) but still applies back on the ground. Don't try to power your washer, dryer, dishwasher, oven, heater, TV, Hi-fi set, PC and electric leaf blower at the same time. Not a single house or factory is calculated on 100% load. Instead, a simultaneity (concurrency) factor is used. That's how you can have a total load of 450W on a generator providing less than 300W. Makes the generator etc. a lot cheaper. $\endgroup$
    – Mast
    Commented Oct 29, 2021 at 7:29
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    $\begingroup$ @Mast but they had three generators each providing ~300W, which is where I was confused regarding why they couldn’t lower them. I’d understand if Cassini only had one RTG, or there were other unlisted power consumers in excess of 150W. Was mostly curious because saying a budget of 600W-900W, but not being able to run everything even though they total to only 450W was not quite clicking. $\endgroup$
    – fyrepenguin
    Commented Oct 29, 2021 at 15:48
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    $\begingroup$ @fyrepenguin The 450W is just the science instruments. Bus services (communication, attitude control, power conditioning, data storage, etc.) aren't included. Together, these services typically require power comparable to the instruments they support. $\endgroup$
    – John Doty
    Commented Oct 29, 2021 at 18:49
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The power budget, at least what is public, can be seen at https://sci.esa.int/documents/33648/36003/1567254416901-cassini-huygens.pdf . The scientific instruments included are up to 450.9 W, as shown on page 31. That doesn't include the supporting equipment. The highest of these was used by the radar system, at over 100 W. I'm fairly sure the bus power was higher than that. In order to run all of the instruments it would require 3 RTGs as a result. Uwe points out that at the end of 20 years the electrical power supplied from the RTGs was 633 W, so it seems like a reasonable margin.

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    $\begingroup$ In addition to the instrumentation, Cassini needed a good amount of power just to keep that large vehicle sufficiently warm. Cassini also needed a good amount of power to transfer data from the vehicle to Earth. Ulysses and Galileo were much smaller vehicles and had much lower data transfer rates. $\endgroup$
    – David Hammen
    Commented Oct 27, 2021 at 20:33
  • $\begingroup$ Usually with an RTG heat isn't a problem. Data transfer is, of course, but may not happen during data gathering times. $\endgroup$
    – PearsonArtPhoto ♦
    Commented Oct 27, 2021 at 21:01
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    $\begingroup$ Perhaps I wasn't clear. In addition to providing electrical power, another purpose of an RTG is to provide heating. Cassini not only had three RTGs, it had a bunch of strategically places pencil eraser sized blobs of plutonium whose sole purpose was to provide heat. Cassini was big. $\endgroup$
    – David Hammen
    Commented Oct 28, 2021 at 1:09

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