According to the BBC's Nasa confirms ocean moon mission:

Europa's orbital path takes it deep into belts of intense radiation that surround the giant planet. This radiation fries spacecraft electronics, which limits the durations of prospective missions to months or even weeks.

So rather than orbiting Europa, Clipper will make repeated close flybys of the moon, to reduce its exposure to the energetic particles trapped by Jupiter's magnetic field.

If Clipper orbited Europa then every moment that it was in the high radiation environment it would also be taking data on Europa.

But since Jupiter's radiation field is much, much bigger than Europa, each hour-long (or less) flyby would involve an orbit around Jupiter and probably would involve tens of hours of radiation exposure during it's perijove.

Question: How does not orbiting Europa maximize Clippers coverage of Europa for a given radiation dose? This seems backwards to me.

Related items involving spacecraft passing through Jupiter's radiation field:

enter image description here

above: "Animation of Jupiter's Synchrotron Radiation Torus. Credit: NASA/JPL - Caltech". From the Vatican Observatory Blog


1 Answer 1


It seems to be a duty cycle thing:

Because Europa lies well within the harsh radiation fields surrounding Jupiter, even a radiation-hardened spacecraft in near orbit would be functional for just a few months. Most instruments can gather data far faster than the communications system can transmit it to Earth because there are a limited number of antennas available on Earth to receive the scientific data. Therefore, another key limiting factor on science for a Europa orbiter is the time available to return data to Earth. In contrast, the amount of time during which the instruments can make close-up observations is less important.

(From Wikipedia quoting a Planetary Society article)

Flybys intermixed with time spent sending back data maximizes the data sent back because you can send for longer.

The article provides a numerical example:

Studies by scientists from the Jet Propulsion Laboratory show that by performing several flybys with many months to return data, the Europa Clipper concept will enable a \$2B mission to conduct the most crucial measurements of the cancelled \$4.3B Jupiter Europa Orbiter concept. Between each of the flybys, the spacecraft will have seven to ten days to transmit data stored during each brief encounter. That will let the spacecraft have up to a year of time to transmit its data compared to just 30 days for an orbiter.

I can only speculate why a “sit until your electronics is not quite dead, then fly out to a safe distance and transmit” isn’t used, but I suspect the “not quite” part is nerve wracking: guess wrong and you lose the bulk of the mission data. The flybys are being done via gravity assists from other moons, so it’s also possible this takes less fuel than going into and out of orbit.

  • $\begingroup$ Added the example from the article as a quote. $\endgroup$ Aug 21, 2019 at 5:34
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    $\begingroup$ Speculatively, gathering data over a a longer total timespan might have scientific advantages (spotting variabiity or trends). $\endgroup$ Aug 21, 2019 at 6:39
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    $\begingroup$ I think the "sit until...." idea doesn't work because of orbital mechanics. Getting into Europa orbit (compared to a Europa-grazing Jupiter orbit) takes fuel, and getting out again takes more fuel. Also, the proposed mission lets them adjust the observing plans for later orbits based on the data from earlier ones. $\endgroup$ Aug 21, 2019 at 6:40
  • $\begingroup$ @SteveLinton see Flying fuel tanks! Which deep-space spacecraft had the largest fuel mass fraction? and also Unravelling Cassini's “ball of yarn” orbit around Saturn, tabulation of propulsive maneuvers? If Clipper were of the "flying fuel tank" variety that NASA has gotten so good at, then I don't think there would be any problem. $\endgroup$
    – uhoh
    Aug 21, 2019 at 7:25
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    $\begingroup$ “How much flexibility does Clipper have for its flybys?” Might be an interesting question. Gravitational assists depend on where the other moons are. But it’s a complex system, perhaps there are lots of opportunities. Maybe somebody could find mission planning docs? $\endgroup$ Aug 21, 2019 at 16:45

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