But there's another radically different mission that has been proposed as a New Frontiers-class preliminary Europa mission -- a craft that would enter the Jupiter system, fly past Europa at low altitude without stopping, and eject a smaller "impactor" to crash into Europa's surface and kick up a huge rarified cloud of gas and fine ice and rock particles.

The main craft would then either make spectral observations of the cloud -- or would actually fly through it, collect a tiny sample of it with a low-density aerogel "catcher's mitt" that could preserve some of the particles even when they smacked into it at over 36,000 km/hour, and then fly on beyond Jupiter and return to Earth with the sample, in the same way that the Stardust craft returned samples of Comet Wild 2.

Back in 1997, a team led by the Ames Research Center's Christopher McKay actually proposed "Europa Ice Clipper" -- a mission to return a sample of Europa's surface to Earth in this way -- as one of the candidates for NASA's low-cost Discovery planetary program.

The Clipper would have been launched in December 2001, arc outwards from the Sun to a short distance beyond Jupiter's orbit, intercept Jupiter and Europa on its way back toward the Sun in mid-2009, and fly over Europa's surface at only 50 km altitude.

Shortly before doing so, it would eject a 20-kg copper ball that would crash into Europa's surface ahead of the main craft and kick up such a fine debris cloud, which the main craft would fly through -- collecting fine particles with a duplicate of the Stardust craft's aerogel collector, collecting water vapor from the cloud with another collecting surface, and analyzing some particles on the spot by using a mass spectrometer to detect the puffs of vapor produced when they slammed into a metal plate (as Stardust did with grains of the debris from its comet).

The Clipper would then finally return to Earth after 12 years and parachute its collectors back to Earth in a return capsule.

The whole spacecraft, in fact, would have been a close copy of Stardust (including being powered by solar panels), and the team hoped that it could be flown for only $255 million. It seems likely that such "smash-and-grab" sampling mission may have a future for airless worlds -- one that would return samples from both of Mars' moons has been a Discovery finalist twice, and the idea has also been suggested for Mercury and asteroids.

But the Discovery proposal review team quickly rejected the Ice Clipper, largely on the grounds that precisely navigating the craft for such a low-altitude flight over Europa's surface during a first-time approach to that moon would be too risky.

One can also argue that the sample returned might not be large enough for some of the most interesting possible substances in Europa's ice -- organic compounds that might be indicators of life -- to be detected, even with the giant super-sensitive instruments that could analyze the sample back on Earth.

However, more recently Karl Hibbitts of Johns Hopkins' Applied Physics Lab has proposed a modified version of this concept that actually bears a closer resemblance to the Deep Impact comet mission.

It would release a much bigger impactor -- weighing at least 100 kg, and preferably around 370 kg like the Deep Impact impactor -- that would actively guide itself to an impact on Europa, using small maneuvering jets and a TV or radar guidance system -- and kick up a much bigger, denser cloud of debris than the little 20-kg passive ball proposed for the Ice Clipper, after which the main craft would analyze the cloud from a distance using infrared spectrometers as the main Deep Impact craft did for Comet tempel 1.

The main craft might fly by Europa without stopping, like Ice Clipper -- or it might brake into orbit around Jupiter or even Europa to take high-resolution photos of the fresh crater left by the Impactor to observe the hardness and near-surface layering of Europa's surface (something which Deep Impact was unable to do, simply because the initial dense debris cloud didn't clear until after the craft had departed from the comet.

This mission would have some obvious advantage over the Ice Clipper concept. Its bigger impactor would not only kick up a much bigger debris cloud -- but most of that debris would be excavated from well beneath the upper meter or so of Europa's surface, which is thought to have any organic compounds in it seriously scrambled by Jupiter's intense radiation.

It would, so to speak, be a fresh sample from Europa's subsurface, where biologically interesting compounds may still be intact -- a sample that could otherwise be collected only by a lander equipped with a deep drill or "cryobot" ice-melting probe to dig up samples from well below the surface layer.

And the Impactor's ability to actively steer itself at a downwards angle after its release from the main craft -- along with the fact that it would kick up a much bigger debris cloud, and the fact that the main craft would observe the cloud only from a distance -- would allow the main craft to fly over Europa's surface at a higher, safer altitude.

But the ability of infrared spectrometers to analyze complex compounds (including non-organic ones) in such dust and gas cloud is seriously limited, as the Deep Impact mission itself ended up proving (much of its useful science came from its observations of the comet nucleus before the impact).

An improved version of this mission might have the main craft fly directly through the debris cloud and use a dust-impact mass spectrometer to analyze the materials in the debris grains in vastly more detail than an IR spectrometer could.

Or one might combine this concept with the Ice Clipper concept: have a Deep Impact-type craft fly through the Jupiter system and over Europa, release such a big active Impactor to kick up a really big debris cloud which the main craft could fly though at a safe altitude above the surface to collect a sample in its aerogel pad, and then return this sample -- considerably more massive and easier to analyze than the one planned for the Ice Clipper -- to Earth.

Such a mission could almost certainly be flown within the New Frontiers budget -- especially since the main craft could be very closely based on the design of the Deep Impact craft, with the additional sample collector and Earth return capsule copied from the Stardust mission.

Moreover, such a craft -- although it would make only a single flyby of Europa -- could make other scientific observations that could be useful in planning later, more advanced Europa missions.

Even the Ice Clipper was supposed to carry a modest-sized camera that could photograph details as small as 1.5 meters from 50 km altitude; and the much bigger camera carried on Deep Impact -- if anti-blurring image-motion compensation was added -- could observe objects as tiny as 20 cm or so from that altitude, giving us crucial new insight in how to design a survivable lander for Europa's apparently very rough surface.

And-- if the craft also carried wider-angle cameras, near-infrared spectrometers to map surface composition, and thermal IR cameras to look for warmer surface spots where recent liquid-water eruptions might have occurred, plus a very high-volume data-recording system allowing it to record hundreds or even thousands of images and spectra at a very high rate during the flyby -- it could also survey a considerable area of Europa's surface for promising landing sites, in a way much more detailed than the obervations that Galileo was able to make even during its 11 Europa flybys.

One other early Discovery proposal -- "Firebird", proposed by the Jet Propulsion Laboratory's William Smythe -- would have made a single flyby of Io with such a set of cameras and spectrometers to map a large area of its surface at high resolution and very high speed, and then spend months afterwards playing back all the data it had accumulated during that one flyby -- fully 50 times more data than can be stored on the Cassini spacecraft's recorders.

Finally, the returned Europa sample would allow an estimate of the amount of salts and rock grains in Europa's surface ice, which could be vital knowledge to properly design a subsurface ice-sampling "Cryobot" probe for a later biological lander.

Obviously such a mission has limitations. It's seriously questionable whether even the bigger Deep Impact version of a Europa smash-and-grab sample return mission could return anywhere near a large enough sample to allow Earth researchers to look for biological compounds, which would probably exist in the ice only in an extremely diluted state. Even given the supersensitivity of the giant analytical instruments that exist on Earth, they would, after all, be trying to analyze a total sample weighing, at absolute most, a small fraction of a milligram.

Ice Clipper's returned sample would largely be examined instead for lighter organics that might have come from either biological or non-biological sources, for those aforementioned salts and rocky materials, and for the amount of deuterium in Europa's ice -- which could tell us just where the ice that originally formed Europa and Jupiter came from in the infant Solar System.

And the sheer speed with which those grains of Europan material smashed into the aerogel collector might break down any of the more complex and interesting organic compounds within them. Even given the fact that the extraordinarily low-density aerogel can slow down such particles very gradually, the sheer heat from their impact and friction with it could break down the organics.

Dr. McKay has told this writer that the Clipper team's studies indicated that the collected material could still preserve biogenic compounds: "In fact, we concluded that there would be some ice [in the grains] that was not even melted." But this is still open to debate.

Stardust's aerogel only had to collect particles slapping into it at 22,000 km/hour -- and one recent study by M.K. Spencer and Richard Zare has revealed the disturbing fact that just firing a laser beam at a clean sample of the aerogel used by Stardust generates organic compounds from the gel itself that resemble those extracted so far from the tracks left by comet dust grains that plowed through the gel, suggesting that we may never be able to get a usable sample of the organic compounds from Comet Wild 2 itself out of the Stardust aerogel.

It's been suggested that another kind of aerogel layer -- thicker and less dense than that in Stardust's collector -- could collect organics intact at impact speeds of up to 36,000 km/hour (the figure for a Europa sampling flyby). But who knows?

In short, this kind of specialized low-cost Europa mission could be a risky gamble; it might well end up not providing as much new knowledge about Europa as had been hoped, and its usefulness in picking good landing sites for later Europa landers is certainly seriously limited.

But it would nevertheless have a high science return even in the worst case -- which is why a similar mission is being seriously considered for Saturn's moon Enceladus, as we'll learn in a later chapter. And my next chapter will begin examining NASA's current effort to identify relatively cheap missions that might nevertheless be scientifically worthwhile for exploring even worlds as distant as Enceladus and Titan.

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