The indispensable "Group 1A" objectives for the Europa Orbiter include determining whether it does have a subsurface ocean, determining (if possible) the thickness of its overlying ice crust and locating local thinner regions that could make good landing sites for future Europa landers, and understanding the past history of its ice layer (did it have an ocean during its warmer early days that is now frozen?)

The likely instruments for this include cameras for much better mapping of its surface features than Galileo obtained; a long-wavelength radar sounder that might be able to punch through as much as 15 km of ice to locate a liquid water layer underneath (or isolated pockets of liquid water in the ice); and a laser altimeter that should settle the overall question of an ocean even if the ice is too thick for the radar to penetrate (by measuring the degree to which Europa's ice crust flexes up and down with the changing strength of Jupiter's tidal pull during each orbit).

Less crucial "Group 1B" investigations -- which will be included only if the craft's very limited payload weight allows them -- would include attempts to analyze and map the substances mixed with Europa's ice. This might be done with a more sensitive near-IR spectrometer than Galileo's -- which has discovered hydrogen peroxide, carbon dioxide, and what appears to be either Epsom salt (magnesium sulfate) or sulfuric acid, depending on which scientist you ask.

Or it might be done by using a mass spectrometer to analyze the faint aura of gases around Europa produced when its surface compounds are "sputtered" off the surface by Jupiter's intense radiation. Such analyses would be important both to look for organic compounds that might serve as the food for Europan microbes (or even be their remains), and to understand just how hospitable for the possible evolution of life a Europan ocean might be.

It has seemed both to me and to some scientists that such an instrument is increasingly important. Leading astrobiologist Dr. Jack Farmer told me at the recent Ames Astrobiology Science Conference that he would like to see a tiny lander added to the Orbiter to analyze the ice directly, in detail.

Another possibility might be to have the Orbiter -- during one of the series of flybys of Europa it will make before finally settling into orbit around it -- eject a metal weight that would crash into the surface at several km per second to throw up a faint debris cloud that the Orbiter's mass spectrometer could then analyze (which would include material excavated from below the upper meter or so of Europa's ice, where Jupiter's radiation has long since broken down the original organics into other chemical forms).

Alas, these ambitious ideas have gone aglimmering. Bergstralh told me flatly that no investigator had proposed either a lander or an impactor for the Orbiter to carry -- and, given the weight of such devices, this isn't really surprising. Howver, he was noncommittal on whether some less complex surface composition instrument might be selected -- and some such instruments are very lightweight. We'll see in a month.

As for the Pluto Express, its likely trinity of instruments have been recognized from the start: a camera to photograph both Pluto and its oversized moon Charon (an almost equally important target), a near-IR spectrometer to analyze their surface ices, and a UV spectrometer to analyze Pluto's faint atmosphere (and see whether Charon has one). These devices are near-certainties; but given the craft's tiny size -- it will weigh only about 210 kg -- it's rather unlikely that any more instruments can be crammed onto it. Again, though, we'll see this summer.