Los Angeles - Jan 23, 2001
A new program of Medium-class, or "Discovery Plus", missions to explore the Solar System is being increasingly favored by NASA. It has considerable application to the inner planets and the Asteroid Belt. But its most important application is likely to come in the vast reaches of the outer Solar System, where space exploration up to now has been very expensive simply because those regions are so vast and so distant.
This reporter attended not only last November's meeting of the Survey's central Committee, but also the December meetings of the two Survey subcommittees directed at exploring both the four giant planets and their larger moons.
In 1997, NASA's Solar System Exploration Subcommittee (SSES) listed the five Solar System missions not directed at Mars, which it considered most crucial for the near future. One such need - a Mercury orbiter - has since been filled by a fully funded mission - Messenger - that will launch in 2004.
But the other four all involved either the outer Solar System or (in the case of comets) objects that come from there. And two of those - a Europa orbiter and a mission to land on a comet nucleus and return intact samples of it to Earth - are now clearly missions which, in their current form, will exceed a billion dollars each, as will many other outer Solar System missions regarded as important in the medium term.
In NASA's increasingly tight fiscal situation, these simply cannot be funded, except rarely. So perhaps the central Committee's most urgent business at its November meeting was to examine and judge concepts for exploring the outer planets with as much scientific effectiveness as possible using Medium-class missions costing only $500 to $700 million each.
Some of these concepts have been around for a while - most notably the Pluto flyby which has been the subject of so much fuss and dispute, and whose latest incarnation ("New Horizons") I described in a previous installment of this series.
It might fly in 2006 using a Jupiter gravity-assist to reach Pluto - in which case it will cost $450 to $500 million - or (as the White House and NASA currently wish) it might be delayed several years, in which case it will require a solar-powered ion drive module to reach Pluto, lowering its near-term cost but increasing its total cost by $100 to $200 million and reducing its likely scientific return as Pluto recedes from the sun.
In either case, though, it falls into the Medium-class category, and remains one of the key missions recommended by the SSES in 1997.
The other mission fitting both those categories is the "Jupiter Multiprobe", which would drop three entry probes into Jupiter's atmosphere simultaneously. Its rationale is that - while the Galileo entry probe of Jupiter can hardly be called a failure - it wasn't as successful as it might have been, simply because it had the bad luck to parachute down into one of the atypical "hot spots" that cover only 8% of Jupiter's surface area in the equatorial region.
These hot spots are believed to be due to local downflows of stratospheric Jovian air which strip away almost all of Jupiter's multiple cloud layers, exposing the warmer depths beneath, and thus confusing any measurements of water vapor and ammonia in the Jovian atmosphere and perhaps the wind-speed measurements as well.
Scientists would very much like to obtain accurate measurements of these gases' average levels, both for meteorological reasons and because they can provide data on the composition of the ices in the original solar nebula from which the giant planets formed.
They would also like to see simultaneous measurements of Jovian weather in several different spots, since a single such measurement from one entry probe can be as misleading as such a measurement from one spot on Earth would be.
This Medium-class mission would involve a flyby craft releasing three entry probes on different trajectories - striking Jupiter at the equator and at 25 degrees north and south - and radioing back data to the main craft as it flies past Jupiter without stopping.
Scientists are seeking composition and wind data from as deep as 100 bars, far below the 23-bar level at which the Galileo probe finally failed - and since Jupiter's air temperature at that depth is fully 390 deg C, the probes (unlike the vented Galileo probe) will need insulated pressure hulls.
Further adding to their weight problem, they will need thick heat shields to survive an initial 160,000 km/hour plunge into Jupiter's atmosphere. The Galileo probe's heat shield weighed as much as the probe itself -- and estimates are that we could whittle only about 20% off the shield's weight today using new technologies.
So these probes require a lot of miniaturization in their systems and instruments, in order to lower their non-shield weight to only 150 kg each as against the Galileo probe's 340 kg, while taking similar measurements.
But the cost of this mission is still estimated to be only about $500 million, as the carrier spacecraft itself can be fairly simple and lightweight, using solar panels for power and perhaps not even carrying a camera.
It might be a modified version of the "INSIDE Jupiter" craft which was the rejected finalist for the latest selection of a low-cost Discovery mission -- a Jupiter orbiter that would carry only a magnetometer and radiation detector, but which would still enable a great deal of data to be obtained by entering an orbit with a periapsis of only 4500 km, so that simply tracking it would provide detailed data on the planet's interior structure.
This design could be turned into a probe carrier by removing its big orbital-entry fuel tanks and adding the probes instead. Indeed, the same craft, with some further changes, could later be used to carry similar entry probes to any of the other three giant planets.
There was some interesting debate at the later Subcommittee meetings over whether a cheaper short-term substitute for the Jupiter Multiprobe might be "JASSI", another recent Discovery proposal in which a craft would fly over both of Jupiter's poles at low altitude and use a high-quality microwave spectrometer to precisely measure its water vapor and ammonia.
JASSI and "INSIDE Jupiter" seem to be the only two viable concepts left at the moment for a low-cost Discovery mission to the outer Solar System; the former could obtain important information on the planet's composition, while the latter could provide much better data on its gravity and magnetic fields and thus on its basic internal structure.
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