Galileo says...

THE plucky American spacecraft Galileo,which went into orbit around Jupiterafter successfully delivering a 'suicideprobe' in December last year, has gradually begun to unveil the mysteries surrounding the planet's atmosphere.

, After a reasonably long wait, scientists at the us National Aeronautics andSpace Administration (NASA) have nowbegun receiving the data collected by the'suicide probe' following its 57-minutesojourn in the Jovian atmosphere. Theysay that the probe's measurements maynecessitate a re-evaluation of existingviews on how Jupiter evolved from thesolar nebula.

The composition of the Jovianatmosphere offered some surprises tothe Galileo project scientists. The 'suicide probe' found significantly lower levels of helium, neon, and certain otherheavy elements, such as carbon, oxygenand sulphur. It also did not encounterany solid objects or surfaces during itsentire 600 km descent; this was expectedin the case of a, giant gas-planet such asJupiter, say scientists.

"The probe detected extremelystrong winds and intense turbulenceduring its descent through Jupiter'sthick atmosphere. This provides evidence that the energy source drivingmuch of Jupiter's distinctive circulationphenomena, is probably the heat escaping from the interiors of the planet,"said one scientist with the Galileoproject. "The probe also discovered anintense new radiation belt approximately 49,879 km above the planet'scloud tops and a veritable absence oflightning," he observed.

What are the implications of the information disclosed by the 'suicide probe'?Most researchers at NASAbelieve that Jupiter has abulk composition similar tothat of the, gas and dustcloud of the primitive nebulafrom which other planetsand the sun were formed.The. lower-than-expectedlevels of helium and neon onJupiter compared to the sun,influence scientific understanding of the process offractionation - the 'rainingout' of helium and neonduring planetary evolution.

During the probe's high-speed atmospheric entryphase, deceleration measurements high in the Jovianatmosphere revealed that thedensity was much higherthan previously believed.Corresponding temperatures were also much higherthan predicted. The hightemperatures appear to require an unidentified heating mechanism for this region of the planet's atmosphere, say NASA scientists.

During the parachute deploymentphase, six scientific instruments onboard the probe collected data throughout the 156 km of its descent. Duringthis phase the probe endured extremewinds, periods of intense cold and heatand strong turbulence. The extremepressure and temperature in the planet'senvironment caused the probe's communication subsystem to terminate thedata transmission operations to themother craft.

Earth-based telescopic observationssuggest that the probe's -entry sight maywell have been in one of the least cloudyareas on Jupiter. At this location, theprobe did not explicitly detect the threedistinct layers of clouds - a top layer ofammonia crystals, a middle layer ofammonium hydrosulfide, and finally athick layer of water and ice crystals -that the researchers had anticipated. Butthere are some indications of a high-level ammonia ice cloud being detectedby the net flux radiometer. Evidence of athin cloud, 'suspected to be that ofammonium hydrosulfide, was also provided by the scientific instruments onboard the space probe.

The data flow from the space probehas provided new impetus to the ongoing debate on the nature of the Joviansystem. A clearer picture is expected toemerge after the mother craft completesits two-year study mission around thegiant planet and its retinue of moons.

On the day of Galileo's rendezvouswith Jupiter, NASA chief David S Goldinhad predicted that by the turn of thecentury or soon afterwards, the agencywould be launching a spacecraft onceevery month - small cheap machinesdesigned to explore the solar system,and search for earth-sized planetsaround other stars. "The best is yet tocome," he asserted.

Plans are underway to dispatch twospacecrafts to Mars late this year - oneto orbit the planet and other to land anautomated rover on the surface.Another ambitious mission, christened'Cassini', is set for launch in 1997 toSaturn and its gaint moon Titan. NASAscientists have already begun work for aflight to the moon, more missions toMars in 1998, and still another designedto fetch some dust from a comet.Progress is also being made in devisingan affordable mission to Pluto, the onlyplanet in the solar system yet to welcome a spacecraft from earth.

Goldin's optimistic observationsabout the future of planetary exploration probably came as a surprise tomany NASA Sceptics for whom the daysof extensive planetary scouting seems tohave become a thing of the past. ThePioneers and Voyagers are coasting tothe edge of the solar system, with theirdiscoveries behind them. The Vikingshave been relegated to the dustbin ofhistory. Galileo, which was launched sixyears ago, is the only spacecraft activelyengaged in studying a planet.

Notwithstanding the scepticism, theagency has managed to sectire the WhiteHouse's approval on its philosophy ofsmaller, better and less expensive missions. But congressional support maybe very difficult to come by. Growingdemand for reduction in federal spending could jeopardise these and otherNASA programmes.

Wesley T Huntress Jr, NASA's associate administrator for space science ,says there is hope for new plans as longas they are achieved without increasingthe agency's annual budget devoted toscience. In the current budget, the allotment for new plans is us $2 billion outof the total us $13.8 billion. WithGalileo at its destination and Cassininear completion, expenditure on twohigh priced projects is sharply diminished, leaving room for embarking onnew missions.

To evolve a new approach to spaceexploration that is less expensive andfocused in nature, NASA planners havecome out with a concept called thePluto Express, at an estimated cost of us $400million, which the agencyofficials presume, will beaffordable. Instead ofpacking several instruments in a single craftweighing several tonne,engineers have designedtwo light-weight spacecrafts weighingas little as 137 kg (approximately) each.This was made possible by incorporating new microelectronic technologyand limiting the number of scientificinstruments on board. Further, a highdegree of automation is expected toreduce the costs of operating the craft inflight.

The Pluto Express involves launching of two crafts seperately, -perhaps byRussian Proton rockets, on 'fast' trajectories. They should be able to reachPluto in six to eight years compared toVoyager-2's travel to Neptune in 12years.

The Discovery programme of NASAis a starter in the direction of small, low-cost projects. The idea is to develop spacecraft with narrowly focused and precise goals for flight in less than three years at a cost of less than US $150 million, including the cost of launching four such missions. The first mission craft is Stipulated to orbit and study an asteroid, Eros.

Another approach to the future is being studied at the Jet Propulsion Laboratory (JPL) in Pasadena, under what is called the New Millennium Programme - aimed at identifying the likely science missions of the next century and determining the new technology that would be required to accomplish the objectives at reasonable costs. The first New Millennium flight, scheduled for 1998, would involve a 100 kg (approximately) spacecraft equipped with miniaturised antenna and automated controls. It will be the first spacecraft that would depend on solar electric propulsion for its main source of thrust, instead of conventional solid or liquid propellents. With electric propulsion, the craft can get continuous thrust, rather than coasting, and this would reduce the time of flight substantially, Say JPL scientists.