WHEN it punches into the atmosphere just north of the equator, the probe will be travelling at 170 000 kilometres per hour, more than a hundred times the velocity of a bullet from a gun. As it passes through the outer atmosphere, friction will slow the vehicle to below the speed of sound in less than two minutes. Then, at a depth of 68 kilometers, the craft will discard its heat shields and continue its descent by parachute. A few minutes before sunset, the Galileo probe will take the first measurements of conditions beneath the thick clouds that cover Jupiter. For an hour after that, it will broadcast the results of its experiments to the main Galileo spacecraft as it approaches the planet 200 000 kilometres above the probe.鈥漌e鈥檙e going to be huddled there with white knuckles, hoping that the spacecraft locked on to probe,鈥� says Richard Young, an atmospheric scientist working on the probe project and based at NASA鈥檚 Ames Research Center at Moffett Field in California. The mission will end 150 kilometres beneath the cloud tops when the extreme atmospheric conditions destroy the probe.
The probe is part of the $1.4 billion Galileo mission to Jupiter that left Earth almost six years ago. Next week, the spacecraft will release the probe on a five-month collision course for the largest planet in the Solar System. The remaining vehicle, known as the Galileo orbiter, must adjust its flight path to prevent it following the probe into Jupiter. This must be done carefully so that the orbiter arrives at the planet just as the probe penetrates the atmosphere on 7 December this year. The timing is crucial because the probe can only broadcast signals in a narrow beam straight up. If the orbiter arrives early or late, it will miss the data.
In any case, the orbiter is preprogrammed to stop listening after an hour and prepare to brake prior to entering orbit around Jupiter. As the probe dies, the orbiter will begin a two-year study of the planet and its moons. 鈥淓ven if the probe is still working after 75 minutes, we cannot afford to listen to it,鈥� says Charles Sobeck, an engineer on the probe programme.
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Galileo鈥檚 journey to Jupiter has been long and arduous. The mission was originally scheduled for launch on board the space shuttle in 1982. But budget alterations, changes in the design and the explosion which destroyed the Challenger in 1986 caused years of delay.
In 1989, the mission finally got off the ground on board the space shuttle Atlantis only to face another serious setback when the craft鈥檚 main antenna failed to open thereby reducing the amount of information that it could send back to Earth by a factor of ten thousand. To increase the data rate, scientists at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, where the mission is coordinated, are currently reprogramming the vehicle鈥檚 computers with software that will compress the data before it is sent to Earth. 鈥淚t is still down by a factor of a hundred but this is a hundred times better than it would have been with no compression,鈥� according to William O鈥橬eil, who heads the Galileo project at JPL.
Even the vehicle鈥檚 trajectory has been complex. Safety changes after the Challenger accident prevented Galileo from being fitted with a rocket motor powerful enough to send it directly to Jupiter. Instead, engineers fitted a less powerful but safer model and worked out a series of gravitational slingshots, once around Venus and twice around Earth, that would provide the energy to reach Jupiter. During the journey, Galileo has studied the asteroids Gaspra and Ida, and last year it took the only photographs of the comet impact on the far side of Jupiter, the side not directly visible from Earth.
In March, scientists at the Ames Research Center who are responsible for the probe, carried out checks on the scientific instruments it is carrying and the batteries that will provide power when it is released. 鈥淓verything looked normal, so we鈥檙e happy,鈥� says Marcie Smith, who is in charge of the probe. 鈥淲e鈥檙e ready to go for the full mission.鈥� The probe does not have any thrusters for manoeuvring so it must be accurately oriented before release so that it enters the atmosphere nose first. To keep it stable during the journey, the probe will also be spun at 10.5 revolutions per minute.
On Monday, the power and communication cable connecting the probe to the main spacecraft will be severed with an automatic guillotine. Two days later, ground control will trigger the three explosive bolts that hold the probe in place and a set of springs will push it gently on its way. Confirmation of separation will take 38 minutes to arrive back on Earth.
Several weeks after the release, scientists will test the main engine with a two-second 鈥渨ake up鈥� burn. If all is well, this will be followed by a five-minute burn on 27 July that will set the orbiter on course to reach Jupiter just as the probe enters the atmosphere.
The test is essential. 鈥淲e don鈥檛 dare commit ourselves to the five-minute burn without first verifying that the engine is working properly,鈥� says O鈥橬eil. If there is a problem, the team has roughly 50 days to explore and execute contingency plans. One option could be to use the craft鈥檚 dozen smaller thrusters which were originally intended for minor manoeuvres. The main engine, however, will still be needed in December to position the craft in orbit. 鈥淭he best bet right now is that everything will go beautifully,鈥� says a confident O鈥橬eil.
During its approach, the orbiter will pass close to the two Jovian moons, Europa and Io. Four hours later 鈥� and some six hours before it is due to enter the atmosphere 鈥� the orbiter should 鈥渉ear鈥� the probe come to life.
Bolts from the blue
The measurements will begin before entry. They will examine Jupiter鈥檚 radiation belts and look for radio emissions produced by lightning. In 1979, the Voyager spacecraft spotted 鈥渟uperbolts鈥� of lightning which were typically a hundred times more energetic than their earthly counterparts. The Galileo probe will provide more data on their frequency and strength.
During the entry, the shock wave in front of the craft will reach temperatures of 28 000 掳C. 鈥淭hat鈥檚 two-and-a-half times the temperature of the Sun,鈥� says Young. The high temperature will ionise the gas surrounding the probe, blocking radio signals for approximately two minutes. 鈥淚f we have a problem during entry, we鈥檒l never know why,鈥� admits Smith. The instruments are protected by a 145 kilogram-heat shield made of a carbon-based material which will carry away the heat as it vaporises. About 90 kilograms of shielding material will be melted away during the probe鈥檚 fiery arrival and the deceleration will create forces up to 350 times Earth鈥檚 gravity.
When the speed drops below Mach 1, the probe will deploy a parachute and discard the remains of its heat shields. During the rest of the mission, descending at roughly 180 kilometres per hour, it will discover the composition and structure of the atmosphere by analysing samples with a mass spectrometer and measuring the temperature and pressure. 鈥淭here is still controversy over whether there are water clouds on Jupiter,鈥� points out Young. 鈥淲e have models but we don鈥檛 know for sure. The probe will give us direct information on that,鈥� he adds.
This data should also answer other questions. The strength of Jupiter鈥檚 gravitational field, for example, depends on the planet鈥檚 composition. Knowing the atmospheric composition will help, but the probe will also measure the acceleration due to gravity.
Other instruments will look for the energy source that drives the Jovian winds by monitoring the solar radiation that penetrates the cloud layers. 51动漫s also hope to find clues about the primitive mix of gases in the early Solar System by measuring the relative abundance of helium and hydrogen in the Jovian atmosphere. During this time the probe will beam data at a rate of 128 bits per second that the orbiter will be able to handle with ease.
Crushed and melted
The Galileo team expect atmospheric conditions to deteriorate quickly as the craft descends, but the probe鈥檚 ultimate fate is unclear. By the end of the 75-minute mission, the probe鈥檚 batteries will be running low and the atmosphere will have begun to severely impede the radio signals. The temperature will also have risen from 鈭�175 掳C to over 130 掳C and the pressure will have increased to 25 Earth atmospheres 鈥� either could destroy the probe.
Will 75 minutes of data at 128 bits per second be worth the years of waiting? 鈥淭he entire set of data could fit on a 3.5 inch floppy disc,鈥� says Sobeck, who has worked for more than 15 years on the Galileo probe. 鈥淏ut, as we鈥檙e fond of saying, 鈥榥ot all bits are created equal鈥�.鈥� (see Diagram)
