NASA's Galileo space probe will shortly be released into a 170,000km/h plunge into Jupiter's atmosphere.
Tim Furniss/LONDON
On 13 July, an 340kg instrumented probe, will be released from NASA's $1.4 billion Galileo spacecraft, 80 million kilometres away, from the planet Jupiter. The 1.25 x 0.86m probe will be directed at a point 6¡N of the giant gaseous planet, the largest in the solar system. On 7 December, it will plunge into the turbulent atmosphere at a speed of 170,000km/h.
The Hughes-built probe will encounter deceleration forces exceeding 350g as it penetrates 640km into the atmosphere, and is expected to survive for up to 75min until the atmospheric pressure exceeds 20 times that of the Earth's. The probe will return data from inside the chemical cauldron of Jupiter.
The probe is equipped with an atmospheric-structure instrument, a neutral-mass spectrometer, a helium-abundance detector and a net-flux radiometer, as well as a lightning and energetic-particles instrument.
Data from the first direct measurements of Jupiter's clouds, lightning, winds and other features will be transmitted to the Galileo mother ship, which will be under preparation for a 46min-long engine burn to place it into an initial orbit around the planet, the first spacecraft to do so.
For 23 months, the 2,200kg, Jet Propulsion Laboratory-built, orbiter, equipped with an array of ten science instruments, will conduct the most detailed survey of Jupiter and its major moons ever undertaken. It will make ten huge orbital tours of the Jovian system, with a low point to the planet, called the perijove, of 200,000km, and an apojove (high point) of 10 million kilometres.
The Galileo (named after the Italian scientist Galileo Galilei who, in 1610, discovered four moons orbiting Jupiter) was deployed from the Space Shuttle Atlantis/STS34 on 18 October 1989. The spacecraft was used to explore two asteroids, Gaspra and Ida, en route to Jupiter, via one Venus and two Earth fly-bys. By the time it reaches Jupiter, it will have traveled 3.8 billion kilometres.
Jupiter lies in an orbit with an average distance from the Sun of 778.3 million kilometres, and makes one orbit in 11.9 years. With a diameter of 142,800km, this huge ball of gaseous hydrogen and helium, with chemicals including ammonia, hydrogen sulphide and phosphorous, has an extremely turbulent atmosphere, with swirling bands of clouds, which reach speeds of up 35,000km/h. Its major feature is the Great Red Spot, an enormous anticyclone measuring 32,000 x 12,000km.
Previous explorations by the NASA Pioneer 10 (December 1973), Pioneer 11 (December 1974), and Voyager 1 (March 1979) and Voyager 2 (July 1979) spacecraft, which flew past the planet, revealed the first close details, particularly its four largest moons, Io, Ganymede, Europa and Callisto. These will be subject to the close attention of the Galileo orbiter.
Indeed, before its orbital insertion, the Galileo will have undertaken the closest fly-bys of Io and Europa. Its only close approach of Io will bring it to within 1,000km of the moon's sulphuric surface, collecting images of features as small as 20m.
Io, with a diameter of 3,640km, has an extraordinary orange, yellow and white surface, with deposits of sulphur and sulphur dioxide as well as volcanoes which spew sulphur dust and gas 280km into space. The Galileo, meanwhile, will also come six times closer to Europa than did the Voyagers at 35,000km.
Transmissions to the Earth will have to be made using the spacecraft's low-gain antenna because the high-gain antenna failed to deploy fully in April 1991. Subsequent attempts failed to nudge free the three, which remain of 18 ribs still slotted in their launch position. Software and ground-hardware improvements will help retain about 70% of the planned science mission, although only 4,000 images are likely to be returned, compared with the 50,000 planned originally. The Galileo's primary computer has been cleared and reloaded with new software.
SCIENTIFIC RESULTS
A full return of data from the atmospheric probe is expected. The probe consists of a 220kg, 45°, half-angle cone deceleration module, which will protect the instrumented descent module during the expected peak heating of entry, losing 90kg of its 145kg carbon-phenolic heat shield. Heating of 40kW/cm2 from the 16,000°C shock wave temperature is expected.
By now travelling at 176km/h terminal velocity, the descent module will then be pulled free by the deployment of a 2.5m-diameter Dacron parachute. The 28kg of instruments will return L-band data to the mother ship, 200,000km away, until the capsule succumbs to the atmospheric pressure.
During its descent through Jupiter's three upper-cloud layers, scientists expect the probe to encounter hurricane-force winds; clouds of white ammonia-ice crystals; brownish, thick, ammonia hydrosulphide clouds; dense water clouds; and even torrential rain. At the bottom of the cloud deck, it will be dark and hot, with the temperature eventually exceeding boiling point.
Unlike the Russian Venera landers, which survived 90 Earth atmospheres pressure on the surface of Venus, the Galileo probe is not designed as a pressure vessel. It will be crushed and vapourised.
Source: Flight International
