David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.
Copyright © David Rothery
The outer Solar System has been less thoroughly explored than Earth's immediate neighbourhood, but the lure of Jupiter and its amazing satellite system is such that nevertheless it has received considerable attention (Table 9.1).
Name | Description | Date of fly-by or operational period |
---|---|---|
Pioneer 10 | fly-by; first close-up pictures, first determination of magnetic field and charged particles, atmospheric measurements | Dec. 1973 |
Pioneer 11 | fly-by; similar to Pioneer 10 | Dec. 1974 |
Voyager 1 | fly-by; first detailed pictures of Jupiter's satellites, discovered active volcanoes on Io | Mar. 1979 |
Voyager 2 | fly-by; similar to Voyager 1 | July 1979 |
Ulysses | fly-by as a means to go into polar orbit about the Sun; magnetospheric measurements | Feb. 1992 |
Galileo | entry probe and orbiter; atmospheric measurements, long duration detailed imaging of satellites | Dec. 1995 (entry probe) - 2003 |
Cassini | fly-by en route to Saturn | Oct. 2000 - March 2001 |
Europa Orbiter [cancelled] | orbiter; imaging, ice-penetrating radar, surface altimetry | 2010 - 2012? |
The first probes to venture Jupiterwards were Pioneers 10 and 11, which were launched in 1972 and 1973. Their safe crossing of the asteroid belt dispelled fears that this region of space might be so full of meteorite debris that it would be impossible for spacecraft to survive passage through it without unfeasibly robust shielding. Furthermore, by determining the intensity of radiation in the zones of charged particles trapped by Jupiter's magnetic field they enabled the design and trajectories of future missions to be planned within safe limits.
In addition to mapping Jupiter's magnetic field, the Pioneers returned the first close-up images of the planet itself. Probably of more importance than the images were the determinations of pressure, density and temperature in the atmosphere made using infrared detectors and by radio occultation, which depended on measuring the way the Pioneers' radio signals faded out as they passed temporarily out of sight behind the planet. In particular these showed that Jupiter's atmosphere is hydrogen-rich but mixed with a significant amount of helium, thereby lending support to the model of Jupiter's growth by scavenging gas from the solar nebula that was described in Chapter 02.
Jupiter and its satellites were explored in more detail by two Voyager probes that flew past in 1979. These recorded detailed time series views of Jupiter's atmospheric circulation and the first ever high resolution views of Jupiter's four largest satellites, revealing active volcanoes on one of them, and also discovering three inner satellites and Jupiter's slender main ring. Each Voyager probe took advantage of Jupiter's gravitational field to alter its trajectory and speed it on its way to Saturn, using the so-called 'gravitational sling-shot' effect otherwise known as a gravity assist trajectory. Voyager 2 must rank as the most productive spaceprobe in history, proceeding beyond Saturn to encounters with Uranus and then Neptune (Figure 9.2).
As the new millennium dawned both Voyagers were still functioning 76 and 60 AU from the Sun respectively, having overtaken the slower-moving Pioneer probes with which communications ceased in 1997, and returning data on the interplanetary environment. Voyager 1 is expected to be the first human artefact to penetrate the interstellar medium (space beyond the influence of the solar wind) in about 2019.
After the Voyagers there was an interval of more than a dozen years before Jupiter received its next visitor from Earth. This was Ulysses, a probe designed to study the Sun. However, in order to get Ulysses into a high-inclination orbit, it was sent first to Jupiter so that by swinging over the planet's north pole the gravitational sling-shot effect would direct it south of the ecliptic plane into an orbit passing over the Sun's south and north poles in turn. This Jupiter swing-past provided a useful opportunity to map the high-latitude parts of Jupiter's magnetosphere, but Ulysses had no imaging system.
For new pictures, the world had to wait for the arrival of Galileo in 1995. This was named after Galileo Galilei, the discoverer of Jupiter's four large satellites, and was the first mission to go into orbit about a giant planet. It swung several times past each of the large satellites in order to produce higher resolution and more complete image coverage than the Voyagers had managed and was still functioning in 2003. Galileo also dispatched a probe into Jupiter's atmosphere. This survived for over an hour reaching nearly 160 km below the cloud tops, where the temperature was 153°C and the pressure was 22 times that at sea-level on Earth. Entering the atmosphere at 6.5° north of the equator, the probe demonstrated that the westerly wind continues below the cloud tops, with its speed actually increasing to about 190 m per second at the depth where the probe lost contact.
The next mission with Jupiter as its goal will probably be one targetted specifically at Jupiter's satellite Europa, listed as Europa Orbiter in Table 9.1. This will initially go into orbit around Jupiter itself, possibly in 2010 [this mission has now been cancelled]. However, by means of multiple gravity assist fly-bys of Europa and other satellites it will ease itself into a position from which it can be captured into a high inclination orbit about Europa. There may be an earlier opportunity to collect close-up data on the Jupiter system in March 2007 when a Pluto-bound probe called New Horizons is provisionally scheduled to swing past Jupiter on a gravity assist trajectory.
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