Jupiter, the nearest, of the giant, planets, in also the largest and most massive. It appears brighter than any other planet apart from Venus; at times it may even cast, a shadow. Some of its surface features, and the four brightest satellites, can easily be seen in a small telescope, or even in binoculars. .Jupiter’s mass is more than twice that of all the other planets put together, arid it m determined by observing the periods of .Jupiter’s satellites and the sixes of their orbits and applying Newton’s law of gravitation to the data It is not essential to use observations of a natural satellite; similar calculations can be performed for any body that passes near the planet. For example, the spacecraft Pioneers 10 arid 11 flew pant on 4 December 1973 and 3 December 1974 respectively. The radio transmitters on these craft broadcast on accurately known frequencies, so that by observing the Doppler shift in the received signal the velocity of the craft, along the line of sight could be calculated. This velocity changed aw each spacecraft was attracted by Jupiter’s gravitational field and the Doppler shift measurements were then used to calculate the mass. These observations show that Jupiter has a mass 317.89 times that of the Earth. How¬ever, although Jupiter is such a massive planet we should note that it is still much smaller than the Sun, whose mass is 1047 times greater.

Jupiter is noticeably flattened. Its equatorial radius is 71 600km, which is 11.23 times that of the Earth, whereas the polar radius is 67300km, about six per cent, smaller. The volume of Jupiter is therefore 1330 times that of the, Earth. Since its mass is only 318 times that of the Earth its mean density is lower: 1314kgm-3 compared to 5518 kg m-3. because the density of Jupiter is so much lower than that of the Earth and the other terrestrial planets it cannot possibly have, a similar composition. If Jupiter did have a terrestrial composition the higher internal pressure caused by its greater mass would result in a greater density than that of the Earth and not a lower one.

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