Jupiter
Ulysses takes advantage of Jupiter's pulling power : The primary goal of the Ulysses mission is to explore for the first time the region of space above the poles of the Sun. Although the importance of this exploration to our understanding of the Sun's environment, the heliosphere, has long been recognised, the practical implementation of such a mission has, until relatively recently, been impossible.

• While we are able to place satellites into a polar orbit around the Earth without much difficulty, the energy needed to launch a space probe into a polar orbit around the Sun is far greater. So much energy is required in fact, that even with the powerful launch vehicles available today, direct injection from the Earth cannot be achieved. This is because the Earth itself orbits the Sun at a speed of 30 kms-1 in a plane perpendicular to the desired solar polar orbit. The energy imparted to a space probe must cancel out this motion in addition to providing the correct polar trajectory.

• True-colour simulated view of Jupiter : A polar orbit can be achieved, however, by taking advantage of a gravity assist by another planet. Jupiter is the nearest body capable of meeting the requirements. The need to make use of Jupiter in order to carry out its primary mission has resulted in the recent flight of the ESA-built Ulysses spacecraft through the Jovian magnetosphere. Even with this gravity assist, the combined power of the space shuttle and three upper-stage rockets were needed to send the 370 kg space probe on its way. As it left the confines of the Earth's gravitational field, Ulysses was travelling at 11.3 kms-1, making it the fastest interplanetary spacecraft ever launched.





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