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The notion of inertia had been well-established by the time Newton began his formulations of the three laws of motion. Dramatic evidence of the fact that even objects subjected to tremendous forces maintain a straight line motion unless acted upon by some external force that differentiates between them is shown in the ``string-of-pearls'' comet, Shoemaker-Levy-9, before its demise. A year before the comet crashed into the planet Jupiter, it made a close pass to the planet. Jupiter's powerful gravitational field provided tidal forces which broke up the comet nucleus into about 21 pieces. These pieces, however, didn't scatter in different directions as might be expected in a terrestrial environment. Instead, in the absence of other forces expected in space, each piece maintained the same trajectory as each other piece (as shown in the artist's rendition below or the real images from the Hubble if you click on the image) up to their fiery end in Jupiter's atmosphere.
You can find other images of the comet's demise here.
Space sciences also benefit enormously from Newton's Laws. Application of these laws allows us to send spacecraft on journeys of millions of miles and end up accurately placed in position to observe planets and moons. As we shall soon see, one of the principle problems with such long journeys is the enormous travel time and the tremendous amounts of kinetic energy they require. Scientists have figured out a method for aiding such journeys however, using what is called the slingshot effect. The effect works by placing a planetary probe onto a path which allows it to absorb a very tiny portion of the almost incalculable kinetic energy present in the orbital motion of the planets themselves. Essentially, the gravitational attraction of a large planet, like Jupiter, on the probe is used to whip the probe around the planet and ``slingshot'' it to its next destination. Essentially every planetary probe targeted for Jupiter or beyond makes use of this method to gain sufficient kinetic energy to overcome the tremendous gravitational pull of the Sun as the probe moves away from earth.
Calculate the effect of a slingshot encounter on
a spacecraft approaching Jupiter in exercise
3-12.