Near-Earth Objects: Finding Them before They Find Us

By Donald K. Yeomans | Go to book overview

CHAPTER 3
How and Where Do Near-Earth
Objects Form?

Comets and asteroids are the leftover bits and pieces from the
early solar system formation process.


The Gravity Assist

Mother Nature is conservative, especially when it comes to the orbital energies of her solar system bodies. During any encounter between two bodies in space, energy is conserved in that if one body gains energy during an encounter, the other one loses an equal amount of energy. For example, when the Voyager I spacecraft careened past Jupiter in 1979, the spacecraft received a huge boost in its orbital energy and Jupiter suffered an equal orbital energy loss. Of course since Jupiter is two trillion trillion times more massive that the spacecraft, it suffered far less of an effect than a speeding semitrailer truck would feel from the gravitational effect of a close approach by a housefly. Nevertheless, the same conservation of orbital energy was key to the initial formation of the solar system when millions of planetesimals made encounters with the far larger planetary bodies.

A planetesimal's orbital energy is proportional to its orbital semimajor axis. A body whose orbital energy is increased will have its semimajor axis increased proportionately. Its orbit will expand in size. Planetesimals orbiting the Sun in the neighborhood of a massive planet can, from time to time, pass close by this planet and in doing so

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