One topic that comes up frequently in orbital dynamics is the Tisserand parameter. So as an interested layman, I'm curious about it, and have several questions about it, like "what is the Tisserand parameter?" or "how is it useful?". Well, first, here is it's definition:
As you can see, it relates the orbital parameters semimajor axis a, eccentricity e and inclination i of a perturbed body (i.e. a small body encountering a larger body) to each other and to the semimajor axis of the perturber ap, like, say, Jupiter, or any larger body in the Solar System. The result is the dimensionless value called the Tisserand parameter Tp, or Tisserand's criterion.
The nice thing about it is that it is a quasi-conserved quantity that stays more or less the same before and after a close encounter of an asteroid, a comet or a space probe with a large body. That makes it a useful tool to identify prior visits of periodic comets, whose orbital parameters may have changed in the mean time, but since the Tisserand parameter stays about the same, old observations can be searched for a similar value.
Another use is the shaping of spacecraft trajectories, since each of the orbital parameters changes through a flyby in a predictable way. It is in fact this property of the Tisserand parameter that made achievements like the Voyager's Grand Tour or Cassini's epic tour of the Saturn system, possible. The former through flybys of all the giant planets, the latter case through regular flybys of Saturn's largest moon Titan.
OK, that's what it does, but I had a hard time visualizing how it behaves for each of the parameters. So I made the formula interactive:
As you can see, the graph lets you select each parameter a, e, and i separately, and chose a value for the other two. Select any kind of trajectory and see how it's Tisserand parameter changes relative to each of the outer planets, which are represented by a red circle. The currently selected parameters are indicated in the resulting formula at the lower right.
For asteroids the value relative to Jupiter is normally Tj < 3 and for comets within 2 < Tj < 3. If you select semimajor axis, you'll see that Tj only gets close to these values if the bodies' orbit goes anywhere close to Jupiter, and if you play around with e or i, the higher you chose them /(i.e. more comet-like) the lower Tj gets.
Maybe this helps somebody else as well.