Reply To: What if?
Hmmm… I’ll start with something that may seem irrelevant to that question.
There is popular confusion over the term “orbit” I think. Technically even a flyby is an “orbit”, it’s a hyperbolic one. That’s the trajectory something follows when it is moving at greater than escape velocity, and it’s symmetrical, something coming “inbound” is moving with the same energy as it will when outbound (and when it’s a given distance away on the inbound leg, it’s moving at the same speed, relative to the primary, as it will be on the way out, at that distance). A hyperbolic orbit, however, doesn’t repeat, it goes off to infinity in either time and space. But usually “orbit” is used to mean something going around and around the other object in a repeating pattern, which is some sort of an ellipse (a perfect circle being a very special case of an ellipse). If you are just below escape velocity, it will be a very large ellipse. Of course that’s the “two body” treatment of the matter; there are more than two objects in our universe, and if the other objects are close and/or large enough, you can see things like objects moving at greater than escape velocity being “captured” into (elliptical) orbits and the like.
Satellites are objects that are in an (elliptical) orbit about another body, and New Horizons was never in such an orbit about Pluto, so I have to say that part of Davis’s statement is wrong.
By the way, when I mentioned hyperbolic (flyby) orbits, I described them in such a way that makes it seem like you can’t get a “gravitational boost” or “slingshot” from a planet (such as New Horizons got from Jupiter) but that’s not true. You don’t gain speed with respect to the planet, but you *can* gain it with respect to the Sun. You have to fly “behind” the planet in its orbit about the sun, and your trajectory ends up bent in the direction of the planet’s motion, your speed plus the planet’s orbital speed added together. If you cross the planet’s orbit in front of it, you will end up slowing down. Here’s Wikipedia on the subject: https://en.wikipedia.org/wiki/Gravity_assist and this animation http://www.theplanetstoday.com/voyager_flight_path.html shows what happened to Voyagers 1 and 2. (There doesn’t seem to be any way to pause, zoom, or restart other than to reload the page, and the planets are drawn too large to make it clear which side of the planet the flyby is.) Slingshots can only be understood by being willing to think in terms of multiple frames of reference. (I tried finding a satisfactory youtube video to *show* this rather than describe it in words, but gave up after three tries.)
