Trajectory Terminology...

Is there a term for when a spacecraft makes its closest approach to a heavenly body on a fly-by?


Periapsis. “Perigee” is the closest approach to Earth, just as “perihelion” is the closest approach to the Sun. “Periapsis” is generic.

Note, though, that periapsis refers specifically to the point of closest approach in a Keplerian orbit around a body of interest.

If the fly-by is just a fly-by, the term may not fit. For instance, a satellite taking a look at Titan could be in orbit around Saturn and may even pass inside of Titan’s orbit around Saturn. The satellite will certainly have a point of closest approach to Titan, but it wouldn’t be a periapsis. I think you’d just call it the “point of closest approach”.

Thanks, Pasta! Kind of disappointing not to have a term for this, huh? :frowning:

Someone could very well still show up with a term, but I can’t think of one myself that’s completely general.

I am pretty sure the standard term just is “closest approach.” It may not be fancy, but it is accurate, generally applicable, and quite succinct.

And as a side comment, the term “periapsis” and its brother “apoapsis” are etymological bastards. The point of closest approach and the point of furthest separation are themselves the apses, not the body that they’re orbiting. So if one tried to take the words literally, they’d mean something like “the point where the satellite is closest to itself” and “the point where it’s furthest from itself”.

If it’s a NASA craft, the term you’re looking for is “crash.”