periapsis

From peri- +‎ apsis.

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periapsis (plural periapsides or periapsises or periapses)

1. (astronomy, astrophysics) The point of a body's (elliptical) orbit in a 2-body gravitational system such that the distance between the two bodies (measured from their respective centres of mass) is at its minimum.
   • 1983, Richard O. Fimmel, Laurence Colin, Eric Burgess, Pioneer Venus, NASA SP-461, page 109,

     Figure 5-19. Sun-Venus-Orbiter geometry illustrates how the periapsis moves around the planet during the Venusian sidereal year to sample day and night hemispheres. Because the planet rotates in a retrograde direction, it takes more than one Venusian sidereal year for periapsis to move over all longitudes of the planet (as explained in text).

   • 2001, D. A. Vallado, Fundamentals of Astrodynamics and Applications, Springer, page 325:

     We don't always want to burn near periapsis. Suppose we want to change the height at periapsis. The most efficient place to change the radius of periapsis is at apoapsis. Consider applying ${\displaystyle \Delta v}$ at some other point in the orbit. In this case both periapsis and apoapsis will change. If we apply the ${\displaystyle \Delta v}$ tangentially at apoapsis, all of the ${\displaystyle \Delta v}$ will go into changing periapsis, and vice versa.

   • 2004, R. S. Saunders, et al., Mars Odyssey Mission, C. T. Russell (editor), 2001 Mars Odyssey, Kluwer Academic Publishers, page 34,

     The main aerobraking phase began once the point of the spacecraft's periapsis had been lowered to within 110 km above the Martian surface.

2. The minimum distance between the two bodies in such a system.

• In the case of some particular celestial bodies, specialized terms are used. See Related terms, below, for examples.

• pericenter

• apoapsis

• apsis

• argument of periapsis
• longitude of periapsis

• Apsis on Wikipedia.Wikipedia
• Apsidal precession on Wikipedia.Wikipedia

Internationalism (see English periapsis).

• IPA^(key): /ˈperiɑpsis/, [ˈpe̞riˌɑ̝ps̠is̠]
• Rhymes: -ɑpsis
• Syllabification^(key): pe‧ri‧ap‧sis

periapsis

1. (astronomy) periapsis