The question "Orion re-entry velocity: Why is it higher than Apollo?" has an unstated but critical assumption: that re-entry is measured at the same point for both missions. The point selected to define "re-entry" is fairly arbitrary: for example, you can orbit with a periapsis below Karman line if your orbit is eccentric enough.

I've been able to find that, for Apollo, "entry interface" was an altitude of 400,000 feet, but I haven't been able to find a definition for Orion (or any other NASA spacecraft). Has it been consistent over time?


1 Answer 1


It’s also 400K feet for Orion:

EI is defined at a geodetic altitude of 400 kft

Orion Exploration Mission Entry Interface Target Line (Rea, Jeremy R., February 2016) p. 1

...as it was for shuttle:

Entry interface, the point where the vehicle enters the atmosphere, is considered to occur at an altitude of 400,000 feet, approximately 4,200 nm from the landing site, and at a velocity of approximately 25,000 fps. The orbiter is maneuvered to 0° roll and yaw (wings level) and a 40° angle of attack for entry. The flight control system issues the commands to roll, pitch, and yaw RCS jets for rate damping.

Shuttle Crew Operations Manual p. 1.1-3

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    $\begingroup$ And also Apollo. That 400 kft definition of EI is a bit arbitrary, but is consistently so. $\endgroup$ Commented Nov 29, 2022 at 4:23
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    $\begingroup$ 400 kft for Apollo is stated in the question, although unsourced. $\endgroup$ Commented Nov 29, 2022 at 12:22

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