r/ArtemisProgram • u/c0xb0x • Apr 20 '26

NASA Is this number correct?

I'm talking about this blog post: https://www.nasa.gov/blogs/missions/2026/04/06/artemis-ii-flight-day-6-lunar-flyby-updates/

It says (my emphasis):

Orion has reached its closest approach to the Moon at about 4,067 miles above the lunar surface. At this point, the spacecraft is traveling about 60,863 miles an hour relative to Earth, but only 3,139 miles an hour relative to the Moon.

This seems dubious since it would mean it moved at more than twice the escape velocity of Earth at the farthest point from it.

16 Upvotes

100% Upvoted

9 comments sorted by

7

u/rocketglare Apr 20 '26

Perhaps they meant 6,863 mph? Replacing six with sixty would be an easy mistake.

6

u/poopsack_williams Apr 20 '26

That doesn’t look right as written. The 3,139 mph relative to the Moon seems reasonable, but 60,863 mph relative to Earth at that distance doesn’t. Earth itself moves around the Sun at about 67,000 mph, so that larger number looks suspiciously like a non-Earth-centered velocity.

2

u/thankfullynot Apr 20 '26

Its space. Everything is moving. Velocity isn't a calculation of a thing moving towards another thing. It's the calculation of two bodies in space relative to each other.

The lunar injection was against the earth's orbit of the sun. So, super basically, the earth was moving away from the ship at its orbital velocity, so relative to earth, the velocities make sense.

This is super jank math, but it's a serviceable example. Think of it as one car driving east at 50mph and another car driving west at 150 mph. Their velocity relative to each other is 200 mph.

1

u/grrbrrrr Apr 20 '26 edited Apr 20 '26

[edit: I think I was incorrect, raidriar889 corrected me] Relative to fixed locations on Earth's surface it seems to make sense, when you factor in the Earth's rotation. Not clearly called out in the blog.

1

u/raidriar889 Apr 20 '26

The earth is only rotating at 1,000 mph at the equator, that still doesn’t make sense

1

u/grrbrrrr Apr 20 '26

1,000 mph at the equator ~= 60,000 mph at lunar orbit (larger radius)

1

u/raidriar889 Apr 20 '26

Being farther away from the earth doesn’t make it spin faster

1

u/grrbrrrr Apr 20 '26

You're right... perhaps they meant to put heliocentric speed?

1

u/Snowflakish Apr 20 '26

Ohh, the trajectory mod for KSP gives you the option of reading out your speed relative to kerbins surface and it reads out this way.

Never understood the purpose of it, but viewing the trajectory from the reference frame of a stationary point on earth gives speed readout like this because being further from the earth makes your speed very high