Orbital Velocity… and great analogies

by Aug 13, 2013

One of my favorite web-comics, xkcd, did an excellent piece titled “Orbital Speed“, in which they answered a few readers’ questions:

What if a spacecraft slowed down on re-entry to just a few miles per hour using rocket boosters like the Mars-sky-crane? Would it negate the need for a heat shield?

Is it possible for a spacecraft to control its reentry in such a way that it avoids the atmospheric compression and thus would not require the expensive (and relatively fragile) heat shield on the outside?

Could a (small) rocket (with payload) be lifted to a high point in the atmosphere where it would only need a small rocket to get to escape velocity?


These are great questions, and the guys at xkcd did a really good job answering them with not only text-based factual explanations, but also a wonderful comic-based visual explanation.


geo-animatedThe bottom line is that staying in orbit about a planetary body (Earth!) is more about VELOCITY (i.e., speed) than it is about ALTITUDE.

Yes, there’s a minimum altitude required to get above the very draggy atmospheric effects, but there’s a difference between a sub-orbital launch (e.g., the first two Mercury program launches of Alan Shepard and Gus Grissom) or an X-15 extra-atmospheric venture and a true Earth-orbiting vehicle (e.g., Space Shuttle, ISS, etc)… and that difference is HOW FAST IT IS GOING!

I’ve given talks to students in the past, and the example I have always used is the fact that the Space Shuttle’s orbital velocity was approximately 25,500 fps (or ~17,500 MPH) and the deorbit burn delta-V to bring the Shuttle out of orbit back into the atmosphere was only ~350 fps (or ~240 MPH).

That’s just a smidge over


between being in orbit…

and “not being in orbit”.

One. Percent.