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.

1 Comment

  1. Anna

    Hello Roger, this is my second cmmoent to you.I have always followed NASA. As a child me and my brothers watched the first landings. I wish there were videos of everything that was televised a set of DVDs. Most of the DVDs that I’ve seen were incomplete!I’m still hoping to be selected for astronaut training with my first application, but my age is 52. After reading my brief profile, would you answer the following question for me. The question is what do you think my chances are of getting selected, and why?Since December 1996, I have been very athletic. My weight-lifting routine involves lifting for a total of 4 hours a week on 4 different days. I do aerobics (jogging and I use a rowing machine to cross-train [prevents accidents]) for 30 minutes each day.To try to improve my life I take about 60 different supplements each day. All of my over the counter supplements have been approved by two good doctors! Anyway, I’m hoping NASA needs a very good software engineer with some electronic experience (discrete components) thrown in. Some of my professional accomplishments are that I built most of the support software for a new large missile testing facility, and I’m currently building support software for the F22 program. The skill that I have to offer NASA is that I can program anything in any well known computer language to do anything.Most of my professional work involves building my own programs from scratch.All the professional jobs that I have had were pre-leadership roles.At home, I have an expensive Lab-Volt system (it costs about $10k new) to keep-up on my electronic university skills. I have three MS degrees in CS (Computer Science), ECE, and a bridge between the two. I have two bachelor degrees. My first BS degree was in geology, and my BA degree is in CS.I was in college for 18 years. And, I lived in the dorms for 13 and 1/2 of those years.My collection of rocks, minerals, and meteorites is in the hundreds.I wish you luck.Will you wish me luck as well!Christopher M. Lusardi


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