In The Cat Who Walks Through Walls (1985) by Robert A. Heinlein, a novel approach to measuring mass in zero G used.

For a mass reading Budget Jets does not use a centrifuge but the newer, faster, cheaper, much more convenient, elastic inertiometer-I just wonder if it is as accurate. Dockweiler had us all get into the net at once (all but the bonsai, which he shook and wrote down as two kilos-near enough, maybe), asked us to hug each other with the Macy's package held firmly amongst us three, then pulled the trigger on the elastic support-shook our teeth out, almost; then he announced that our total mass for lift was 213.6 kilos.

Did he invent the concept, or can it be found in earlier works?

  • Great question, though AFAICT, astronauts use the same basic device but only need to measure half an oscillation, if that. The SLAMMD, for example, simply extends the measuring arm/spring out, gives the astronaut time to brace himself and wrap his body as close to the spring arm as possible (any excess movement will introduce errors), and then lets the spring contract. An optical sensor measures the acceleration of the spring arm as it contracts, giving the astronaut's mass. Commented Aug 1, 2013 at 16:18
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    @Lèsemajesté there is a related question at space.stackexchange.com/questions/911/… you might be insterested Commented Aug 1, 2013 at 16:37

2 Answers 2


I was trying to find if NASA did any zero-G mass measurements prior to 1985 and came across this:

Dr. Thornton returned to active duty with the United States Air Force and was then assigned to the USAF Aerospace Medical Division, Brooks Air Force Base, San Antonio, where he completed the Primary Flight Surgeon’s training in 1964. It was during his two-year tour of duty there that he became involved in space medicine research and subsequently applied and was selected for astronaut training. Dr. Thornton developed and designed the first mass measuring devices for space, which remain in use today.

At first I couldn't find any description of Dr. Thornton's measuring device, but Heinlein's "elastic inertiometer" is a fairly obvious, though still elegant, solution to the problem that surely a scientist like Dr. Thornton with a BS in physics would have come up with on his own.

Coincidentally, the SLAMMD (Space Linear Acceleration Mass Measurement Device) I mentioned in my comment is used primarily to measure the body mass of astronauts to study how zero-G affects astronauts on the ISS, and this is also what Dr. Thornton specialized in as an astronaut.

And I eventually found this patent application for a "NONGRAVIMETRIC MASS DETERMINATION SYSTEM". Its description:

A non-gravimetric mass measurement system having a support structure for holding a mass to be "weighed" attached to an oscillating spring assembly. A device for indicating precisely when the oscillating mass crosses point of zero displacement produces a signal which is sent to a counter for determinating the time period. A substantially frictionless air bearing serves to restrain the motion of the oscillating mass to a single axis under zero gravity conditions in space while serving to support the weight on earth.

This patent was issued Jan. 19, 1971. It was filed May 20, 1968, so quite a bit before the "Cat Who Walks Through Walls" was published.


Albert Einstein wrote something on it some years earlier (1907), in his "Über das Relativitätsprinzip und die aus demselben gezogene Folgerungen". A fascinating work of science fiction, a bit on the hard side (he was very good at providing a plausible world, but didn't indulge in psychological characterization of his characters).

  • Can't believe no one else has commented yet. Great answer! Made me laugh, and then think, and then laugh again.
    – Anthony
    Commented Aug 2, 2013 at 3:41
  • Thanks @AnthonyArnold, I am happy that you enjoyed it. It is always a good thing to go back to the works of the masters and - I am no joking this time - Einstein's works are always very clear and instructive. Sometimes the "original" works are too obscure and you need to "wait" for someone else to clarify them. But in the case of Einstein, he was a master explainer, not only a master scientist.
    – Francesco
    Commented Feb 5, 2016 at 13:38

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