Weighing things in space is complicated without gravity

Mar 4, 2013 21:21 GMT  ·  By

The ISS astronauts will spend the next few days unloading the supplies aboard the Dragon and then a few more loading it with stuff that must get back to the ground, the science experiments that have run their course.

Since the Dragon is currently the only unmanned spacecraft capable of returning to Earth, this part of the mission is crucial.

But loading a bunch of experiments and other cargo into the Dragon spacecraft isn't as straightforward as it may seem.

"We work with a team on the ground, item by item, to control Dragon's weight and balance. It hugely affects how it flies home," astronaut Chris Hadfield explained.

It makes sense, weight distribution is important on a cargo truck, let alone a craft that has to maneuver in space while flying at huge speeds and then splash down into the ocean within a relatively small target area.

But this begs the question, how do you weigh something in space? Everything coming aboard the ISS is carefully weighed, but not everything going back will have a record, some things will need to be weighed.

There's no gravity on the ISS so conventional means won't work. But even a weightless object has mass and that can be measured.

In fact, astronauts employ one of the most basic principles in physics, Newton's Second Law of Motion, which states that force is equal to mass times acceleration. Therefore, to measure mass, you apply a known force and measure the acceleration.

That's what the SLAMMD (Space Linear Acceleration Mass Measurement Device) instrument does aboard the ISS. SLAMMD was designed to weigh astronauts, to keep track of the effects zero gravity has on their bodies.

The device actually looks like fun, though pretty much anything you can do in space is going to be a lot more fun than back on Earth.

An older version of the same type of instrument