A Rant About Kilograms On Mars

An Eccentric Anomaly: Ed Davies's Blog

The CACM have published a reasonably interesting article by Gerard J. Holzmann about verification of the software on the Curiosity Rover: Mars Code.

Near the beginning, though, it says:

On Earth, Curiosity weighed 900 kg. It weighs no more than 337.5 kg on Mars because Mars is smaller than Earth.


On Earth, Curiosity had a mass of 900 kg and weighed 8830 newtons. On Mars, it still has a mass of 900 kg but weighs 3310 N.

Pounds have dual use as both units of mass and weight. Kilograms formally only measure mass and should be kept free of this confusion except in the most casual circumstances and when it's unambiguous what gravitational field is being considered. Most branches of Tesco have a gravitational acceleration of about 9.81 m·s⁻² (or a force of gravity of 9.81 N/kg) so there's not likely to be much carnage as a result of them saying some cheese has a “weight” of 0.324 kg. It still irritates though, until there's an acceptable verb “to mass” in English, it's understandable.

But for an agency which has previously inadvertently crashed a spacecraft into a planet by not being clear about pounds and newtons to extend this sort of muddle to a semi-technical paper is, in my picky opinion, unforgivable.