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Comment by devb | Hacker News Reader

Remember that the rover weighs almost 2000 pounds, so each wheel is responsible for carrying around 330 pounds on average over very rough terrain.

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Is that the weight on Earth or Mars? Sites listing its specs are unclear on that.

Wikipedia says the dry mass is 899 kg, which is about the 2000 pounds of weight (on Earth) mentioned by GP.

https://en.wikipedia.org/wiki/Curiosity_(rover)

Which would make it about 338 kilos on Mars - wikipedia lists Mars' gravity as 0.376g (vs earth's 1)

Technically that would make it 899kg on Mars, the kilogram is a unit of mass. However the Rover would weigh ~8800N on Earth and ~3300N on Mars.

The pound is a unit of mass, same as the kg (it's defined as exactly 0.45359237kg). Just as 900kg is 900kg everywhere, 2000lb is 2000lb everywhere. The colloquial weight is literally a force, and its imperial unit is thus the pound-force.

This is incorrect. The pound is a unit of force, similar to a Newton. The pound-mass(notated lb-m) is similar to the kg. The conversation you gave above is based on Earth gravity for weight.

> This is incorrect. The pound is a unit of force, similar to a Newton. The pound-mass(notated lb-m) is similar to the kg.

That's the pound-force (lbf).

Pound and pound-mass are aliases. Indeed the official facts sheet defines the mass of the rover in kg and pounds.

> The conversation you gave above is based on Earth gravity for weight.

My comment is based on mass being an intrinsic property of matter, gravity does not enter the equation.

There are about a dozen different units that are all called "pounds," and you have to rely on context to know which is meant. When I took physics in high school, pounds were always force (abbreviated lbf) and we used slugs for mass ( https://en.m.wikipedia.org/wiki/Slug_(mass) )

It's the weight on Earth. Just look for a source that specifies in kg - there's no ambiguity about what the SI unit represents.

No, ambiguity was introduced when the measurement was switched from force to mass. The question remains: what is the weight/force experienced on the wheels on Mars? Knowing the mass won't tell us unless we also know the acceleration due to gravity on Mars.

Thanks for the reminder of scale. Without a reference, it's easy to forget how large this rover is, in pictures it looks very cute.

Here's a good reminder:

  http://i.imgur.com/3Hg9O.jpg

This shows engineering test models of Pathfinder, MER (Spirit/Opportunity), and Curiosity. It was taken in the Mars Yard at JPL, which engineered all three.

Yes, but it looks pretty tore up and the treads seem like they're coming completely off. I was under the impression that the wheels would have been built a little more sturdy. I know it's not the same, but car tires last for thousands of miles. So you would think that a hyper efficient tire, battle tested on earth terrain, would be designed to last at least 100. This is after 10 miles.

Edit: To clarify, I know they are thin and to save on weight. I just never thought it'd be designed to only last 20ish miles. Though another comment has said it will run fine without a lot of this material.

It's been four years, the wear could be as much a function of time as of travel. Car tires on earth dry out, crack and disintegrate over time.

The skin is "about half as thick as a U.S. dime". A car tire that thin wouldn't last a 10 mile drive.

The tires are extremely thin to save on weight. Take a look at a dime and think half of that!

the wheel skin is 0.75mm thick, and was specifically designed to a goal of 10 to 20 km with the smallest weight possible: the rover mass is 899kg, the launch vehicle totals was 531000kg - by reversing the payload fraction one can see that for every gram of rover one needs more than half kilo of rocket to get it to mars, so there's that - you can get outside of your launch weight real fast.

this article has all the answers you're looking for:

http://www.planetary.org/blogs/emily-lakdawalla/2014/0819063...

short story - the wheel were tested to specification against expected rocks etc but once on the boogie suspension the front wheel had to support weight plus the force of the other wheel pushing, so there's where the extra punctures come from, this is what force was not accounted correctly.

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