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Perseverance Rover lands on Mars [video]

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Re: Perseverance Rover lands on Mars [video]

#331

Post by sojournerc »

Melkman wrote: post #307
mulmen wrote: post #269

Earlier quoted context omitted.

Ever been on an airplane? Altitude was almost certainly measured in kilofeet. As noted in other comments, NASA (like the rest of the United States [1]) does use the metric system. But it doesn’t matter. Nothing about the metric system makes it uniquely suitable to landing on Mars. Or space travel in general. What matters is a consistent standard. Internally NASA could use Armstrongs. Where 1 is the weight or height o…

Consistency is not the only value of a system of units. Convenience is also of importance. And that is where the metric system shines. Having all measures of a unit in multiples 10 combines perfectly with our decimal calculations. Having as few as possible magical numbers to convert and combine between units makes making mistakes harder. How many calories in kinetic energy has a pound going 10 miles per hour ? I know…

As an occasional woodworker and carpenter, I can tell you having evenly divisable (inch) measurements makes mental division a whole lot easier. It's just a case of using the right tool for the job.

Re: Perseverance Rover lands on Mars [video]

#332

Post by bernulli »

mulmen wrote: post #309
dieortin wrote: post #296

Earlier quoted context omitted.

It does matter. See the errors caused in the past by using the imperial system. The metric system has a number of advantages.

The errors were not caused by the imperial system. The errors were caused by using the imperial system and the metric system. Specifically in expecting one system and getting the other.

The thing is, though, that you have these conversions even within the imperial system (https://en.m.wikipedia.org/wiki/Imperial_units#/media/File%3...) but not within the metric system.

My experience with U.S. students is that they are having a much harder time making sense of the imperial system (that they are used to) than doing problems in metric, even though they don’t use it in everyday life.

Re: Perseverance Rover lands on Mars [video]

#334

Post by [deleted] »

sojournerc wrote: post #331
Melkman wrote: post #307

Earlier quoted context omitted.

Consistency is not the only value of a system of units. Convenience is also of importance. And that is where the metric system shines. Having all measures of a unit in multiples 10 combines perfectly with our decimal calculations. Having as few as possible magical numbers to convert and combine between units makes making mistakes harder. How many calories in kinetic energy has a pound going 10 miles per hour ? I know…

As an occasional woodworker and carpenter, I can tell you having evenly divisable (inch) measurements makes mental division a whole lot easier. It's just a case of using the right tool for the job.

[deleted]

Re: Perseverance Rover lands on Mars [video]

#335

Post by sephamorr »

hedgehog wrote: post #122

I read that the design life of the helicopter is five flights. Does anyone know what the limiting factors are? The brutal cold and abrasive dust both seem like they could contribute but I am curious what the real answer is.

The helicopter doesn't have enough power generation to stay warm through the Martian winter; the solar panel that feeds it is extremely small, so at the very least, it is unlikely to last beyond the summer. Here's the thermal design paper for the helicopter:

https://ttu-ir.tdl.org/bitstream/handle/2346/74036/ICES_2018...

Re: Perseverance Rover lands on Mars [video]

#336

Post by danaliv »

killjoywashere wrote: post #325
Melkman wrote: post #307

Earlier quoted context omitted.

Consistency is not the only value of a system of units. Convenience is also of importance. And that is where the metric system shines. Having all measures of a unit in multiples 10 combines perfectly with our decimal calculations. Having as few as possible magical numbers to convert and combine between units makes making mistakes harder. How many calories in kinetic energy has a pound going 10 miles per hour ? I know…

Can’t talk to the statute mile, but the nautical mile is sublime: one minute of latitude. The amount of math you can do in your head with a system with so many factors of 2, 3, and 5 is truly amazing.

Also the average of one minute of arc on a great circle route, which was the real handy thing about it at sea.

Re: Perseverance Rover lands on Mars [video]

#337

Post by mulmen »

bernulli wrote: post #332
mulmen wrote: post #309

Earlier quoted context omitted.

The errors were not caused by the imperial system. The errors were caused by using the imperial system and the metric system. Specifically in expecting one system and getting the other.

The thing is, though, that you have these conversions even within the imperial system ( https://en.m.wikipedia.org/wiki/Imperial_units#/media/File%3... ) but not within the metric system. My experience with U.S. students is that they are having a much harder time making sense of the imperial system (that they are used to) than doing problems in metric, even though they don’t use it in everyday life.

Ok, so?

First off, you linked to a list of english measures which are not used in the US. Nobody uses fathoms or barleycorns.

Here is the list of actual US customary units: https://en.m.wikipedia.org/wiki/United_States_customary_unit...

Second, none of that is relevant to landing on Mars.

The only problem space where metric has an advantage is in converting between meters, kilometers and millimeters.

That’s great, and it’s easy to learn. But it doesn’t suddenly make all problems of distance easier to solve.

If I am traveling toward Mars at 47 meters per second it doesn’t help me to know that is also .047km per second. And converting to kilometers per hour involves using base 60 twice anyway because metric time is unwieldy.

In reality none of your measurements are going to be nice round numbers. Mentally converting from meters to km might be nice sometimes but it’s essentially a party trick.

It won’t help the lander make decisions. The hardware doesn’t inherently work in base 10.

Does NASA mix meters and kilometers? Isn’t that the same problem that destroyed the Mars Climate Orbiter?

The fact is the units are irrelevant beyond just being defined and used consistently.

Also, I can’t think of a situation where I need to convert miles to feet. My bike ride is six miles, I’m never going to express that in feet. If I need to describe the size of a thing in a room I will probably use feet, maybe inches if it is small. Probably not feet and inches. I wouldn’t use miles at all. Easy conversion between those units just isn’t a problem that comes up. It’s more important to me to have reasonably sized units and that the person I am communicating with understands them.

Re: Perseverance Rover lands on Mars [video]

#338

Post by mulmen »

sojournerc wrote: post #331
Melkman wrote: post #307

Earlier quoted context omitted.

Consistency is not the only value of a system of units. Convenience is also of importance. And that is where the metric system shines. Having all measures of a unit in multiples 10 combines perfectly with our decimal calculations. Having as few as possible magical numbers to convert and combine between units makes making mistakes harder. How many calories in kinetic energy has a pound going 10 miles per hour ? I know…

As an occasional woodworker and carpenter, I can tell you having evenly divisable (inch) measurements makes mental division a whole lot easier. It's just a case of using the right tool for the job.

It’s unfortunate that fractional measurements and the base size of units get conflated.

Maybe metric users do use fractions and I just don’t hear about it. Is that table one and a quarter meters high?

Re: Perseverance Rover lands on Mars [video]

#339

Post by jtsiskin »

unanswered wrote: post #196
DiogenesKynikos wrote: post #185

Earlier quoted context omitted.

It depends on what coordinate system you're using. Simultaneity is ill defined in relativity. There's only future, past and "spacelike-separated" (neither past nor future). When they say, "X is 1 minute from happening," it's actually neither in the past nor the future. It's currently spacelike-separated, but in 1 minute, it will be in our past.

Yes, yes, you have shown you know what relativity is. But the relative velocity of earth and mars — which I can't convince Wolfram Alpha to tell me, but it's got to be on the order of their orbital velocity so let's say 5x10^4 mph — is a tiny tiny fraction of c so their inertial reference frames are essentially identical. So sitting in our reference frame, we can make inferences about what's happening "now" on mars,s…

No, I don’t think you’re quiet understanding what they are saying. They aren’t talking about the different speeds of earth or Mars.

Simultaneity is not “perfectly well defined in a single intertidal reference frame”. That is just a convention.

If the RTT of earth to Mars is 20 minutes, then we can say that it takes us 20 minutes for our message to reach the rover, and the rover’s message arrives instantly, and that’s a consistent definition of simultaneity.

https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/si...

Re: Perseverance Rover lands on Mars [video]

#340

Post by FredFS456 »

xarope wrote: post #318
theelous3 wrote: post #311

Earlier quoted context omitted.

What I'm getting from this, is that you can use weights to balance the mass of the spaceship during cruise conditions.

Ships do this all the time (ballast), and anybody who's ever flown on a light aircraft or helicopter also knows the importance a pilot places on weight distribution. I guess what's surprising is that they needed that much weight (140+kg seems like a lot?) and couldn't redistribute existing componentry; guess the knapsack algorithm wasn't good enough, or that they just couldn't break up enough pieces? And yes, Cruise…

They can't redistribute existing componentry because of competing requirements: during cruise, the spacecraft needs to be balanced around the rotational axis (perseverance rotates at 2RPM in cruise). During re-entry, an asymmetric weight distribution is needed to generate lift.
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