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Perseverance Rover lands on Mars [video]

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Re: Perseverance Rover lands on Mars [video]

#181

Post by PeterisP »

unanswered wrote: post #167

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"? Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X ha…

It's the latter. The Earth-Mars latency at this time is something like 11 minutes, and the landing itself takes about 7 minutes, so when we on Earth first saw the craft entering atmosphere on Mars, by that time all the landing was already over, one way or another.

Re: Perseverance Rover lands on Mars [video]

#182

Post by cjohnson318 »

OliverGilan wrote: post #175
unanswered wrote: post #167

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"? Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X ha…

Doesn’t relativity tell us it doesn’t matter?

There's a lag time in communication due to distance. I don't see what that has to do with relativity.

Re: Perseverance Rover lands on Mars [video]

#183

Post by prox »

raylus wrote: post #152
notum wrote: post #58

Any JPL/NASA HN users that could comment? That would be beyond awesome! Great job, mission!

JPL Engineer here, any questions I could convey to the slack I'll be glad to feed back to HN

I just want to thank you all for the wonderful time tonight, we’ve been watching with the family. Amazing accomplishment!

Re: Perseverance Rover lands on Mars [video]

#185

Post by DiogenesKynikos »

unanswered wrote: post #167

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"? Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X ha…

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.

Re: Perseverance Rover lands on Mars [video]

#186

Post by unanswered »

Sharlin wrote: post #171
unanswered wrote: post #167

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"? Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X ha…

Yes, those are all Earth Receive Time, that is, when they were saying that eg. entry interface is two minutes away, in reality the rover was already sitting on the surface and we were just waiting for the radio signal to get here.

> Earth Receive Time

Ah great, that's a great phrase to make everything clear and provide a kind of "frame of reference" to think & communicate in. Always need these abstractions.

Re: Perseverance Rover lands on Mars [video]

#187

Post by txg »

POiNTx wrote: post #81
kaycebasques wrote: post #78

Earlier quoted context omitted.

N00b question: why is it black & white?

My guess is lower image size, which means image can get transferred faster.

This is the right answer. The camera (and its 8 siblings) are capable of color HD imaging - the sensor has a Bayer filter. This image used a binning mode to produce a downsampled frame that could be more rapidly transferred back over the lower bandwidth comms used during landing. Binning combines the Bayer pattern and so color information is lost.

Also doesn't help that there is a (transparent) lens cover in front of the lens obscuring the view.

Re: Perseverance Rover lands on Mars [video]

#189

Post by whuffman »

nwallin wrote: post #177
kaycebasques wrote: post #78

Earlier quoted context omitted.

N00b question: why is it black & white?

Other posters have pointed out that it's the hazard avoidance camera, but they haven't said why the hazard avoidance camera is black and white. When you do computer vision, the first step you do is convert your color image into a black and white image, and run your CV algorithms on the black and white image. This is because when you're looking at objects and shapes and stuff, it's contrast that tells you where the bo…

Just as a heads up, the HazCams on Perseverance are in fact in color (Source: https://link.springer.com/article/10.1007/s11214-020-00765-9 - "The Mars 2020 Navcams and Hazcams offer three primary improvements over MER and MSL. The first improvement is an upgrade to a detector with 3-channel, red/green/blue (RGB) color capability that will enable better contextual imaging capabilities than the previous engineering cameras, which only had a black/white capability.") Your observations are correct though - the stereo precision is important, so there was additional analysis of the stereo depth computation to make sure it wouldn't cause an issue.

Re: Perseverance Rover lands on Mars [video]

#190

Post by runarberg »

OliverGilan wrote: post #175
unanswered wrote: post #167

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"? Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X ha…

Doesn’t relativity tell us it doesn’t matter?

PBS Space Time recently explained what the present time means within general relativity[1]. As I understand it... it matters in this context.

[1]: https://www.youtube.com/watch?v=EagNUvNfsUI

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