Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Perseverance Rover lands on Mars [video]

youtube.com

Post reply on HN
Print view View original

191–200 of 517 posts

Re: Perseverance Rover lands on Mars [video]

#191

Post by colechristensen »

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 looked like they were quoting time as it would appear for an earth-local observer (i.e. a million light-year away supernova that showed up five minutes ago happened "five minutes ago" not 1 million years and five minutes ago.

With your personal light cone, it's fine to equate "now" with what you see in the moment. It just has to be clear what you mean for situations where communication might be ambiguous. If you have a person on mars, be sure to be precise what you mean when you tell them to do something in five minutes, when they receive the message they won't know if you mean five minutes after they receive the message or anywhere between 17 minutes before and 2 minutes after they receive the message.

When you get into relativistic speeds (and especially very short time intervals), nobody can even agree on when something "actually" happened, different observers have different opinions about what happens when even after you account for light travel time.

Re: Perseverance Rover lands on Mars [video]

#192

Post by davidmr »

teraflop wrote: post #121
_Microft wrote: post #110

Earlier quoted context omitted.

Starlink would most certainly be of little direct use here. What I could imagine is having Starlink satellites around Mars that allow to route data from rovers anywhere on the planet to a dedicated high-performance communications platform that handles communication with Earth.

In fact that's exactly what they're doing: the Mars Reconnaissance Orbiter is serving as a communications relay, as it did for previous landers. It's just that since there have never been more than a handful of spacecraft active on Mars at any given time, there's currently no point in spending huge amounts of money to launch a whole constellation of satellites for continuous coverage.

And a photographer! MRO took what might be my very favorite picture of all time: https://www.space.com/16946-mars-rover-landing-seen-from-spa...

Re: Perseverance Rover lands on Mars [video]

#193

Post by DonHopkins »

tester756 wrote: post #176
sneak wrote: post #149

Earlier quoted context omitted.

> Watching these is always a stressful experience, I can't imagine what it would be like sitting in that room Sitting in any closed space like that one, with other people, masks or no, is stressful right now. It's a shame that NASA can communicate with a rover 125 million KM away but their staff have to all be crammed into one small enclosed space. You'd think we'd be able to communicate just as effectively over seve…

what if those people were tested for covid and there was no risk?

That's how Trump got it, then spread it.

Re: Perseverance Rover lands on Mars [video]

#194

Post by Daho0n »

erulabs wrote: post #120
thelean12 wrote: post #98

Earlier quoted context omitted.

> Also, remind me, did NASA develop reusable rockets? I can't tell if this is sarcasm or not but... yes, of course NASA developed reusable rockets. The space shuttle missions reused the shuttles and the boosters.

They did, you're not wrong at all, but just to add a little bit of clarity the space shuttle was never as reusable as was hoped - it wound up costing a huge amount of time and money to retrofit the shuttle again before each launch. Reusable and Re-usability are different things :P As far as I know no solid state boosters were ever re-used (how would that work?) - but then again SpaceX doesn't re-use solid state boost…

It doesn't really matter much because a look at the actual numbers shows that SpaceX charge more than the cost of launching the exact same payload would have cost using the shuttle. Besides the reusability point is disingenuous when talking cost since SpaceX's cost have actually gone up per (re)launch, not down. So yes, it is more complex than quippy internet back and forth suggests.

Here is a video that explains it in decent details if you are interested, but the TL;DR is that SpaceX is more expensive than the shuttle and way more expensive than they said they would be: https://youtu.be/4TxkE_oYrjU

Re: Perseverance Rover lands on Mars [video]

#195

Post by alkonaut »

andbberger wrote: post #59
pupdogg wrote: post #51

It took them approx. 4,881 hours from launch to land approx. 127,770,000 miles away. Is it safe to say that the average speed of the mission can be calculated as 436 miles/hour?

no. hohmann transfer, not a straight line. also there are no absolute reference frames.

> also there are no absolute reference frames.

"Well, officer, perhaps to you it seemed like I was speeding there..."

Re: Perseverance Rover lands on Mars [video]

#196

Post by unanswered »

DiogenesKynikos wrote: post #185
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.

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,such that these inferences are consistent (to within that tiny fraction of c) with all of our current and future observations in this reference frame; i.e., consistent with a classical(+ finite speed of light) model of the universe. Which is why I left this out of my question and only asked about the consequences of a finite speed of light.

Put another way, simultaneity is perfectly well defined in a single inertial reference frame, and for purposes of my question, earth and mars can be considered to be relatively motionless.

Re: Perseverance Rover lands on Mars [video]

#197

Post by gfiorav »

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…

Waiting for the physicist in the room to point out: there is no such thing as simultaneity!

:)

Re: Perseverance Rover lands on Mars [video]

#198

Post by joeyh »

bluescrn wrote: post #133
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.

Dust on the solar panel seems likely to be a problem after the first landing, unless it's got a clever way to keep it clean. And its got to be tough conditions for the battery, low temperatures and probably deep discharges to make the most of it.

It's powered by plutonium.

Rovers with solar panels deal with the dust by waiting for a storm to blow it away.

Re: Perseverance Rover lands on Mars [video]

#199

Post by Daho0n »

m4rtink wrote: post #156
Daho0n wrote: post #136

Earlier quoted context omitted.

.

Actually, all the Starship flights are fully automated, possibly except a very nervous person somewhere with the self-destruct button and binoculars. If you though there is someone in Boca Chica flying Starship remotely with joystick and steady hand, I'm afraid I need to disappoint you.

That wasn't the point but this is Reddit level snarky commenting so I'll be on my way.

Re: Perseverance Rover lands on Mars [video]

#200

Post by theNJR »

gfiorav wrote: post #197
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…

Waiting for the physicist in the room to point out: there is no such thing as simultaneity! :)

Came here to suggest The Order of Time by C Rovelli, which explains this in such a captivating way.
Post reply on HN