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

NASA proposes new propulsion to cut Mars voyage to 3 days

wired.co.uk

Post reply on HN
Print view View original

71–80 of 105 posts

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#71

Post by mcguire »

kemiller wrote: post #63
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

How would it slow down at the other end?

Wonder if you could use a one shot variant of Orion to slow down?

Fling a device ahead of the craft, detonate it and catch the energy on the back of the sail.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#72

Post by abecedarius »

cowardlydragon wrote: post #53

Can't pulse nuclear (Orion) get there in about as close to practicality as exists? 133 years based on what I've read. And that doesn't do antimatter-catalyzed fusion, which seems near-term practical.

Orion is pretty great, but still suffers from the rocket equation. Well, you could get around that by keeping the bombs back at home base and throwing them to catch up with the craft... I wonder if anyone's worked out that variant. Too complex to be worth it, right?

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#73

Post by owenversteeg »

peter303 wrote: post #60
owenversteeg wrote: post #59

For context, some of the numbers here: c: 300,000 km/sec, 1/3c: 100,000 km/sec Fastest man-made thing in history by far, Voyager 1: 17km/sec Time it took most recent Mars mission to get there (Mars Science Laboratory, 2011): 254 days Closest Mars-Earth: 182 light-sec Farthest Mars-Earth: 1342 light-sec These scientists are proposing to send something at about six thousand times faster than anything man-made has ever…

New Horizons is fastest chemical rocket taking 78 days to cross Mars orbit. (You'd use a different, longer path for Mars orbital/lander insertion.) New Horizons crossed lunar orbit in 9 hours, or about six times faster than Apollo flights.

Sure, but my points in the article only really apply once you change the velocity in a significant way. ke=mv^2, so going 100 times faster means ten thousand times the KE and thus ten thousand times the destruction.

For an example, New Horizons was at Mars around 13km/s. At 13km/s, the impact of that microscopic object, instead of being one hundred million bullets, is now 169 joules, barely a tenth of a bullet's energy. Still harmful, but with New Horizons being car-sized, slower moving, and armored, that repeated tenth-of-a-bullet impact is a problem that can be dealt with.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#74

Post by MaysonL »

rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

The .3c estimate was a best theoretical case, not proposed for Earth to Mars voyage. Note 3 days to Mars is ~50M miles/day ~2M miles / hour ~500 miles/sec or .3%c - 1/100 of his best case estimate.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#75

Post by airza »

venomsnake wrote: post #32
imglorp wrote: post #18

Earlier quoted context omitted.

I like exercise ;-) If we wanted a self-assembling orbital solar array to provide that much power, it would need to be almost half the surface area of the moon. http://www.wolframalpha.com/input/?i=1.5%C3%9710^18+J++%2F+8...

we could use a suitable size black hole to create lensing effect - that way we could capture sizable chunk of the sun's energy

probably making a reasonably large lens would be somewhat easier than creating and controlling a black hole

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#76

Post by api_or_ipa »

kemiller wrote: post #63
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

How would it slow down at the other end?

Aerobreaking could net you some gain. Obviously, this requires a more engineeered vehicle, including possibly an ablative shield.

Even at that, however, you'll be skimming very close to Mars' surface and incurring very high temperatures.

Perhaps (and this is a big perhaps) we could engineer lifting body surfaces to increase the time-in-atmosphere and dissipate heat better.

This is why I love playing Kerbal Space Program.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#77

Post by zwetan »

"if it works"

between parenthesis, in the title ...

so I'm not a scientific but even if it works at the tech level, even if the deceleration problem is solved, even if not colliding with something along the way is solved

what about the impact of travelling at 1/3rd the speed of light on the human body ?

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#78

Post by chiph »

gherkin0 wrote: post #43
chiph wrote: post #35

Earlier quoted context omitted.

You use 2 kites on board the spacecraft. One gathers the transmitted beam and reflects it to kite #2, which then reflects forward again. Careful design is important to ensure the spacecraft in the middle isn't struck by the photons. And you will have significant losses by the double-reflection, so you have to turn earlier than 50% of the way there.

Wouldn't your idea, at best, null-out the forward propulsive force, but not actually decelerate the ship?

Not my idea - it's been around for decades.

It works because you can treat the two kites as one system, because they're both connected to the ship.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#79

Post by nkrisc »

dr_zoidberg wrote: post #65
nkrisc wrote: post #57

So please help me out here. The article suggests a craft could be accelerated to .3c; surely this would not be for a Mars intercept!? This is just an example of the potential, right? Otherwise slowing down a craft at relativistic speeds for Mars capture sounds... difficult to say the least. EDIT: OK, after thinking it through further at .3c the trip would only take about 50 minutes, roughly. So clearly that's not wha…

Sounds like Kerbal Engineering at its best... :P

Relativistic litho-braking?

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#80

Post by stuxnet79 »

gherkin0 wrote: post #45
stuxnet79 wrote: post #37

Earlier quoted context omitted.

Did Mars figure prominently in the plot? I read the book exactly six years ago and loved it, but I have no idea why you are quoting it here.

No, but the aliens were discovered because one of them built a ship based on this idea ("solar" sail with a ground-based propulsive laser) and happened to arrive in a human-inhabited system. https://books.google.com/books?id=lKt5laoj1coC&pg=PA55&lpg=P...

Thanks, I wasn't aware that this was the exact type of propulsion that NASA was proposing. Mote In God's Eye is a classic book! I'm going to have to re-read it sometime soon. Getting a nostalgia kick just thinking about it.
Post reply on HN