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NASA proposes new propulsion to cut Mars voyage to 3 days

wired.co.uk

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

#51

Post by mikeash »

squilliam wrote: post #49
mikeash wrote: post #29

Earlier quoted context omitted.

Given that aerocapture and parachutes have been successfully used for Mars probes in the past, that seems unlikely to be true.

It works great for lighter unmanned missions, but becomes less useful as you increase in mass. NASA has had to come up with some interesting solutions in order to land their rovers. Mars Pathfinder used a giant inflatable airbag to scrub the remaining speed it had when hit the martian surface(Lithobraking.) MSL Curiosity, after it jettisoned it's heat shield and parachutes, used a rocket-powered 'sky-crane' in order…

Landing under a parachute is extremely problematic, but they can still be useful for braking before landing, and there's plenty of atmosphere to brake into orbit from an interplanetary trajectory.

Doing that last bit from a 200km/s arrival speed is going to be challenging to put it mildly, but because of the massive deceleration required and the massive heating.

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

#52

Post by falcolas »

leecarraher wrote: post #2

Jane, Stop this crazy thing! or in other words, how do you decelerate the spacecraft once you near Mars orbit. You'd need either a ton of conventional propulsion fuel , or a mars based laser there too. Worse is that you have a limited energy sources and no one to mine them. Solar might help, but it'd take a helluva alot of planning to install them via some automated system.

In the story "Flight of the Dragonfly", which used the same propulsion system, they simply detached a portion of the sail, and used it to reflect the beam back at the spacecraft to decelerate it.

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

#54

Post by vkou »

TickleSteve wrote: post #33

Shooting a 100kg mass off at 30%C wouldn't be taken lightly by whoever it hit! Planet killer?

Not quite.

Considering an inelastic collision, the impact would dissipate roughly mv^2/2 worth of energy, so...

1/2 * 100kg * (10^8 m/s)^2 = 5 * 10^17 Joules of kinetic energy.

In contrast, the amount of solar energy that impacts the Earth each second is 10^17 Joules.

For a more relevant comparison, the Tsar Bomba (50 megatons) released ~2*10^17 Joules. So, worst-case, the probe would cause the equivalent of a ~100 megaton explosion. Hardly a planet-killer.

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

#55

Post by mbrubeck »

nabla9 wrote: post #36
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…

Another thing to consider: mirrors are never perfect. 100% of the light is not reflected. If the spaceship uses high quality laser mirror with 0.00001% absorption rate, absorption from 5.8 terawatts is still 5.8 megawatts of power to get rid of in space.

Nit: 0.00001% of 5.8 TW is 580 kW. (I assume you just misplaced a decimal point.)

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

#56

Post by scoot »

nabla9 wrote: post #36
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…

Another thing to consider: mirrors are never perfect. 100% of the light is not reflected. If the spaceship uses high quality laser mirror with 0.00001% absorption rate, absorption from 5.8 terawatts is still 5.8 megawatts of power to get rid of in space.

> 100% of the light is not reflected.

That isn't a mirror, it's a black hole!

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

#57

Post by nkrisc »

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 what the article meant.

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

#58

Post by sehugg »

A practical near-term application of this tech is precision formation flying of satellites -- Young Bae has been developing test systems for over a decade: https://www.nasa.gov/spacetech/niac/2013phaseII_bae.html

(It must be annoying when the researchers studying pie-in-the-sky mission architectures get all the clickbait headlines)

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

#59

Post by owenversteeg »

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 gone in history. At this scale, the energies involved become absolutely insane. Guess what would be the energy of a tiny one-gram pebble floating in space hitting this spacecraft? Ten terajoules. That's approximately equal to you walking along on your way to work and getting hit by, oh, I don't know, the International Space Station (13 terajoules) or a sixth of a good sized atomic blast (63 terajoules). Space is a busy place. If you hit even one thing, you're over.

But, you argue, you won't get hit by any pebbles. I'd imagine that you're wrong, but let's humor you. You argue that the largest thing to hit you would be a microscopic 0.001g object. Let's say that your microscopic object is standing perfectly still in space, somehow, even though that's incredibly unlikely.

You hit this object, and instantly your craft is subjected to a ten gigajoule blast. Let's put that into context: at an incredibly small point somewhere on your craft, rocketing along at just north of 223000000 mph, you just experienced a hit equivalent to one hundred million large caliber (.45) bullets. Let's say the scientific payload of your craft is one gram, and the rest is armor. You'd be safe right? No, unfortunately 99.999kg of armor doesn't seem so strong in the face of one hundred million bullets.

I don't think it's possible.

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

#60

Post by peter303 »

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.
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