So if it take an array of laser to get there, how do you get back? Bring lasers with you?
NASA proposes new propulsion to cut Mars voyage to 3 days
41–50 of 105 posts
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#42Earlier quoted context omitted.
Mars is actually a pretty bad planet to try landing on. It has just enough atmosphere to burn up a spacecraft that enters too quickly, but not enough to use for air-braking (skimming or parachutes).
Given that aerocapture and parachutes have been successfully used for Mars probes in the past, that seems unlikely to be true.
What jobu probably means, is that Mars' atmosphere is too thin for parachutes to slow a reasonably sized craft down to low speeds (that's why MSL's landing required additional rocket propulsion [1]). Compare with Earth: Here, the terminal velocity of a reentering capsule is slow enough for parachutes to take over, which in turn slow it down enough for touchdown.
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#43Earlier quoted context omitted.
Propulsion is provided from the earth /LEO/GEO (imagine blowing on a kite with giant fan). turning the kite around will not make it slow down/return to you while you keep blowing on it.
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?
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#44back-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…
Here is the relevant passage in the paper, for the convenience of other readers:
> The photon driver is a laser phased array which eliminates the need to develop one extremely large laser and replaces it with a large number of modest (kW class) laser amplifiers that are inherently phase locked as they are fed by a common seed laser. This approach also eliminates the conventional optics and replaces it with a phased array of small optics that are thin film optical elements. Both of these are a follow on DARPA programs and hence there is enormous leverage in this system. The laser array has been described in a series of papers we have published and is called DE-STAR (Directed Energy System for Targeting of Asteroids and ExploRation). Powered by the solar PV array the same size as the 2D modular array of modest and currently existing kilowatt class Yb fiber-fed lasers and phased-array optics it would be capable of delivering sufficient power to propel a small scale probe combined with a modest (meter class) laser sail to reach speeds that are relativistic. [...] As an example, on the eventual upper end, a full scale DE-STAR 4 (50-70 GW) will propel a wafer scale spacecraft with a 1 m laser sail to about 26% the speed of light in about 10 minutes.
(I like the idea of DARPA putting together a 70GW orbital laser array — "no no, this is purely for scientific purposes, trust us!")
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#45See Larry Niven's Mote in God's Eye (1974) for details..
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...
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#46So over-under on this being viable before fusion power? Nice idea, but we are years away from this being viable. If we visit Mars it will be with chemical rockets first? This may be viable for some sort of highway system when travel is more common.
> So over-under on this being viable before fusion power?
There's nothing magical about human-built nuclear fusion power generation that'll give you noticeably larger or cheaper power output than nuclear fission.
Therefore, commercial fusion power will not bring us any closer to viability of this concept.
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#47Another back of the napkin thing - if one manages to maintain +1G acceleration for the first half of the trip and -1G for the second half, one can get to Mars in 1 to 3 days, with the top speed of about 1000 km/s.
Which does provide the interesting feature that if we could method of propulsion that gets to c/3 in a day or two, we could just use the acceleration of the ship to provide artificial gravity on-board, and flip the ship around mid-trip to provide artificial gravity during the deceleration.
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#48See Larry Niven's Mote in God's Eye (1974) for details..
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.
Minor spoilers for others
The first ship sent by the moties (the aliens in the book) uses this exact type of propulsion. A huge laser pushing a solar sail ship across space. It is not a new idea i think was the GP's point.
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#49Earlier quoted context omitted.
Mars is actually a pretty bad planet to try landing on. It has just enough atmosphere to burn up a spacecraft that enters too quickly, but not enough to use for air-braking (skimming or parachutes).
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 to slow down the last 200 MPH for a soft touchdown.
This is one of the reason SpaceX is focusing on rocket-powered landings of boosters and capsules vs. parachutes. This is a vital technology required to be able to put large amounts of mass on the surface of Mars
Re: NASA proposes new propulsion to cut Mars voyage to 3 days
#50> Lubin supposes that the system could propel crafts to an "unheard of" 30 percent the speed of light. > " There is no known reason why we could not do this " Yeah, no. There is a known reason. What do you think happens to a spaceship hitting something at 30% c? What do you think happens to the front of a spaceship or craft hitting something the size of bead or pebble at 30% c? The ship will disintegrate. Granted, th…
Not sure its that bad. This thing would be mostly akin to a big mylar sail. A micrometeorite at high relative velocity will punch right through making a nice cutout. As long as it doesn't hit the (small) payload I don't think it should disintegrate. But I would worry about all the atoms and ions hitting too. Space isn't empty!