NASA proposes new propulsion to cut Mars voyage to 3 days | Hacker News Reader

NASA proposes new propulsion to cut Mars voyage to 3 days

wired.co.uk

128 pointsleecarraher100 comments

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 proposal is left as an exercise to the reader.

Edit: here's their proposal with physics. Just to be clear, I don't doubt the theoretical possibility, I doubt the economic and experimental reality. http://www.deepspace.ucsb.edu/wp-content/uploads/2015/04/A-R...

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

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.)

    5,800,000,000,000 W × 0.00001 =

           58,000,000 W  = 58 MW

I misplaced decimal point, but it's for different direction.

0.00001% != 0.00001

> 100% of the light is not reflected.

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

Sounds like fusion and better lasers need to be viable before this is viable.

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!")

11% of the energy output of the US...I don't doubt the theoretical possibility, I doubt the economic and experimental reality.

If we're going to be doing space travel on time scales where decades don't come into the picture, we're going to need devices with this kind of energy. Access to that kind of energy isn't ridiculous if your infrastructure is already in space, so it's only ridiculous in our current context.

The notion of creating a web of railway networks across an entire country was completely impractical before the Bessemer steel process.

I haven't even gotten into dispersion of a laser beam.

Also, diffraction lenses of ridiculous size become practical in micro-gravity so diffraction isn't an issue either, so long as your infrastructure is already in space. This is precisely how Planetary Resources will eventually make their giga-fortunes; by pioneering entirely new forms of infrastructure for an entirely new context of civilization. (This new context may not have much to do with earthly civilization, or even with humans.)

How would it slow down at the other end?

Crash into the planet.

Further Research Is Needed™. The paper says:

> A very difficult challenge is to slow the spacecraft to typical planetary orbital speeds to enable orbital capture once arriving. This task is extremely difficult as the initial entry speeds are so high (~ c) and the orbital speeds are so low (~ 1e-4 c). Dissipating this much energy is challenging. We have considered using the stars photon pressure, the stellar wind (assuming it is like our own solar system), using the magnetic coupling to the exo solar system plasma. None of these techniques appears to be obviously able to accomplish this task and much more work and simulation is needed. A simple fly-by mission is clearly the first type of mission to explore in any case to assess the environment in a given system to design (if possible) an optimized braking strategy.

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.

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.

Aerobraking from .3c would require materials many orders of magnitude more heat-resistant than known to humanity.

Yes. Stopping from 90000km/s in 100ms (more or less earth radius) might prove to be a bit hard :-)

We just build a second megalaser on mars. Super easy.

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.

Good point. Cuts the acceleration/deceleration down to about 1 G, which would actually be useful.

Exactly how do the deceleration work... ? Hoping someone put a really big haystack on mars?

I assume they can change the angle so the thrust vector is negative. It depends on the geometry between Earth, Mars, and the spacecraft. Or maybe they accelerate to just match the orbital velocity of Mars as they arrive.

I think it is good that people are thinking very far out of the box for this. Reason? What we think we can do now isn’t enough, so if we want to go to Mars, some ridiculous idea must eventually turn out to be practical.

For that reason, I would not worry too much about that 11%. _If_ we go into space, we will have to build enormous solar arrays in space, and they could provide order of magnitude more power than we produce now.

I would not aim for Mars, though. Instead, send it out to overtake Voyager. To do that, you do not need to decelerate, and you do not need to goto 0.3c, either.

(Actually, some billionaire should put a huge price on the first one to overtake Voyager and send a video of it back to earth. That could become a fun race, with later, faster, spaceships overtaking early entries.)

Not to mention the ~350 G's of acceleration/deceleration you'd experience over those three days.

> 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, the ship could be shielded or employ some sort of plasma/magnetic deflector technology (still in proof of concept stages of development), but now you're talking technology that's just as far away, if not further than the propulsion system they're talking about using. It's also why things like project Orion really never got off the ground or are feasible in their current forms. Sure, we can get up to those speeds, it's surviving at those speeds that's the challenge.

Also, they talk about the weight of a ship being ultra light. If you had to shield it to survive the impact of tiny particles and other space debris you'll inevitably encounter on your trip, it's no longer going to be ultra light.

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!

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.

I had the same question. One thing that crossed my mind is that if speeds like these are possible, you could let the spaceship fly a gigantic loop and slow it down on the way back to earth. Same laser for acceleration and deceleration. (Things I've learned from the Martian #1.)

Did Hermes use a solar(laser) sail?

I know the ISV Venture Star from Avatar did (with antimatter engines for the Pandora side of the trip.)

Hermes used an ion engine, so no sailing, just steady (but slow) acceleration.

Regarding slowing down upon arrival, could they enter a low orbit and skim the atmosphere of Mars to reduce velocity?

Definitely not. Back of the napkin shows that to get to Mars in 3 days would need a speed in excess of 200 km/s. That is an order of magnitude larger than Mars escape velocity. Your only chance of being captured by Mars is by pancaking.

Make the first vehicle nothing but a giant mirror, crash it into Mars and then reflect the laser off the debris to slow down the second vehicle?

I think you're having trouble grasping exactly how fast 200km/s actually is.

The "debris" would a fine dust scattered over millions of square kilometers at that speed.

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.

[1] https://en.wikipedia.org/wiki/Mars_Science_Laboratory

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

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.

I assume that all fuel taken with you could be used to stop, though at those speeds it might not be a wash. So perhaps atmospheric braking?

Still I could see the value for sending problems out of the system without having to do the orbital boost tricks with gravity

Turn the craft around halfway to your destination and burn to decelerate at the same rate you accelerated. Keeps a nice 1G on the body the entire time.

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.

Sail boats are able to sail into the wind. This, however, requires the drag of the keel to work. It might be possible to use a traditional rocket engine to act like a keel to allow using the laser energy to slow down the craft.

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.

> 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.

Care to supply some citations?

Physicist Robert L. Forward mentioned it in his book Rocheworld (later republished as The Flight of the Dragonfly)in the early 1980's, with the concept work being done in the 1970s.

https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight

That's a science-fiction book, though. I'm still not getting how the ship could basically sail straight into the laser that's pushing it (e.g. decelerate). The main sail is still getting pushed forward by the laser, even while the second sail is pushing backward (with the reflected light from the main sail).

His credentials in the field are pretty good, and his books are always hard-science (no elves or wizards, and very little hand-wavy stuff).

https://en.wikipedia.org/wiki/Robert_L._Forward

If I recall the design correctly (forgive me - it's been over 15 years since I read it), there's a significant size difference in the sails. The larger sail collects photons from the distant laser and focuses them on the smaller "backwards facing" deceleration sail.

Yeah, for this idea to work as transportation, you really need lasers at both ends: one to accelerate the ship, and other to decelerate.

So it doesn't really solve any problems of initial colonization (beyond fly-by surveys), but it could be used for commerce once a colony has already been established.

You'd also probably want the lasers based on the moon: no atmosphere but large enough not to fly off in the opposite direction.

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.

This doesn't make sense to me. The detached portion would be subject to radiation pressure from the ground-based propulsion lasers, and would quickly be pushed away from the spacecraft it was meant to act on.

Yes, but it's reflecting the light back at the spacecraft. So, even though it would accelerate quickly away from the spacecraft, the light reflected back at the spacecraft would be used to decelerate the spacecraft. So long as the daughter sail never hits C (thanks, physics), it's always reflecting back photons back at the spacecraft (though red-shifted).

You'd still have to account for the light coming from the source as well as from the daughter sail, but a black coating on the back of the spacecraft's reflector would take care of that; it would give you n force pushing you away from the original light source, and 2n pushing you towards the source.

The coordination and planning required for such a device would not be trivial, but it would be possible.

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.

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.

I think the "Mars in 3" days comment is just for scale, not a serious proposal.

This is more about getting probes to whiz past other solar systems. As obviously, you can't stop the craft.

you can't stop the craft.

Zubrin and someone devised a way to use magsails to decelerate starships without using onboard fuel. That's magic, as you're not subject to the rocket equation. (Really, it was a failed attempt to develop a Bussard ramjet, but they figured out you'd never overcome the "friction" with the interstellar medium. So they were like, let's go with the friction!)

Whenever I read a story about using sails for propulsion in space I'm reminded of the short story Sunjammer by Arthur C. Clarke

https://en.wikipedia.org/wiki/Sunjammer

To reduce travel time from 9 months to four would be incredibly helpful to sending people. To halve that again to two months even more so. But after that, faster has lessened benefits. One month makes it easier on the human body, less than that is more convenience than anything. We'd be better served putting our money elsewhere. An Earth-Mars cycler would be a great start. https://en.m.wikipedia.org/wiki/Mars_cycler

And for unmanned missions, faster is of marginal benefit.

If I were setting up such a system for planetary flybys and extra-solar missions, I'd put the laser on one of the lunar poles .

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)

Okay, so what I read from others is that stopping is gonna be a PITA -- so maybe Mars isn't the best target.. Why not send some huge telescope in the direction of that solar system that looks like it has an Alien Megastructure- - get as close as we can to that thing and take closer pictures of it to find out what's really going on (and other parts of the galaxy) -- imagine having a telescope like a couple solar systems away sending back data to us? -- we may to create some sort of galactic internet though - maybe using lasers and relay stations? (I honestly have no clue... we'd want to get data back and it to NOT take 50 years per data dump.) ..

It's likely very much easier to build a telescope array in this solar system than to send a telescope "a couple solar systems away".

KIC 8462852 is 1480 light years away. At 10% light speed it would take over 10,000 years to get within a few solar systems of the star.

On the other hand, a space-based interferometer with a baseline of 15 AU would have an angular resolution of:

  1.22 * 500 nm / (15 AU) = 2.7E-19 radians

giving the ability to see things at KIC 8462852 which are

  1480 light years * 2.7E-19 = 3 meters across

That's also fantastically advanced, but working backwards, to see objects 100km in size:

  (1.22  * 500 nm) / (100km / 1480 light years) = ~100,000 km

I think we can pull that off within 1,000 years, much less 10,000.

Another 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.

As much as I love the idea, it is fairly impractical for the reasons that other commentors have noted. Call me greedy, but within the constraints of a balanced budget, I would like to see an increased portion dedicated to science (be it a 'cancer moonshot' or an actual marsshot) vs social programs ... maybe we could find some real solutions to these issues. In my own life I have a tendency to skimp on the 'now' in the name of investing in the future, so this is probably just me applying my own financial philosophies at a larger scale.

"Preaching to the choir". Since the government is not going to spend significantly more on science because most citizens don't want it, can you come up with a different solution that might work? For example, a Kickstarter for science that actually works. Crowdfunding X-Prizes, YC (incubator) for basic science companies, etc.

The US economy is $15 trillion. How to redirect some of that into more research?

> Call me greedy, but within the constraints of a balanced budget, I would like to see an increased portion dedicated to science (be it a 'cancer moonshot' or an actual marsshot) vs social program

Note that this basically happened already. As Apollo wound down, the War on Cancer wound up https://en.wikipedia.org/wiki/War_on_Cancer

Turns out it's far more difficult to cure cancer than to reach the moon.

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?

See 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...

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.

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.

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.

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

Relativistic litho-braking?

"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 ?

Nothing, you'll feel only the acceleration which is is equivalent to a gravitational pull. If it's 1 G you won't feel any difference from being in a room on Earth.

You probably need to manufacture the terawatt class space lasers from asteroid materials, at least the majority of mass. One can't afford to lift them from any deep gravity well.

This is the major reason why asteroids are in total much more important than the moon or even Mars.

So 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.

See also: Aurora, by Kim Stanley Robinson

http://www.amazon.com/Aurora-Kim-Stanley-Robinson/dp/0316098...

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.

But still pretty impressive.

I say we start flinging these things at all the extra-solar planets we've discovered. Think of it as grafitti or an art project. Leaving our mark on the universe.

That sounds like the kind of art that Black Hat Guy from XKCD would aprove of.

Could you make any meaningful observations if you flew through a neighboring stellar system at that speed? Seems you could reach Proxima Centauri in between 10 and 20 years that way but you could not slow down.

How long would it take to catch up to the Voyager probes?

So if it take an array of laser to get there, how do you get back? Bring lasers with you?

Very light on details

Yep, that is wired for you. As far as I can tell, this plan was first proposed in the '80s. The technical challenges of a 1.21 jiggawat laser, and a gigantic sail have led to it never being attempted even though it is probably theoretically feasible.

So what changed?

Nothing as far as I can tell.

How to stop a spacecraft propelled by a laser?

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