
NASA proposes new propulsion to cut Mars voyage to 3 days - leecarraher
http://www.wired.co.uk/news/archive/2016-02/22/laser-propulsion-system-mars-in-3-days
======
rubidium
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...](http://www.deepspace.ucsb.edu/wp-
content/uploads/2015/04/A-Roadmap-to-Interstellar-Flight-15-h.pdf)

~~~
kemiller
How would it slow down at the other end?

~~~
api_or_ipa
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.

~~~
05
Aerobraking from .3c would require materials many orders of magnitude more
heat-resistant than known to humanity.

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

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mahranch
> _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.

~~~
dzdt
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!

------
leecarraher
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.

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

~~~
arijun
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.

~~~
zelos
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?

~~~
tomswartz07
I think you're having trouble grasping exactly how fast 200km/s actually is.

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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.

~~~
peter303
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.

~~~
owenversteeg
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.

------
ldpg
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.

~~~
stcredzero
_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!)

------
RLN
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](https://en.wikipedia.org/wiki/Sunjammer)

------
CapitalistCartr
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](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 .

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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](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)

------
gremlinsinc
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.) ..

~~~
dalke
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.

------
huhtenberg
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.

~~~
rubidium
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.

------
danjayh
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.

~~~
chriswarbo
> 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](https://en.wikipedia.org/wiki/War_on_Cancer)

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

------
cowardlydragon
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.

~~~
abecedarius
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?

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grymoire1
See Larry Niven's Mote in God's Eye (1974) for details..

~~~
stuxnet79
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.

~~~
gherkin0
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...](https://books.google.com/books?id=lKt5laoj1coC&pg=PA55&lpg=PA55&dq=mote+in+god%27s+eye+laser&source=bl&ots=QTucO9WK2p&sig=fwQ9HNOZjtl4at5mi9aWZpuKpQs&hl=en&sa=X&ved=0ahUKEwjDva2m8YvLAhVEpx4KHWz0BfsQ6AEILzAB#v=onepage&q=mote%20in%20god%27s%20eye%20laser&f=false)

~~~
stuxnet79
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.

------
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.

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

~~~
nkrisc
Relativistic litho-braking?

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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 ?

~~~
pmontra
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.

------
Gravityloss
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.

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dexwiz
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.

~~~
adwn
> _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.

------
c54
See also: Aurora, by Kim Stanley Robinson

[http://www.amazon.com/Aurora-Kim-Stanley-
Robinson/dp/0316098...](http://www.amazon.com/Aurora-Kim-Stanley-
Robinson/dp/0316098108)

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

Planet killer?

~~~
vkou
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.

~~~
mcguire
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.

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

------
maxxxxx
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.

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

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boggie1688
So if it take an array of laser to get there, how do you get back? Bring
lasers with you?

------
Kinnard
Very light on details

~~~
Skunkleton
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.

~~~
wmfiv
So what changed?

~~~
Skunkleton
Nothing as far as I can tell.

------
amai
How to stop a spacecraft propelled by a laser?

