How hard is space travel, in principle?

http://michaelnielsen.org/blog/how-hard-is-space-travel-in-principle/

by timf • 16 years ago

39 16 16 years ago

Halfway to anywhere: A really good book on the subject, including an interesting story about development of low cost space launch capability. It is a bit old now with SpaceshipOne and SpaceX in progress, but still worth a read: http://www.nss.org/resources/books/non_fiction/review_013_ha...

I was actually expecting some analysis on interstellar space travel. But you can find an entertaining attempt at this by Charles Stross: http://www.antipope.org/charlie/blog-static/2007/06/the_high...

arethuza• 16 years ago

I found myself reading Charlie's article and thinking that 400Mt of energy to get something the size of a family car up to 10% of light wasn't that bad.

Of course, any real propulsion mechanism is going to require a lot more energy than that but that actually sounds reasonably achievable - at least if we allow Orion style ships.

retube• 16 years ago

"In fact, nearly all of the energy expended by modern rockets goes into lifting the rocket fuel itself, and only a small amount into the payload. An unfortunate consequence of this is that you need a lot more than four times as much fuel"

Yes this is where theory != practice, and why in fact, it's incredibly difficult and expensive to get somewhere like Mars.

adaml_623• 16 years ago

I think really this should be titled, 'How hard is moving an object in space, in principle'.

For me space travel involves humans in space and that's really difficult. Think about being in a little bubble of air surrounded by hard vacuum that would kill you in seconds. Plus the cold/radiation/general plumbing problems. Space travel is really hard.

AngryParsley2• 16 years ago

The guy is right. If you can send stuff to Mars, sending stuff to Saturn isn't much harder. In fact, we already do send probes around the solar system.

The problem is moving people in space. Space is the ultimate desert. People need all kinds of supplies to stay alive and healthy (air, water, food, gravity, etc). They also need to be shielded from cosmic rays and solar events.

I've said it before and I'll say it again: I doubt large numbers of biological humans will ever participate in interplanetary travel. I bet brain-in-vat or uploading technology will show up before payload costs get low enough for average people to buy tickets to Mars. A brain in a vat doesn't need gravity or exercise or space to roam around in. It needs less food and water than a full human body, and it can be shielded easily.

puredemo• 16 years ago

By the time technology gets to the point where a brain can be put into a vat I suspect all the neural connections could just be fully virtualized. Neural processing would become a whole lot faster at that point. Space travel could hypothetically take place inside a laser.

Even at that, the Milky Way alone is 100k light years in diameter.

rlpb2• 16 years ago

If my brain were in a vat, my need to travel would be significantly reduced. The only reason I can think of would be if I wanted to interact with a distant environment with less latency.

AngryParsley2• 16 years ago

Good point, but I think it's more general than that: A brain in a vat's need be in a particular location is significantly reduced. Interaction with remote things is limited only by light speed, and possibly bandwidth.

Travel costs for brains in vats are significantly reduced. Not only is travel cheaper monetarily, but the experience of travel can be completely hidden. Instead of sitting in a seat and looking out a window, you could spend your time in a VR world or interact via robots. The only reason you would want to stay in one place is to interact with the nearby environment at low latency.

In the end, uploads are the way to go. You can run them faster and travel cost is just data transfer cost.

iwr• 16 years ago

Aside from propulsion, which is solved at least for space probes, the great problem of outer-planet exploration is bandwidth. Using a microwave high-gain antenna, the data rate at Saturn is on the order of 10kbits/s and New Horizons would have to cope with 1kbit/s at Pluto. This is incredibly small for instruments capable of taking high resolution pictures.

So the next revolution in space exploration would have to tackle the data problem; optical or near-optical lasers for transmitters and standard photosensors (CCD) for receivers. It is technically possible to use off-the-shelf components, the remaining difficulty being the beam aiming system.

khafra• 16 years ago

These dudes making a Bose-Einstein Condensate out of photons might be able to give us coherent light at UV or even X-ray frequencies, which would be rather nice for communication: http://www.nature.com/nature/journal/v468/n7323/full/nature0...

Jabbles• 16 years ago

In fact, nearly all of the energy expended by modern rockets goes into lifting the rocket fuel itself, and only a small amount into the payload.

Which would mean the fuel needed is quadratic in the GPE needed, which would put Saturn right back up there at ~100 times harder to reach than the moon (ignoring constants).

RyanMcGreal• 16 years ago

>In theory there is no difference between theory and practice. In practice there is.

-- Yogi Berra.

ANH• 16 years ago

Regarding his underplaying of fuel mass, check out the rocket equation to get an idea of how hard it is to achieve the delta-v required for various maneuvers (e.g. suborbital, orbital, escape): http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Essentially, your fuel mass requirements increase exponentially as you need more delta-v. It's very much harder to get into orbit than to stay sub-orbital. And you will need a significant amount of fuel left over if you wish to orbit or land at your destination. You can mitigate that with atmospheric aerobraking, but that's not possible on, say, the Moon.

rlpb2• 16 years ago

There seems to be an assumption here that the difficulty of going somewhere is proportional to the energy required.

What about time, and provisions for life support during that time? What about launch windows, in terms of the amount of waiting time required before a return trip and the difficulties of a rescue mission?

Others have already mentioned that the amount of fuel/weight you need is worse than linearly proportional to the energy required.

icegreentea• 16 years ago

If your willing to take a loooooong time, you can take the interplanetary highway. Basically, by carefully picking your path, you can essentially move from Lagrange point to Lagrange point, where changing directions is nearly free.

jpa• 16 years ago

Really, really good book on this subject: http://en.wikipedia.org/wiki/The_Millennial_Project:_Coloniz...

http://www.amazon.com/Millennial-Project-Colonizing-Galaxy-E...

The book does a nice job of capturing all the hard engineering problems. His proposed solutions are interesting but a bit superficial.

How hard is space travel, in principle?

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