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Comment by bobsy | Hacker News Reader

While very interesting I don't see the point of not landing on Mars. It seems like a long way to go for a bit of sightseeing.

Humans have only landed on the moon once. I would have thought going to moon more often. Establishing some sort of base on the moon and improving space technology would increase the speed in which we do anything meaningful with Mars.

I mean, surely there are minerals on the moon? Rare minerals perhaps? Metals? It seems like a far better place to launch ships from due to the lack of atmosphere and less gravity.

Do we actually have a method of propulsion apart from rocket fuel?

While this is a noble mission, I don't see why there is so much interest in the red planet when we haven't even found a relatively efficient way to get to and from our own moon.

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> Humans have only landed on the moon once.

12 people have been to the moon, spread over 6 different missions.

http://www.universetoday.com/55512/how-many-people-have-walk...

> Humans have only landed on the moon once.

Not to be nitpicky, but humans have landed 6 times on the moon, from '69 to '72. (http://en.wikipedia.org/wiki/Moon_landing)

I agree with most of what you've said though. As for the moon and its minerals, I don't think it has any valuable ones but it's an excellent starting place for future long-distance space flights.

> While very interesting I don't see the point of not landing on Mars

You need a lot of propellant to escape from Martian gravity (versus, say, the moon or an asteroid).

Besides the possible problems with carrying it during the whole mission and being sure it will work in Mars after the whole trip, lifting all that propellant out of the Earth is in the best of cases extremely difficult and expensive. Its an enormous hit in the mass budget for your spacecraft.

You can try and do the math, it is not by any means the hardest part of rocketry:

http://en.wikipedia.org/wiki/Delta-v

http://en.wikipedia.org/wiki/Rocket_equation

You can manufacture fuel in-situ on Mars, something not possible (or significantly more difficult) on the Moon. This, coupled with the atmosphere of Mars that provides delta-V savings on the way down (with little drag on the way up) makes Mars somewhat more accessible, energy-wise.

Mars is interesting because it has the ingredients for humans living there: water, minerals for industry and agriculture, energy. The Moon has only solar energy, even that dampened by the 2 week long night.

The problem is we don't really know how we'd do that yet - we have no prototypes on Earth, so it would all have to be invented. I'd budget a decade for that, cautiously, since it has to work once we get there.

A much better plan would be an expedition with no set return date (other then intention) - if you were going to land people. A reasonably self-sustaining habitat could probably be set up if we could prove a few key things (namely - can you grow things in Martian soil if you provide an atmosphere, and can you find extra water while you're there?)

The big benefit of a fly by mission - the one we should take advantage of - would be to put people close enough that we could do more frequent rover moves since they'd have real time control for a period while in orbit.

You can get someone (ok, some couple) in a can and on their way by 2018, the next near approach. The following near approach will be enough time in the future to define a distinct mission, enough that current interests and abilities may no longer be capable (even multi-millionaires only live so long and/or retain mental capacity so long, or even possibly their money and influence), and enough time in the future that it will not provide the current boost and motivation that is intended.

TL;DR: This is in part the "just fucking do it" approach to manned space exploration including travel. Some people argue that it / a return to it are long overdue.

Trying to actually get to Mars will surely push technological advancement needed for space travel. It will also inspire people to get involved in space travel.

While this mission might not produce other results than getting better at space travel, I definitely see the point of it.

Its possible but I don't see how sending essentailly a flying camper van on a 1000 day mission will bring anything new to the table apart from seeing how crazy 2 people get in a confined space for such a long time.

Exactly my thoughts, other than the fact we've landed several times and not just once on the moon, but that's besides the point. It's been nearly 44 years (!) since the first manned moon landing and we have done practically nothing on the moon. 44 years is a HUGE amount of time when it comes to technology, and yet all we can do with the moon so far is talk about how we sent people there once (okay, a few times).

Frankly, it's hard to get excited about manned missions to Mars if we won't do a thing with the planet in the following 40 years.

As to minerals: according to some, mining the moon for helium-3 will be profitable (http://www.popularmechanics.com/science/space/moon-mars/1283..., http://science.howstuffworks.com/environmental/green-science...)

> Do we actually have a method of propulsion apart from rocket fuel?

There are several alternatives (solar sails, tethers, nuclear), but the engineering details are yet to be worked out, and all of them still ultimately depend on the same principles as rockets: something has to go in the opposite direction for the vehicle to accelerate.

The idea on solar sails is that they don't need to send something in the opposite direction, they steal momentum (vectored in the travel direction) from the gases and/or photon radiation emitted from the Sun.

> Do we actually have a method of propulsion apart from rocket fuel?

Absolutely. Hayabusa and Dawn (among others) have already demonstrated the efficiency of ion propulsion systems.

http://en.wikipedia.org/wiki/Ion_thruster

Can those work in the upper atmosphere? If not, why not?

I'm working on a new way to get into orbit and they'll play a part if I can figure out if it's possible to operate them in the upper atmosphere.

They're mass ejection thrusters, so yes. But the amount of thrust produced is minuscule compared to a hydrocarbon burning rocket. So I don't think they'd be useful so deep into earths gravity well.

No. Electric propulsion works by bringing ions to very high energies/speeds. This isn't practical in a dense atmosphere, because ions make too many collisions with air molecules. Their energy is dissipated almost immediately, as heat.

Moreover, it's not useful for escaping a planet's gravity well, atmosphere or not. Ion thrusters are extremely weak. E.g. the Dawn probe has a thrust/weight ratio of 0.000007. At the earth's surface, the gravity pulling it down would be 135,000 times stronger than the thrusters pushing it up. It can't fly.

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

Hmm, maybe you can help with my math? (and keep an open mind)

Here's what I was thinking. Float an ion thruster powered ship tethered to a weather balloon with solar panels up to 53km altitude (world record) and fire up the thrusters gradually reaching orbital velocity.

Here are my calculations:

altitude = 53 km

Air density = 5.3 kg/m^3

max speed =7 km/s

Coefficient of drag = .02

Cross section area = 130 meters

I used this equation to calculate the drag:

(aird/2) * (speed2) * coeffdrag * area

And I got around 340 newtons of drag. If an Ion thruster could produce something greater than that, we might be in business.

Your calculation is off by 10^6 -- I think you mixed up m/s with km/s? Also the air density should be far lower -- at your altitude (53 km) it's 7.2e-4 kg/m^3 or so. I don't think streamlining makes much difference at hypersonic speeds -- air molecules will hit your craft regardless of how it's shaped, there's no time push them out of the way. So a drag coefficient of 0.02 isn't likely; it's probably at least 1. You could look up stuff on spacecraft reentry to find more accurate figures.

https://www.wolframalpha.com/input/?i=density+air+53+km

Put together, I get something like 2 million Newtons or 200 tonnes force. This is about half the thrust of SpaceX's Falcon 9, so clearly this won't work.

https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+...

https://en.wikipedia.org/wiki/Falcon_9

The drag power at this speed is 16 gigawatts, which needs about 100 square kilometers of solar panels.

https://en.wikipedia.org/wiki/Drag_(physics)#Power

https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+...

Another problem: at this altitude, a sphere with a 130 m^2 cross section has a lifting power at most 0.8 kg (the mass of the displaced air).

https://www.google.com/search?q=4%2F3+*+pi+*+(130+m%5E2+%2F+...

This really won't work.

Wow. Thanks for fixing the math. Gosh I was really sure the atmosphere would be so thin that it just might work. Oh well back to the drawing board.

Btw what did you mean about the lift? I was proposing a helium balloon/blimp to supply the lift.

Right, but helium doesn't just magically "go up". It provides lift in proportion to the difference in density between the gas in the balloon and the surrounding atmosphere.

As the atmosphere thins, the amount of lift capable of generated by a helium balloon decreases to the point where, at the altitude you've mentioned, it could lift a total of about 0.8kg (according to GP's calculations). The point being that you wouldn't be able to have much of a payload lifted to those heights.

Oh that's a very good point. So I'd also need a whole lot more helium than I realized. Which would increase the drag even more.

Oh well on to the next idea.

Mars gravity is much more powerful than the Earth's. If they landed with current technology (especially considering the budget) they'd likely be unable to reach orbit.

This is false. Mars gravity is only 38% of Earth's, owing to the fact the Mars has only 10% the mass of Earth.

http://www.universetoday.com/22603/mars-compared-to-earth/

No. The escape velocity on Mars is 5 km/s, whereas on Earth it is 11 km/s. There is also a lot less of an atmosphere on Mars (though the atmosphere is smaller on Earth) and Martian gravity is significantly smaller than Earth's. It is far easier to leave Mars than it is to leave Earth.

http://en.wikipedia.org/wiki/Earth

http://en.wikipedia.org/wiki/Mars

Actually Mars has a considerably smaller gravitational pull than the Earth owing to its lower mass (5.972E24KG -vs- 6.39E23kg).

Mars gravity is weaker (so taking off again is easier), but the atmosphere is much thinner, so landing is actually harder. It's relatively easy for lightweight/robust things, but not particularly easy for humans. Parachutes, low-angle aerobraking, etc. all don't work so well. Expending rocket fuel in a retro burn is very expensive (since you have to carry it with you all the way from earth -- a lot of the "take off again" plans involve fuel either cached on Mars or made on Mars).

We have no idea how to build a spaceship that can withstand the radiation in space; especially unpredictable Coronal solar events (as mentioned in the article.)

This mission would be an excellent test of building and using a long-haul space vehicle.

> We have no idea how to build a spaceship that can withstand the radiation in space ...

No, that's not true, we know how to build it, but we can't afford it. A fully shielded spacecraft is easy to imagine and draw on a piece of paper, but not so easy to build and launch.

In defence of my comment, we have never tested a human-carrying craft in non-earth-orbit-space, during a solar mass ejection event (or other unexpected radiation event). We may know how to guess at building one, but actually trying it is the only way to know if we're right (i.e. if our model fits the universe.) Maybe I was wrong to say "no idea" - but it's still not a solved, easy problem.

> In defence of my comment, we have never tested a human-carrying craft in non-earth-orbit-space, during a solar mass ejection event ...

That's true, but (a) we know exactly what radiation levels take place during such an event, and (b) we know how to build radiation shields. It's just not practical to launch an adequate shield.

> but actually trying it is the only way to know if we're right ...

No, we have enough experience with radiation to know our model corresponds to reality.

> but it's still not a solved, easy problem.

It's solved, but it's not easy.

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