watervole | Space travel question

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Space travel question

Can anyone help?

I'm working on a story in which an alien ship is entering the solar system. The crew have been in relativistic flight from a plant at a convenient plot distance (close to us, but not next door). The ship is now decelerating (and may have been for a long time).

It can have any drive system that anyone cares to propose as long as it is reasonably plausible.

How far out is it likely to be spotted? (Consistent with your proposed drive mechanism)

How long would it take to reach Earth orbit from the point where it is spotted?

(For some strange reason, being ill seems to have released the writer's block that I've been plagued with for ages. Maybe it's because my brain knows it's not quite well enough to work on anything serious - though I do seem to be gaining ground on the email now - and thus it's freed me to write)

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By 'relativistic flight' do you mean in compliance with the Theory of General Relativity as proposed by A. Einstein?

If so, how far out they would be spotted depends on how much deceleration the occupants can stand, or are willing to use. A low-energy ion drive, for example, would produce only a gentle deceleration and not be too visible. If the ship were of low albedo it might not be noticed until near Earth orbit where tracking radar would pick it up.

If, on the other hand the crew can survive 20G for protracted periods they might use a fusion drive which would be visible much further out. Exactly how far would depend on the size of the ship which would determine how much power they would need to decelerate at that rate. Maybe as far out as the asteroid belt, for a large ship with a diffuse high-power fusion drive?

Of course they could use X-ray lasers for deceleration, which would not be visible by optical telescope.

But there are no hard rules. In one of Larry Niven's stories ('Protector', I think) a light-sail was used for very gentle deceleration. It was spotted a long way out by visible light spectroscopy because it had the same spectrum as the sun, slightly blue-shifted.

Yes, I mean obeying general relativity. They'll go home by non-relativistic means, but they have to get there the long way to set up the base point for jump travel.

The travellers are humanoid, so could not stand severe deceleration. I haven't yet decided whether to allow them artificial gravity, but am inclined not to.

For plot purposes, I'd ideally want it to be discovered at least several weeks out, though there's scope for a lot of variation either way in that.

A light sail is possible, but wouldn't that take several years between detection and arrival? That would be too long.

It's going to be very difficult for you to arrange deceleration from relativistic speeds that isn't visible for years in advance, whether you use rockets or light sails. The mathematics is not on your side.

Sorry, the note below about "looking in the right direction" reminds me I should have been more clear: I mean it's going to be hard for you to arrange for it not to be naked eye visible for years, let alone by someone with a telescope. You'll be lucky for it not to be something the whole Northern Hemisphere can't read their newspapers by for a decade.

The interstellar rocket ship that arrives in Earth orbit before anybody notices is one of those clichés of science fiction that isn't physically plausible.

If by relativistic you mean just a tad under lightspeed, it will take 3 times ten to the seventh seconds at one G of deceleration This equates to approximately one earth year of thrust. This is beyond the ability of even fusion power, and way outside the parameters of ion or Orion type drives; your only options are matter/antimatter and for this to work you need antimatter in industrial quantities. The other option is a plasma sail - akin to a Ramscoop field as used by Larry Niven in the Known Space sequence but is viable as a means braking from extreme velocities(Check the New Scientist Archives for details). I think the plasma/intersteller medium interaction will produce light in the UV spectrum which is not visible to earth based astronomical instruments so you can tune the 'advance warning' by chance observation by the Hubble of other UV sensative orbiting telescope

Actually observing an interstellar spacecraft is akin to obital observation of ships at sea - You look for the wake and the ship is the speck at the fromt of the Vee. Earthbound radar (SpaceGuard analogue) can reach several million miles but detection distance is dependant upon size with a maximum range around 20 million miles for something about the size of Ceres. The Arecebo dish can and has radar ranged Venus when alignments are right. Once you spot the wake you can hunt fo the actual craft.

I'm not worried if they get nowhere near light speed. I should have said Einsteinian rather than relativistic, in retrospect.

I'm assuming they've probably been decelerating for decades.
As it happens, Hubble has already had a mention in the story, so it would be nice to use it for the detection.

OK Einsteinian/ normal space approach, if you chose a low C approach it can be done by either ion or Orion technologies (I'm discounting fusion drives as they have similar efficiencies) wih a maximum velocity of 10% of C. Orion is noisy and highly visible over a wide spectrum range including neutrino as well as Alpha, beta and Gsmma, visible light frequencies etc Ion is less obtrusive but accelerations are low - say a peak of between 1/0th to 1/100th G so you're either stuck with a big noisy approach which is short or a more subtle one which takes a year or so.

At low C velocities you don't benefit from time dialation effects so it's either a generation ship or some form of hibernation/stasis system.

The best candidate is somewhat overused but ideal foe a non generation ship is Barnards star at 6.? light years which gives a flight time of about 60 -70 years aside from Alpha/Proxima Centauri you're looking at multicentury flight times.

Oops, should be 1/10th to 1/100th G theoretical specific impulses available to ion drive range from 5.000 seconds (already achieved in the comet borally(sp?) mission to better than 50.000 using higher potential gradients, more efficient grid designs, a way of stripping more electrons off the atoms used as fuel - oh and a space borne nuclear generator. To get this latter figure though the ion particles are moving at a substantial fraction of C and darned unhealthy to be in it's path even at a substantial range it would be like being on the receiving end of a medical cyclotron for months!

To decelerate from the orbit of Jupiter and arrive in about 12 weeks, they would have to be travelling at about 1/1000 of light speed at Jupiter orbit and decelerate at about 1/20G. That's simple Newtonian mechanics.

A rough calculation, assuming a hypothetical high-efficiency nuclear motor using water as reaction mass with a specific impulse of about 200:
If they are bringing with them a machine big enough for a base point I'd assume several thousand tons payload. The amount of reaction mass required would probably be the equivalent of a large asteroid. To get something that massive to 1/1000 of light speed would need even more reaction mass, perhaps the equivalent of a large moon.

Ion drives have a higher specific impulse (up to 5000) but the thrust of any design I've seen is way too low for 1/20G, even with only the motor itself as payload.

So you need a novel design. Something capable of a specific impulse in excess of 20,000 but with a power source and drive mechanism which doesn't outweigh the payload and fuel. Ideally with an exhaust velocity in excess of the maximum spacecraft velocity, which means about six times that of an ion drive. The exhaust would probably be hot enough to be seen by optical telescope, and it might look like a comet travelling backwards.

This article may be of interest:

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

If you have them decelerating for a long time before being spotted, that probably puts them further out at first sighting. Given the same time from first sighting to arrival, the further away they are, the greater the deceleration required (the more powerful drive making them visible even further out) and the greater the fuel requirement.

I'd have them coasting to Jupiter orbit (or closer) and decelerating from there. Unfortunately that makes a journey time of several thousand years.

A compromise would be a two-stage deceleration. A giant ship travels at a sizeable fraction of light speed and carries enough fuel to decelerate to 1/1000 c. This drops a smaller (50,000 tons?) vehicle which coasts in from the Oort cloud to within Jupiter orbit and then decelerates from there.

The giant ship might carry on to another system and drop a second vehicle there, or just hang about and wait, or be abandoned.

Of course all this could change if your base point is manufactured at the destination from locally available materials. The masses involved could be orders of magnitude smaller.

Think you slipped. Nuclear propulsion engines run higher than a specific impukse of 200, hell you get a specific impulse of 380 using methane/Oxygen chemical propulsion (see Zubrin's The Case For Mars) and the shuttle enginesare rateed at better than 440 seconds.

Wasn't the early NERVA engine rated at 800 plus?

Just Nitpicking:-)

It depends on the design. I was thinking of a nuclear powered steam rocket with an exhaust temperature of 800 kelvin.

See: http://www.neofuel.com/optimum/

Where Anthony Zuppero claims that a specific impulse of 160 is optimum.

But he was analysing it in terms of a much lower delta-v for a Mars mission.

In any case the difference between SI of 200 and 800 isn't enough to make an appreciable difference to the order of magnitude of the fuel requirement. I think you need an exhaust velocity several orders of magnitude higher, well beyond even what an ion drive produces. Ideally, the exhaust velocity should be roughly equal to the total delta-v, i.e 1/1000 c.

And to all those someone would have to be looking in the direction of it's approach. There's an awful lot of sky out there, if no one was looking in their direction I imagine it could be quite close, within the orbit of Jupiter perhaps.

Larry Niven is your friend here - get hold of his novel Footfall which follows a lot of the postulates you have given.

It will really depend on a number of factors - particularly the drive mechanism - the big problem with interstellar is fuel - you either have to use some form of nuclear/ion drive in which case the problem is reaction mass, or use something like a ramscoop (Look up Bussard Ramjet) where essentially an electromagnetic field is used to collect interstellar hydrogen.

Detection - well - it'll depend on whether anyone is looking in the right direction - in Footfall, the aliens were detected as they'd set up a mining base at Saturn and were disturbing the rings.

As to time - shortest time is to assume a skew flip - constant acceleration/180 degree flip/constant deceleration -and a zero velocity start and finish, Newtonian mechanics gives s=at^2 with peak velocity at t/2. Rearranging gives t=sqrt(s/a) Saturn is roughly 1.5x10^12 metres from Earth - 1G constant acceleration/deceleration would take about 4.5 days if my maths is correct. Pluto would be about 9 days.

Sorry - didn't sign in to post this.

Alastair.

There's some good thoughts there. It's a while since I read Footfall, but it's a good novel and I think I've still got a copy somewhere.

Sadly, it's now been calculated that the drag from a Bussard Ramjet would more than counterbalance the thrust generated.

Bussard theories and calculations are based on current thinking about the interstellar medium - that thinking changes regularly! (and science shouldn't stop a good storyline - it only needs to be plausible!)

Don't forget that scientists are known to say "it can't work" and then engineers make it happen - "It'll never fly Orville!"

Indeed so. the science isn't the main drive of the story, so I shall aim for plausible rather than perfect.

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