# How far can spacecraft fly?

How far can spacecraft fly? Why ?

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Pengwuino
Gold Member
Until they run into something?

Given that the mean free path of stars in the universe is many orders of magnitude larger than the size of the universe, the answer is essentially forever. I suppose there would be some kind of thermodynamic breakdown due to interactions with the interstellar medium, but I'm not sure how to calculate that.

HallsofIvy
Homework Helper
Note that we are assuming that you get into space, where there is very little matter to cause friction, and then cut off the engines. The space craft would continue along the same trajectory with almost constant speed (relative to the earth). Of course, with with no other bodies near by, how would tell if you "came to a stop"?

Well the farthest distance was Cassini to Saturn. And why it can't explore any further ?

russ_watters
Mentor
Well the farthest distance was Cassini to Saturn.
Incorrect. Voyager 1, Voyager 2 and Pioneer 10, all launched in the 1970s, have all left the solar system. Voyager 2 provided pretty pictures of Neptune when it passed in 1989.
And why it can't explore any further ?
We can explore as far as we want. We can send a probe to another galaxy if we want....it'll just take a very long time to get there, so we don't bother.

We can send a probe to another galaxy if we want....it'll just take a very long time to get there, so we don't bother.
Aha. That's what i wanted to know.

dav2008
Gold Member
We can explore as far as we want. We can send a probe to another galaxy if we want....it'll just take a very long time to get there, so we don't bother.
Would we be able to overcome the milky way's gravity?

russ_watters
Mentor
Er....good point. I'm not actually sure. Googling, I find the Milky Way's escape velocity is 525 km/sec, which is about an order of magnitude greater than we've done so far. It's probably possible, but would take a lot of effort.

I didn't knew about the Milky Way's gravity.

Chronos
Gold Member
How much fuel do you have? Anything is possible with enough rocket power. The problem is you would need the equivalent of an earth mass of fuel just to reach nearby stars in less than a thousand years. For example, Voyager will take around 70,000 years to travel 4 light years.

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Interesting, I wasn't aware of the average mean path of stars being that large (by this you do, loosely speaking mean the amount of empty space between stars on average is so great that you have a very unlikely chance of colliding with anything, right?) though I kind of had a feeling it must be. That probably conclusively answers a question I was pondering in the past. I was thinking about line of sight and stars. I thought about visible objects at the edge of the visible universe and I realize that due to technological limitations we're looking at very large and/or bright objects (galaxies, supernovae) but presuming that we could resolve individual stars from that distance. I would think that perhaps it would be puzzling that we could see anything so far because of other, much closer stars that appear much larger to us blocking the line of sights. But I figured that, as you said the average mean path of stars must be great enough that this would not be the case. Also considered the sheer distance between the star blocking sight and the one we were looking at would mean that the one blocking the sight could be incredibly close to 'perfectly' (I realize you can go possibly for an infinite amount and closer to perfect, or at least the planck scale so I don't use the term very loosely) blocking it from our line of sight but again due to the very long distance even the tiniest deviation would be big enough that it wouldn't shield out the other star.

Can someone clarify that for me? Thanks.

Also, as for the gravity well of the milky way: what would happen, if, say, voyager one were to make it out of our galaxy presuming that it were just escaping now, so for ease let's say that the galaxy were in the same state that it is in now. Would it just eventually slow down to the point of reversing direction and then accelerate back towards the center of the galaxy?

If we assume the spacecraft doesn't escape the disk of the Milky Way and use an estimate I found for the density of stars in the galactic disk of .05 solar masses per cubic parsec, the mean free path is:

MFP = 1/(n*A_g) where n = .05 solar masses/parsec^3 and A_g is the gravitational cross section of the sun.

Using conservation of momentum and energy, we get for the effective gravitational radius of the sun:

R_g = R_s * f * sqrt(1 + 2*G*M/R * 1/v^2 * 1/f )

Here v is the speed of the spacecraft far away from the star, M and R are the mass and radius of the sun. I have used the factor f to represent the multiple of the radius of the sun at which the spacecraft would burn up. So we get for gravitational cross-sectional area:

A_g = pi * R_s^2 * f^2 * (1 + 2*G*M/R * 1/v^2 * 1/f )

Using v = 17 km/s (speed of Voyager 1), f = 10 because some googling made that factor sound reasonable, I get

A_g = 2 * 10^22 m^2

Plugging this in to the formula above we have

MFP = 1/A_g * 1/ (.05 stars/parsec^3) =~ 3 trillion light years

So this means the spacecraft will travel for about 5*10^16 years before it collides with a star!

TheTechNoir:

Do we know everything that's surround us ?

russ_watters
Mentor
Of course not. But what do you mean by that question?

russ_watters
Mentor
Direction of the scientists? I don't understand the question.

Regarding cosmology and astronomy.

Certainly not!! It's impossible to know everything about the universe!! We don't know what is lying under our bed!!!

phinds