# Answer a galactic math problem for me?

Asked by Blondesjon (33976) May 10th, 2013

Say that a long time ago, in a galaxy far, far away, two planets collided . . .

If the event occurred 3 billion years ago, and a hunk of debris the size of a refrigerator entered Earth’s atmosphere today, how many light years away did the collision take place?

I know that gravity and probability dictate that this hunk never makes to Earth but let’s just say it did. I’m looking for a rough distance here.

Thanks.

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At what velocity is this chunk traveling?

bolwerk (10327)

The size of the chunk of debris isn’t relevant but the speed is. If it was travelling at near the speed of light the collision would have occurred 3 billion light years away. If slower then it would have been closer.

flutherother (31104)

It is possible the two planets were located at the L4, or L5 Lagrangian point of Jupiter’s orbit, in which case they would only be 70 -90 light minutes away.

LuckyGuy (39464)

@bolwerk . . . At roughly the velocity debris would be flung in to space if two Earthish sized planets collided.

Blondesjon (33976)
Response moderated (Off-Topic)
Response moderated (Off-Topic)

There’s not enough information to calculate any distance. There’s an upper limit, however, on how far away it could have formed in order to get here within 3 billion years, given that all massive objects must travel at less than the speed of light. So 3 billion light years makes a reasonable upper limit. I’ve never read of planetary collisions ejecting material at relativistic speeds, but maybe it’s possible. Originating in another galaxy might conceivably add a large relative velocity component, I suppose.

Comets and asteroids more typically travel thousands of times slower than light – still blazingly fast by human standards. So at 0.1% the speed of light (about 700,000 mph) the object in question came from 3 million light years away, still outside the Milky Way but within our “local group” of galaxies.

gasman (11315)

@Blondesjon: concur with @gasman. I would assume it’s a non-relativistic speed, and the trajectory is irrelevant (did it just go in circles around the center of the galaxy?). But if you don’t give way to figure out velocity, or provide it, there isn’t a way to give a meaningful answer other than there is an upper limit to how fast it may have traveled. And @gasman‘s answer is probably about as good as you can come up with.

bolwerk (10327)

The upper limit is exactly what I was looking for. I could have sworn I used the term ‘roughly’ but further inspection has proven me wrong.

Thanks for the help.

Blondesjon (33976)

@bolwerk thx.

gasman (11315)

Never. If it’s a galaxy far far away, first of all the planetary fragments would never escape the gravitational pull of the galaxy. And even if they could, there is no way they would have time to reach us.

Rarebear (25162)

Well since we are talking “hunk”, I am going to say “faster than a speeding bullet”, “far greater than the speed of light”, and possibly..

“If Superman traveled 8.734 light years, then he traveled 513,428,174,000,000 miles (513.4 trillion miles). If we divide that number by 20 (the time it took him to get there and back) then he was traveling at a speed of 25,671,409,000,000 miles per second (25.6 trillion miles per second). That’s 92,417,072,400,000,000 miles per hour (92.4 quadrillion miles per hour).

And that’s 137,809,097 times the speed of light. Not to mention, he can vibrate his molecules so fast that he can vibrate through anything.”

Superman (supercharged) can travel 92.4 quadrillion miles per hour or 137.8 million times the speed of light.

The Man of Steel is real, damn you! Let the dream live on!

Screw attack! :P

@Rarebear . . . yeah. i addressed that. i was just looking for a number that was within the realm of possibility. a perfect world and all that.

Blondesjon (33976)

One other thing to consider here is that no matter in space moves in a straight line. Even light itself, which we think of in terms of “a beam” or “a ray”, meaning something linear, is curved in space. All of the “stuff” of the universe revolves around “other stuff”.

To put some perspective on this, the Earth has been revolving around the Sun for several billion years and hasn’t run into it yet.

So the example of “two planets colliding” probably means that they were revolving around a common star of their own – maybe even our own Sun – and the fragments that were ejected continued to revolve, albeit in a somewhat different orbit after the collision, around the same star. Even assuming some kind of forces that would have propelled your hypothetical fragment into some kind of escape velocity, the path taken would have been a helical one as the fragment spun through space over those billions of miles. It could have (I expect “would have”) spun in a more and more eccentric orbit for billions of years before it managed to escape.

So there’s absolutely no way of knowing “how far away” this could have happened, although I like @gasman‘s assignment of an absolute limit on how far away it could have been. Since that assumes some sort of “straight line” travel (and remember that the Earth itself and the Solar System it resides in are all in relative movement, too) that distance could be considerably longer if the Earth’s movement in the intervening years has been toward the location of the collision… and if everything was traveling in straight lines.

Or, as @LuckyGuy has suggested, this could have occurred (in galactic terms) “next door”.

CWOTUS (26082)

@CWOTUS . . . I’m working on a story where a chunk of debris, containing a flu virus, reaches the earth. The mechanics behind how this chunk reaches the Earth are very Rube Goldberg in terms of various collisions, gravitational slingshots, and a generous helping of Ed Wood style ‘suspension of disbelief’. I just want to be able to write down a time and a distance that fit together well enough to not be ridiculous.

In other words, I don’t want to pull the numbers out of my ass like I did everything else.

Blondesjon (33976)

Because of dark energy, stuff from some galaxies far, far away will never be able to reach Earth, because it would require speeds faster than light.

mattbrowne (31648)

@mattbrowne I’m not sure about that. It is true that distant portions of the universe are receding from us (due to accelerated expansion induced by dark energy) at faster than light speed. The “cosmic speed limit” applies to travel through space, but not to space itself. That’s why the observable universe, 14 billion years old, has a radius greater than 14 billion light years.

Photons emitted from distant objects, however, still cross a finite distance at the speed of light and eventually reach us. Particles with mass take even longer, but in principle can still eventually arrive in our neck of the woods.

gasman (11315)

@gasman – I was talking about new photons created “right now” at the core of some star in a far, far away galaxy. Old photons are still reaching us, of course. Am I right?

mattbrowne (31648)

Just as a point of interest, a photon will take an estimated 10,000 years to go from the core to the surface of the sun.

Rarebear (25162)

As @gasman says space itself can expand faster than the speed of light, so if an object is far enough away its light will never reach us even though it is at a finite distance.

flutherother (31104)

@Rarebear – You’re right. I should have written about new photons reaching the surface escaping into space. A couple of thousand years don’t matter, though, to be able to reach our telescopes…

mattbrowne (31648)

or