19

After the Pioneer, Voyager and New Horizons spacecraft complete one orbit of the Milky Way (in approx. 230-250 million years or one "galactic year"), how close will they then be to our solar system?

(Editing my question for clarification) The reason for asking the question is to understand whether these spacecraft that are now in orbit around the center of the galaxy will behave analogously to an object that was thrust away from the ISS and then meets up with the ISS exactly one orbit later. I am assuming that the gravitational pull of the galactic core will prevent the spacecraft from rising above or below the galactic plane for ever and that they will be in a similar orbit to the solar system, just inclined to it. Is that correct?

Charlie
  • 191
  • 1
  • 4

2 Answers2

31

Oversimplifying by taking the current velocity of each probe and multiplying it by 250 million years, I get:

Admittedly this ignores slowing down due to the sun's gravity (definitely significant). It also ignores any gravity effects from any stars it passes along the way (probably significant, but as as @GdD mentions we don't know).

For comparison, Wikipedia puts the diameter of the Milky way at 150,000 to 200,000 light years.

Eugene Styer
  • 1,852
  • 1
  • 13
  • 16
  • 18
    This is pretty good answer actually. "Somewhere in Milky way, and still on the same side of it as earth." – jpa Jun 02 '20 at 17:43
  • 2
    @jpa unless it gets a gravity assist from a freak black hole that shoots it out of the galaxy at 10% the speed of light. While definitely extremely unlikely, it's possible! – John Dvorak Jun 02 '20 at 19:37
  • Wouldn't actually running into something be more likely? And depending on the nature of the impacted object the probe might or might not reasonably be considered to exist afterward. Although in that kind of time frame it would also be possible for whatever the probe impacted to also be flung out of the galaxy. – SoronelHaetir Jun 02 '20 at 20:45
  • 2
    @JohnDvorak Wouldn't that require a black hole moving at 5% the speed of light? – lirtosiast Jun 03 '20 at 08:00
  • @lirtosiast it would. And yes, I have no clue what could cause a black hole - even a small one - to be launched at that speed. Maybe a close approach to an orbiting pair could give it enough boost? – John Dvorak Jun 03 '20 at 08:04
  • 1
    Aren't those velocities heliocentric? If they are, it should be noted that in 250 million years the probes are expected to have completed a galactic orbit, although being still near the Sun. – Pere Jun 03 '20 at 08:36
  • 4
    @SoronelHaetir to quote hitchhiker's guide to the galaxy: "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." (One of the rare occassions Adams was probably underestimating a fact). The chance that you run into something outside of our solar system is almost zero, so you can with almost certainty say none of the spacecraft will hit something large enough in a given timeframe. – paul23 Jun 03 '20 at 11:14
  • @SoronelHaetir adding to what Paul said, a satellite going through space is roughly analogous to a neutrino going through the Earth. What looks solid to us is essentially empty space to a neutrino. Likewise while there are many billions of objects in space, it is so big it looks essentially empty to anything traveling through it. – eps Jun 03 '20 at 16:34
  • I'm aware of all that, however actually running into something still seems like it would be much more likely than getting launched out of the galaxy by a black-hole interaction simply because black holes are a tiny fraction of all the possible objects. – SoronelHaetir Jun 03 '20 at 19:31
  • 1
    Perhaps more likely would be aliens discovering them and taking them to their planet :) – Barmar Jun 03 '20 at 20:12
  • @Barmar it's actually an easter egg in . Sadly, you can't actually take it home with you, but it's definitely out of place in the game's solar system. – John Dvorak Jun 06 '20 at 06:37
21

We don't know as there's no way to calculate it exactly. To do so we'd have to have extremely accurate data on every gravitational interaction these space probes will ever be exposed to. This would require accurate information on the location, mass and vector of every single body in the galaxy, that means every star, every gas cloud, nebula, planet then all the way down to small rocks. Then we would have to perfectly model every gravitational interaction between every single one of these bodies in order to map stellar shift and the movement of objects for the next several hundred million years, and be able to account for the gravitational influence of extremely distant objects. Then we could plot the spacecrafts' paths through this gravitational map.

We don't have nearly enough data to do this. The milky way galaxy has an estimated 250 billion stars plus or minus 150 billion. We don't even know how many stars are in the galaxy, much less their exact mass, position or other direction of movement. That's just stars, we can estimate the number of planets from our existing observations, without knowing their positions and mass. Even if we had the data we don't have the computing power to calculate it.

GdD
  • 20,226
  • 6
  • 65
  • 85
  • I commend you for even making an effort to write this answer. I know there are no stupid questions but I don't know what OP was expecting. – OrangeDurito Jun 02 '20 at 11:10
  • 4
    And we could add the problem of dark matter & dark energy, discrepancies in estimates of cosmological constants,... Hey I know! We'll use TensorFlow! :-) – Carl Witthoft Jun 02 '20 at 11:27
  • 2
    Well, you would hope we will have those solved in 200 million years @CarlWitthoft. – GdD Jun 02 '20 at 11:43
  • @GdD At which point we could observe; we wouldn't need to predict their position :-) – Carl Witthoft Jun 02 '20 at 12:22
  • 1
    @CarlWitthoft ok, first let's measure the universe 500000 times from big bang to heat death, so we have enough data for learning our neural network. – leftaroundabout Jun 02 '20 at 15:07
  • @CarlWitthoft - no experiment and measurement show ANY variation in your "cosmological constants" - what makes you assume those have any influence – eagle275 Jun 03 '20 at 08:00
  • @eagle275 .....yet. – Carl Witthoft Jun 03 '20 at 12:58
  • Do we really need to know about all the stars, verus just the ones in the general vicinity of the Solar System? The rest of the galaxy is far enough away from the spacecraft that they can be aggregated. – Barmar Jun 03 '20 at 20:11
  • @Barmar that's what I thought too, but looking at this answer we see that in 250 million years they go pretty far; roughly 10,000 light years! We probably do know something about most of the stars in this volume using redshift and proper motion for movement and brightness and classification to estimate approximate mass and distance, so one might be able to hazard a guess. But all it takes is one fairly close encounter with an object of some uncertainty to make the guess invalid. – uhoh Jun 04 '20 at 00:05