# When - and how - would we detect an interstellar object the size of the Moon headed right at us? **URL:** \ **Category:** Factual Questions\ **Created:** [February 28, 2022, 10:12pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256 "2022-02-28T22:12:30Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [March 2, 2022, 6:36pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/21 "2022-03-02T18:36:48Z")
> [@Gyrate](#): > > Mars is probably the closest one could be, and that’s assuming it wasn’t subsequently in line for detritus blown off Earth. There is no ambiguity about it—Mars would be pounded. In the Ordovician, just a collision between two asteroids caused a massive increase in the rate of meteorites hitting Earth. > **[PSRD: Tiny Traces of a Big Asteroid Breakup](http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html)** > > Fossil meteorites and chromite grains record a hundred-fold increase in the number of meteorites that fell 480 million years ago compared to the meteorite influx today: Planetary Science Research Discoveries (PSRD) educational on-line space science... ---
**Author:** ![Gyrate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gyrate/32/3588_2.png) [@Gyrate](https://boards.straightdope.com/u/Gyrate)\ **Post date:** [March 2, 2022, 6:41pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/22 "2022-03-02T18:41:15Z")
And of course Earth’s moon has helped block or divert some moderately big hits over time. Phobos and Deimos ain’t blocking shit. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [March 2, 2022, 8:21pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/23 "2022-03-02T20:21:43Z")
> [@engineer\_comp\_geek](#): > > But it does demonstrate the point that the sun creates a bit of a blind spot as we’re looking for things. The rocks in our blind spot mostly have orbits that are almost entirely inside the orbit of Earth. But an interstellar object has to cross the entire Solar System before it gets inside Earth’s orbit. It’d have to be going really fast, even by interstellar object standards, to do that while still on the other side of the Sun from the Earth. An object the size of the Moon, which is really huge compared with most space rocks, will be observable when it’s well outside the orbit of Neptune. Your typical long period comet will take years to go from that far out to the inner Solar System. We usually don’t spot them until they get somewhere inside Jupiter’s orbit because they’re typically really small. Once they develop a coma and tail they become much easier to spot, but they don’t do that until they get close to the Sun. But we still usually have about half a year or longer between them being spotted and close approach. Now the orbital difference between interstellar objects and long period comets is that the interstellar object will have additional velocity, so it travels a hyperbolic orbit. But unless it’s going really super fast, an interstellar object should still take a year or more to cross that distance. So it won’t be in our blind spot for that entire time. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 2, 2022, 8:22pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/24 "2022-03-02T20:22:44Z")
> [@Darren\_Garrison](#): > > There is no ambiguity about it—Mars would be pounded. But how much does that matter? As long as the Mars base was not hit who cares if the planet gets pounded by rocks? There is no ecosystem to wreck. It is already cold. Maybe sunlight could be blocked more which would hamper solar panels but, with the much thinner atmosphere, I would expect dust to settle back down to the planet much faster. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [March 2, 2022, 8:34pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/25 "2022-03-02T20:34:44Z")
> [@Whack-a-Mole](#): > > As long as the Mars base was not hit who cares if the planet gets pounded by rocks? And a Mars base should be located at least a few feet underground for radiation shielding in the first place. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [March 2, 2022, 9:23pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/26 "2022-03-02T21:23:07Z")
A Moon base is one thing. A fully self-sufficient moon base that will survive without any supplies from earth is another matter entirely. Given a few years’ notice, the first is merely impossible. The second is - even more impossible. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [March 2, 2022, 9:27pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/27 "2022-03-02T21:27:37Z")
> [@Whack-a-Mole](#): > > But how much does that matter? That asteroid collision increased the frequency of meteorite strikes up to 100x. An Earth-moon(sized) collision would, I bet, increase strikes by 1,000 or 10,000 times. And while Mars has a little atmosphere so that not all strikes are cratering strikes like they would be on the moon, a large percentage of them will. I think it would matter a whole hell of a lot. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 2, 2022, 9:35pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/28 "2022-03-02T21:35:02Z")
Well…I think we’d have to try anyway. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 2, 2022, 9:45pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/29 "2022-03-02T21:45:05Z")
Yeah, it’d be ludicrously difficult to build a self-sustaining Mars base in that kind of timeframe… but it’s still many orders of magnitude easier than deflecting a Moon-sized object. ---
**Author:** ![Max\_S](https://avatars.discourse-cdn.com/v4/letter/m/46a35a/32.png) [@Max\_S](https://boards.straightdope.com/u/Max_S)\ **Post date:** [March 2, 2022, 9:50pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/30 "2022-03-02T21:50:14Z")
> [@engineer\_comp\_geek](#): > > That depends on what direction it comes from. Both known interstellar interlopers have entered at a high orbital inclination (1I/'Oumuamua at 122° and 2I/Borisov at 44°), from north of the ecliptic plane, rather than passing through the bulk of the Solar System before reaching us. So let’s assume the object in question here comes at a high inclination, too. That way the Sun probably won’t be blocking the view. Here is a graphic from Wikipedia of the two known objects’ trajectories: [![](https://upload.wikimedia.org/wikipedia/commons/5/58/A_comparison_of_two_interstellar_objects_passing_through_our_solar_system.gif) ](https://upload.wikimedia.org/wikipedia/commons/5/58/A_comparison_of_two_interstellar_objects_passing_through_our_solar_system.gif) ~Max ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [March 2, 2022, 10:30pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/31 "2022-03-02T22:30:49Z")
> [@Andy\_L](#): > > > [@md-2000](#): > > > > But because earth moves in its orbit, the night sky actually “moves” from our point of view. At a certain point we would at least see some parallax from the object. > > Agreed. It’s just that there are a lot of people looking for comets or for new planets, both of which would move across the sky. Looking for an unusually large (and growing) parallax circle is more of a specialty, I think. But wouldn’t our lunar-sized interloper _also_ be apparently moving across our sky? The Earth _is_ a moving target, after all; it’s not headed straight at us, it’s on a trajectory to intersect with our path. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 2, 2022, 10:39pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/32 "2022-03-02T22:39:13Z")
> [@Chronos](#): > > Yeah, it’d be ludicrously difficult to build a self-sustaining Mars base in that kind of timeframe… Yeah. But, hopefully, the whole world would bend its collective efforts to this one task. Money is no object kinda thing; all industry dedicated to that one task. Still not easy at all but better than phoning Elon Musk and asking if we can go to Mars tomorrow. And, as mentioned, we’ve got to try. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [March 2, 2022, 10:47pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/33 "2022-03-02T22:47:41Z")
Could we do it technically, _maybe_. But, how many people are going to go to work every day, building rockets and colony materials, so that other people can go to Mars while they stay behind to die? Some, sure, but enough to accomplish the mission? Hard to say. And that’s not counting people looking to sabotage it, under the rationale of “If I can’t have it, no one can.” ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [March 3, 2022, 12:39am UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/34 "2022-03-03T00:39:06Z")
Yes, it would be moving across the sky, but in a very different pattern than planet or comet watchers are looking for - killer moon would be moving in a (growing) circular pattern instead of a (mostly) linear way. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [March 3, 2022, 2:15am UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/35 "2022-03-03T02:15:03Z")
It would still be detectable with a blink comparison of the star photographs - a moving object against the background of stars. Unless it was coming straight down from exactly the north or south pole directly? Even then, there would be parallax issues. Also, an interstellar object would still take years to traverse the distance. It would likely be doing close to sun escape velocity, 42km/sec. It would take years to cross the solar system. At the size of the moon, we should see it coming years away. That could make for interesting social consequences. I’d be more worried about a lesser but still devastating meteor, like the dinosaur killer or slightly smaller. You know it will hit, you know some people will likely survive initial impact, but then what? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 3, 2022, 2:32am UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/36 "2022-03-03T02:32:12Z")
> [@md-2000](#): > > I’d be more worried about a lesser but still devastating meteor… You’re probably right but this is an eye opener (to be fair it is a big galaxy but still…): > **[Our Galaxy May Have 50 Billion Rogue Planets, Just Doing Whatever the Hell...](https://www.popularmechanics.com/space/deep-space/a26788244/milky-way-rogue-planets/)** > > And we'll probably never find them. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [March 3, 2022, 2:38am UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/37 "2022-03-03T02:38:14Z")
> [@md-2000](#): > > Also, an interstellar object would still take years to traverse the distance. It would likely be doing close to sun escape velocity, 42km/sec. It would take years to cross the solar system. At the size of the moon, we should see it coming years away. This is all true except for the velocity. I assume that 42 km/s the escape velocity for a body at 1 AU from the Sun. That’s how much the body will gain as it falls into the Solar System. But it will have excess velocity above that, basically its velocity relative to the Sun before it starts falling in. For example, 'Oumaumau had an excess velocity of 26.3 km/s and Borisov had 32.2 km/s. This excess velocity is reflected in their orbits as they went through the Solar System. See the graphic posted by Max\_S above. The higher the velocity, the flatter the trajectory, which you can see on that graphic. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [March 3, 2022, 5:00am UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/38 "2022-03-03T05:00:20Z")
Back of the envelope calc - So split the difference, let’s say when the killer reaches earth it’s doing 60km/sec. 1AU is 150M km, so it would take about 29 days to go 1AU. Jupiter is 10AU away, so if we could see the menace when it reaches Jupiter’s orbit, we’d have 10 months or more depending on the amount of acceleration you expect. So around a year of knowing that we need to repent because the end is near. Depending on the object’s albedo and the alertness level of our detection systems, possibly years to accept the realization that we are royally and rectally screwed. Maybe until about the 6 month warning, it will be “it may be close, it may miss” but at a certain point the trajectory will be pretty definite. I don’t think we’d want an extremely close encounter either, say a miss of 10,000 miles or less with an object the size of the moon. I assume that would have tectonic repercussions not to mention tidal destruction. It would take over 3 hours to cross the distance of the moon’s orbit, miss us by that much, and leave crossing the moon’s orbit. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [March 3, 2022, 5:24am UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/39 "2022-03-03T05:24:27Z")
> [@md-2000](#): > > I don’t think we’d want an extremely close encounter either, say a miss of 10,000 miles or less with an object the size of the moon. I assume that would have tectonic repercussions not to mention tidal destruction. Not tidal. It wouldn’t be around long enough to do that. Possibly tectonic, but unlikely.\* At that distance, it’d likely take out some navigation and communication satellites. Many communication satellites orbit at geosynchronous altitudes (36,000 km) and navigation satellites are around 20,000 km (give or take a few thousand). So it would cross their orbits, both incoming and outgoing, to get to 10,000 km. The worst effect of it coming in that close is the mass freakout among just about every part of society. I’d hesitate to guess what kind of strange behavior it would generate. . \*I don’t think it’s ever been proven that graviational effects from other astronomical bodies have ever caused earthquakes, landslides or whatever. They’ve tried, though. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [March 3, 2022, 2:24pm UTC](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256/40 "2022-03-03T14:24:53Z")
> [@dtilque](#): > > Many communication satellites orbit at geosynchronous altitudes (36,000 km) and navigation satellites are around 20,000 km (give or take a few thousand). So it would cross their orbits, both incoming and outgoing, to get to 10,000 km. It probably won’t hit them, but it would certainly disrupt their orbits. > [@dtilque](#): > > The worst effect of it coming in that close is the mass freakout among just about every part of society. I’d hesitate to guess what kind of strange behavior it would generate. Seems it could also alter the Moon’s orbit. > [@dtilque](#): > > I don’t think it’s ever been proven that graviational effects from other astronomical bodies have ever caused earthquakes, landslides or whatever. They’ve tried, though. I don’t think that it’s been _proven_, as that’s a pretty complicated science, but I do believe that most of the consensus seems to be on the side that the Moon does in fact influence tectonic activity. Another body of the same size, but passing closer, seems like it would cause some effects, even if only temporary. [Previous page](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256.md?page=1) [Next page](https://boards.straightdope.com/t/when-and-how-would-we-detect-an-interstellar-object-the-size-of-the-moon-headed-right-at-us/960256.md?page=3)