This is another branch of my previous question.

Currently, in real life, space agencies around the world charts and tracks asteroids and other objects that orbit the sun. They project the object's orbits and basically do simulations of their future orbits to determine whether or not the object will intersect with the Earth in the future. That's how their "prediction" works.

However, in my scenario, the 20-km object appears out of nowhere (a dimensional rift that closes soon after, so no one knows about it) as shown in the diagram below. The Earth and the asteroid are both traveling counter-clockwise.

Meteor Diagram

More Asteroid Information: The asteroid has a reddish crystalline surface, with a mass of approximately 2.5 times Chicxulub despite being double the diameter.

Question: Given current technology, around how long would it take the Earth to realize that they will be hit?

To be more generally applicable, given no past orbital data, how long would it take astronomers to figure out whether or not a given object will strike Earth in the near future?

Accepted answers will give a rough time-frame and justification.

Optional but Relevant Question: If the above question addresses the combined capabilities of the agencies of the world, how would the answer change as access to information decreases? Say, a stargazing guru or some guy with a telescope.

(This might belong in Space exchange, but given the sudden appearance of the meteor and the dimensional rift, I thought I might try here first. Vote to close if it doesn't belong here.)

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    $\begingroup$ Your illustration is gravitationally incorrect -- the sun must be at one focus of any orbit. This may lead to incorrect intuition of what gets where, when. Lots of software around to simulate orbits... $\endgroup$
    – Zeiss Ikon
    Jun 15, 2021 at 12:34
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    $\begingroup$ Ignoring gravitation between Earth and the asteroid, I think the drawing could be made correct by changing the orange oval to a near-circle nestled inside Earth's orbit. Since the asteroid's orbit is smaller (to be precise, it has a smaller semi-major axis), it will orbit faster and will catch up with Earth. However, gravitation between the Earth and the asteroid will change that situation, and I have no idea exactly how. (My knowledge of orbital mechanics all comes from Kerbal Space Program, in which objects are only gravitationally attracted to one celestial body at a time!) $\endgroup$ Jun 15, 2021 at 13:36
  • $\begingroup$ Ill find a simulator, but its quite difficult to find one that allows 3 bodies. Ill have to overlap them manually $\endgroup$
    – Henry Shao
    Jun 15, 2021 at 13:42
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    $\begingroup$ Another way to have the object "appear" is to have it come from interstellar space at a high angle and get "sling shotted" by the sun to impact the earth. The first interstellar visitor detected, Oumuamua was first observed after it had already come in and passed the sun. Look also at the path of the Chelyabinsk meteor which came at the earth from very close to the sun and thus was not detected. Even today, it might be difficult to detect such a meteor before striking. $\endgroup$
    – David R
    Jun 15, 2021 at 14:12
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    $\begingroup$ Note that estimating the mass of something that small is actually very hard (because it's gravity is so weak it's almost inconsequential). $\endgroup$ Jun 21, 2021 at 15:23

5 Answers 5


According to your diagram, the 20-km asteroid pops out of nowhere while less than 0.2 AU from earth, and 6 months before impact. It is at about 1 AU from the sun, thus presenting a well-lit half-hemisphere to observers.

It will be detected that same night.
The asteroid will be visible as a magnitude 7.5-9 point. This is dimmer than the naked human eye can see, but well within observation levels for even backyard amateur astronomers, of which there are tens of thousands scanning and photographing the skies each night.

It will have a rather high proper motion against the sky, making it glaringly obvious on any photograph of that part of the sky. However its motion will be too small to fit any normal (not GSO) satellite. Any astronomer seeing it on a photo will think they have discovered a new comet, and will repeat observations over the next few nights, thus quickly figuring out that something was irregular about this object.

So 6 months (minus a few days to detect, identify and characterize its properties) worth of warning.

6 months is not enough time to deflect a 20-km rock! Not without preconstructed assets ready to launch immediately. In the normal run of things, even now, our observation capability is good enough to detect such a large object at least 5 years ahead of time, and that is assuming it snuck up on Jupiter and got slingshotted at Earth. Any other trajectory the expected detection time would be multiple decades.

They will be able to hit it with a nuke, fragmenting it, causing much of the asteroid to miss. But a hefty percentage will still hit Earth, and a fragmented asteroid of that size is possibly even more dangerous than the intact thing.

As for the time from detection to prediction of impact, it depends on the quality of data available.
For something THAT bright, and THAT close to Earth, they could hundreds of observations from around the globe, thus providing actual parallax measurements which will nail down the distance, direction and speed to very tight limits. Close enough to identify it as a potential impactor withing a day or two.

Knowing it to be a potential danger, it is close enough that they could then ping it with a radar, giving distance and position and speed information down to millimeter accuracy. Yes, the Goldstone Solar System Radar can and does make such observations. It is normally booked in advance like crazy, but for a discovery like that, the Powers will prioritize immediate examination.

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    $\begingroup$ I didn't realize that something so small would be so bright even at such a distance. That makes my question kind of dumb in hindsight. $\endgroup$
    – Henry Shao
    Jun 15, 2021 at 12:12
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    $\begingroup$ @HenryShao Questions that increase your insight and understanding are never dumb. $\endgroup$ Jun 15, 2021 at 14:04
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    $\begingroup$ "a fragmented asteroid of that size is possibly even more dangerous than the intact thing." - plus, it's now radioactive ... and blankets one half of the entire planet... even before air currents and suborbital ejecta spread it further. $\endgroup$ Jun 15, 2021 at 19:08
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    $\begingroup$ @HenryShao a 20-km object anywhere inside Mars' orbit is a glaringly obvious thing, to astronomers. Unless it is also (much) closer to the sun than Earth, in which case it is both shaded and in the glare of the sun. Just move your asteroid to a much more elliptical orbit, hugging the sun yet passing Earth's orbit. $\endgroup$
    – PcMan
    Jun 15, 2021 at 19:39
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    $\begingroup$ @bytepusher a 20km meteor will mass something like 6-10 trillion tonnes. It will resist being moved to a quite ludicrous degree. While small movements in space add up wonderfully over enough time, 6 months in not enough time. For a 20km rock, we want a lead time of decades, not years and definitely not mere months. $\endgroup$
    – PcMan
    Jun 16, 2021 at 8:15

The dominating time issue is detection, which is measured in years. Tracking the objects is faster, on the order of months. There's simply a lot of empty space out there, and it takes time to sweep it for objects. As an example, consider the space surveys of Near-Earth objects (NEOs). There are a collection of detection activities out there, collectively termed as "spaceguard:"

As a result [of spaceguard activities], the ratio of the known and the estimated total number of near-Earth asteroids larger than 1 km in diameter rose from about 20% in 1998 to 65% in 2004,[8] 80% in 2006,[64] and 93% in 2011

This can be used to determine detection time probabalistically. If we can detect on the order of 10% of objects in 1 year, then it should take on average 10 years to observe the object.

This is just to identify a NEO. The next step is to identify the likelihood of an impact. We don't really know this for certain until much too late. N-body orbits are too messy on the scale of one Earth radius. However, once again we can do things statistically. There are two scales that we grade NEOs on for impact risk, the Torino Scale and the Palermo Scale. Your object would get graded on these scales. This process happens rather quickly, with a reasonably small number of observations. Obviously we haven't had any large strikes on our planet to test the positive direction (the direction you care about), but we do have evidence for objects moving in the negative direction, in which observations demonstrate that the NEO is not a high impact risk. And, needless to say, any object that is "interesting" is going to get a lot of observations rather quickly. As an example case, the comet C/2013 A1 was on a potential intercept course to Mars. In March of 2013, it was predicted to have an impact probability of somewhere on the order of 1:1250. By April 2013 this was reduced to around 1:120000 (an almost certain miss), so we can see that these numbers resolve on the order of single months.

Obviously your storyline can fudge these numbers. If the object does something "noisy" as it appears from the dimensional rift, we would point a telescope towards it sooner, and thus cut down on the detection time. A stealth coating on the object would increase it. And more interest in spaceguard tasks would obviously devote more resources to cataloging objects. On the tracking side of things, a NEO with a high risk of Earth impact is going to receive considerable attention.This glut of raw observations could be processed into a good track faster, increasing its Torino and Palermo scores more rapidly. But these are some baseline numbers you can use for the real life process astronomers are undergoing today.

As for your single-stargazer question, they can't really affect the timeline for tracking the object all that much. We depend on much more sensitive equipment to do this. Much of the spaceguard activities done in the USA was due to congressional mandate, and a corresponding budget. A rogue individual just doesn't have the precision needed to do this on a short interval. However, they could observe the object sooner. As a classic example, the Messier objects are objects catalogued by Charles Messier in the late 1700s. They are objects like galaxies and nebulae that are popular targets for amateur astronomers to look at. He wasn't actually trying to look at these objects. He was actually trying to identify comets, and developed his catalog of comet-like objects to help other comet enthusiasts to identify objects that are certainly not comets. So never underestimate the ability for an individual to look at the sky in a different way, and see something valuable and unexpected.

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    $\begingroup$ So a key to early detection is if the rift itself does something weird that can be detected, which would attract interest to that region of the sky? Does the newly included composition of my asteroid change anything since its now (presumably) brighter? $\endgroup$
    – Henry Shao
    Jun 15, 2021 at 12:22
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    $\begingroup$ @HenryShao That is quite bright. I think I'd defer to PcMan's answer here. I vastly underestimated its brightness as well. $\endgroup$
    – Cort Ammon
    Jun 15, 2021 at 13:30

Asteroid detection is rather difficult, due to the normally small apparent magnitude of the object. For this reason the largest body of asteroids are detected when they are across from the sun, giving them the brightest reflection in the darkest part of the sky. Everything stated by Cort Ammon still holds, but two there are some things to consider. First, an asteroid of that size is rather large compared to most of the objects being tracked. For comparison, Bennu and Apophis measure 500 and 370 meters respectively, and they were relatively easy to find. Secondly, the appearance of such a massive object at such proximity might be noticed by inertial instruments.

In any event, if detected when across from the sun, your diagram suggests three months of awareness of the object's existence. But because of it's size and proximity, as long as the moon is in the right place, you could probably see it with a backyard telescope, maybe even the naked eye (depends on how it's angled, what it's made of, etc.)


I'm not answering your direct question however I'd like to comment on your statement:

... the 20-km object appears out of nowhere (a dimensional rift that closes soon after, so no one knows about it)

You don't need any dimensional rift or wormhole or magic for asteroids to appear "out of nowhere".

This page lists all asteroid "close approaches" that we know of either in the past or future (objects that came closer than the moon's orbit): https://en.wikipedia.org/wiki/List_of_asteroid_close_approaches_to_Earth

Between 2010 and 2020 there were 22 asteroids that were discovered after they nearly miss hitting Earth. So they basically "appeared out of nowhere" because the human race did not know about them until it was after "too late".

In the same period of time there were 16 objects that were discovered less than 24 hours before they nearly miss hitting Earth. I still think it qualifies as "appears out of nowhere" if we discover something today that will hit us tomorrow.

Between 2010 and 2020 there were only 7 objects that nearly missed Earth that were discovered more than 24 hours before. Out of the 7:

  • 3 was discovered the day before
  • 4 was discovered less than one week before

Out of all the asteroids that nearly missed hitting us between 2010 and 2020 none were discovered more than one week before its close approach.

There is very high probability that if an asteroid were to hit Earth we will not see it coming more than one week before hitting us. So you don't need any excuse for appearing out of nowhere - that's just the reality we live with today.

Your readers however may not be familiar with our history of detecting objects that may hit our planet. So instead of conjuring a portal or wormhole or some magic I'd just explain to the reader our real-world history then say something along the lines of:

... and so it was that we discovered asteroid ABC123 just under one month before it would hit Earth

  • $\begingroup$ The dimensional rift is there for a story reason and is mandatory, and a few months or even a year is to give enough time for a slow descent into chaos. $\endgroup$
    – Henry Shao
    Jun 16, 2021 at 13:04
  • $\begingroup$ So you have the choice of anything between discovering the asteroid almost instantly to not discovering it at all until it makes impact (the Tunguska event is a good example of not discovering until impact). You can choose basically any discovery time for story reasons $\endgroup$
    – slebetman
    Jun 16, 2021 at 13:07
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    $\begingroup$ None of these objects were anywhere remotely close to the size of 20 km, though. Tunguska was estimated at 100 m, but that was long before anything like modern observational equipment was available. Of the ones between 2010 and 2020 that you mentioned, only 3 were possibly more than 20 m and none over 31 m. You can't really extrapolate from failing to detect 1-20 m objects in advance to failing to detect a 20 km object in advance. That's on the same order of magnitude as saying "We couldn't detect these moths at 100 m, so we won't be able to detect a Boeing 747, either." $\endgroup$
    – reirab
    Jun 16, 2021 at 22:32

It depends on where it is and how fast it's coming. For most situations it's bright enough some comet hunter will find it pretty quickly.

However, if it's coming from not quite out of the sun and coming in fast it might not be detected until very soon before impact.

If it actually occludes the sun some solar telescope would spot it. However, in the zone near the sun but not in front of the solar disk there are very few observations due to the light spilling in from the sun. Big scopes are never pointed anywhere even near the sun and few scopes are pointed within a few degrees of the sun at any time.

  • $\begingroup$ Not even the 4m Daniel K. Inouye Solar Telescope counts as a big scope? Sure, it's only 1/3 the diameter of the biggest scopes, but it's still in the top 25 as of this moment. $\endgroup$ Jun 16, 2021 at 17:03
  • $\begingroup$ @GaryWalker I meant big astronomical telescopes. Of course a solar telescope is pointed at the sun--but it's not going to see an object that's simply near the sun. $\endgroup$ Jun 16, 2021 at 19:04
  • $\begingroup$ Wouldn't it have to already be pretty close to Earth at its appearance in order to remain near the sun in the sky long enough to matter, though, given Earth's orbit around the sun? $\endgroup$
    – reirab
    Jun 16, 2021 at 22:13
  • $\begingroup$ @reirab The angle is going to drift somewhat but you have a fair range where the sun messes up observations and thus few observations are made. $\endgroup$ Jun 16, 2021 at 22:46

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