You are standing on the ground and a meteor 6 miles wide is hurtling to earth. It will wipe out 50 - 70 % of life.

Short primer on meteor impact on a nice planet.

You know much about this event, but what you do not know, because the dinosaurs did not record it nor did the rocks, and the maths people have not yet put their calculations online, is how long this meteor would be visible in the sky.

A day?

A week?

I'd like to know this so that I know how much time my people have, to evacuate to a sister planet. Thanks!


3 Answers 3


It can be a few days since the asteroid becoming visible to an unaided eye, however, only in the last minutes this will turn into something spectacular.

From which distance a 6km asteroid is visible? We have to estimate several factors, such as its albedo and angle of approach to Earth.

Using Size and Magnitude calculator and assuming albedo is 0.15 (15% of the light is reflected), we can estimate Absolute Magnitude of our asteroid about 13.8

Next, let's assume that Apparent Magnitude should equal to 5, meaning that celestial body is becoming visible to unaided eye.

Using Formula here and assuming that our asteroid is approaching from outer Solar System, we have the following equation:

$ 5 = 13.8 + 2.5 \times \log \left(\frac{3}{2} \times x^2 \right) $ ($x$ here is the distance from Earth to the asteroid, in AU)

$ -3.52 = \log \left(\frac{3}{2} \times x^2 \right) $

$ 0.000302 = \frac{3}{2} \times x^2 $

$ 0.000201 = x^2 $

$ x = 0.0142 ~\text{AU} $ or $ x = 2\,120\,915 ~\text{km} $

This is the distance at which our asteroid should become visible.

If Earth and asteroid are on converging trajectories, speed of approach can be as low as 10 km/s and even less. At 10 km/s this would translate to about 2.5 days during which it would be seen as a moving and brightening star.

However, it would not be immediately noticeable to regular people. Only during the last hour it would become bright enough to draw attention, and only in the last minutes it will enter Earth's atmosphere and become a meteor.

In a different scenario, when the asteroid would approach the Earth from the direction opposite the Earth own movement, it would be much less visible, and for a much shorter time.

P.S. If your people can evacuate to a different planet, it would be logical to assume that they have astronomy advanced to at least early XXI century Earth level, so they can predict asteroid impacts decades in advance.

  • $\begingroup$ On converging trajectories, the new star will appear to be almost stationary, changing location relative to the stars only due to the rotation of the earth. I don't think it would be noticed until at least 3rd or 2nd magnitude. At 2 million km the parallax shift would be about .006 radians, about a third of a degree. 2/3 of the angular diameter of the moon, and that's if you could see it for 12 hours. $\endgroup$ Nov 17, 2017 at 3:01

I think the asteroid that killed off the dinosaurs was more meteorite than meteor, up until the entry into earth’s atmosphere it would have been a lump of solid rock and or stone, but that’s beside the point. How long would it have been visible for before impact would depend on the level of technology available. If they have the technology to move off planet then I would assume they would also have some very sophisticated telescopes and would probably be able to detect this asteroid far in advance of when it hit probably years or decades or more.

Exactly how long would depend on what you assume about their telescopic technology and the orbit of the asteroid. If it had a very dark surface and was in a very elliptical orbit it might miss detection until quite late, may be a month... but it depends on your assumptions. How long have they been actively looking for asteroids for?

Edit taking in further comments
The people aren't astronomers and don't have telescopes so they will not see the object until very late. It still depends on the angle of approach and the side of the planet the observers are relative to the impact, but at an approach velocity of 10 or even 20km/sec with luck they get a night when they can see an unusual star in the sky. If it’s day time during the final approach then they get even less time. When it finally becomes a daylight visible object they have just hours left. The real fireworks happen in the last minute.

  • $\begingroup$ Hmmm. I was more thinking standing on the ground and noticing something strange in the sky. Like, those nights when you see the ISS and realize you are looking at something up there, with your naked eye, only instead of the ISS it turns out to be a meteor(ite) and is about to kill you. I guess the scientists on my world aren't astronomers - at least I don't have those (or telescopes) written into my world. (they might need magic to leave, or just evacuate away from the impact site and try to survive the fallout.) $\endgroup$
    – SFWriter
    Oct 9, 2017 at 22:23
  • $\begingroup$ I think in that case they're not going to get much warning. You can argue over the details but Alexander's post is more or less what's going to happen. If the albedo was higher it would be seen sooner, if the speed faster it would have been seen later. It is critically important on how attentive they are to the sky. If they are, they may get a day or two’s warning if there not, depending on orbits and time of day arguably hours or minutes. At 10km/s atmospheric entry to impact is going to be much less than a minute 10-20 seconds perhaps. $\endgroup$
    – Slarty
    Oct 10, 2017 at 8:41
  • $\begingroup$ This doesn't really answer the question. $\endgroup$
    – kingledion
    Nov 16, 2017 at 15:07
  • $\begingroup$ @kingledion Yes, but there again the question was initially a bit vague. In light of some of the further comments I have amended my answer - how’s that? $\endgroup$
    – Slarty
    Nov 16, 2017 at 17:57

If it happened at night, then unless they were very attentive to the night sky they likely wouldn't notice until it was very close. If it's on a collision course it won't move much relative to the stars -- only paralax shift as the earth moves changing your observation point. At that point it would initially look like a new, but dim planet. The reaction would curiosity, maybe some fear, but with no experience what would you do.

It wouldn't show a tail until it hit the atmosphere. At about 60 miles or so elevation, it would be visible for about 1150 km away. At 10 km/s it would be visible only 10 seconds. At a slanty angle you can multiple this by a 2-4. At 70 km/s (about the maximum impact speed a solar system object can hit the earth, the times are far shorter.

So, if you are close to the impact point you see this dot expand to a burning ball in a few seconds. Further news at 11:00

If you are far away from the impact point you won't notice this tiny star near the horizon until it hits the atmosphere. Then you see a pillar of flame for a few seconds. It will disappear for a few seconds under your horizon, then you will see that part of the sky light up. If you knew what it was, you might take action in terms of huddling away from trees or anything that can get knocked down.

At 1100 km away (seeing only the start of the atmospheric hit) it will take 3300 seconds (not quite an hour) for the shock wave to hit you at the speed of sound. I'm not sure how violent that shock wave will be. (Initially the shock wave travels faster than that: At higher temps the speed of sound is faster.)

Beyond that distance then the shock wave will be the first notice that you don't have to go to work tomorrow.

Things are slightly different if you are on the day side. (No rule says meteors land at night. Just easier to see.) Won't notice the star. It will be a bit later after it hits atmosphere for it to brighten enough to notice.

This also assumes that the meteor hits land. If it hits water you may still be in the hurricane of steam generated. Beach front property on that ocean will depreciate sharply.

Lucifer's Hammer by Larry Niven posits a strike by a comet. It breaks up before hitting the earth so instead of one large object, it's a solar system scale shotgun blast. Good read.


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