I was talking to my 9-year-old son about the Florence asteroid that is passing close to the earth. He wanted to view youtube videos about that asteroids, and we saw the starting of this one https://www.youtube.com/watch?v=UzxMbkrNbcg and when I realized that the title video has 2036 as the year in which an asteroid crashes into the Earth. I explained that that was science fiction (like Back to the future) and only an animation

He asked me: "If an asteroid or similar object was to collide with the Earth,.... how many years or months before the crash occurs would the scientific community know?"

I suppose that the advance time depends on technology, so assume the current level of technology.

  • $\begingroup$ You need to clarify the size of the asteroid. For a small 20-m asteroid like the Chelyabinsk meteor, none. But their damage is local at worst. Florence is 4 km in diameter, 2000 times larger diameter and 8 billion times more massive. Is this question limited to asteroids that would be a significant threat to humanity and cause a mass extinction? It also depends on the orbit of the asteroid. $\endgroup$
    – gerrit
    Sep 2, 2017 at 10:25
  • $\begingroup$ @gerrit yes, I refered to a significant threat to humanity. But also interested in other disertations/reasoning/links about little asteroids/objects. $\endgroup$
    – Mika Ike
    Sep 2, 2017 at 10:46
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    $\begingroup$ Next time the first thing to say to your son is "I don't know, I'll look it up" before going on the Internet with him and confronting him with nonsense videos. You don't want to burden a 9-year old with doom scenarios. $\endgroup$
    – user1569
    Sep 2, 2017 at 11:10
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    $\begingroup$ @JanDoggen , I usually do that, but I don´t think that a youtube video about a simulation was so doom, when I was with him. I do that in other many cases, but if you see tv, you can´t control what image is comming in 5 seconds and viewing nes you must change channel sometimes(I do). $\endgroup$
    – Mika Ike
    Sep 2, 2017 at 19:50

1 Answer 1


Asteroids come in all shapes and sizes, and generally the bigger they are the easier they are to detect.

Small asteroids, from a the size of sand grains (properly called meteoroids) to a few meters across hit the Earth everyday without causing harm and are undetectable until they hit (we call the trace as they hit the atmosphere a meteor) They cause no damage. For such objects we get 0 hours warning.

Somewhat larger objects can cause some local damage For example the Chelyabinsk asteroid caused damage from broken glass in Russia. Objects this large may be detected a few hours to a few days. The Chelyabinsk asteroid wasn't, and hit with no warning. We have observed some small asteroids just prior to them hitting the atmosphere.

We have found large numbers of objects whose orbit can bring them close to Earth. Every object we have found has an orbit that misses Earth. With each new object that is located we can forecast its orbit for several centuries. However new objects are being found all the time. It is possible that a object will be discovered that has the potential to cause damage to a wider area. If the object happened to be on an orbit that had it collide with Earth then there could be less than a year's warning. That is very unlikely. It is slightly more likely that an object will be discovered to be on a collision course only after several orbits, in which case there would be years or decades warning.

Very large objects that could threaten the survival of humankind are rare. There are some such objects, (larger than Florence). We hope that we have now detected all very large near Earth asteroids, and discoverd them to be non-threatening (like Florence is not a current threat to Earth)

Finally a comet may be on a collision course with Earth. Comets that are spotted are usually detected a few months before they could threaten Earth. But comets are much rarer than asteroids.

In summary, between 0 hours and a century, with several decades being likely for an object that could do widespread damage.

The particular object referred to in the video is Apophis. It is roughly 400m asteroid that is not going to hit the Earth for as long as we can forecast. It will, however, pass close by in 2029, and again in 2036. In 2005 the uncertainty in orbit was such that we could not rule out a collision in 2036. Further observations in 2006 and after allowed and a more accurate orbit to be calculated, and showed that it would certainly miss.

  • $\begingroup$ Objects the size of sand grains are not called asteroids. From memory the threshold size is in the order of a meter diameter. $\endgroup$ Sep 2, 2017 at 12:59
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    $\begingroup$ Yes but the division is arbitrary. $\endgroup$
    – James K
    Sep 2, 2017 at 13:58
  • $\begingroup$ @james-k Thank you for the detailed answer! my is very gratefull to heard such a detailed answer. :-) Two more things if you know. ONE.- In case of forecasting a crash... are there solutions to erase that objects? or is science ficition? :-) TWO.- What films are recommended about this (or similar)issue?, depp impact, armageddon, ... $\endgroup$
    – Mika Ike
    Sep 2, 2017 at 19:45
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    $\begingroup$ About Diverting asteroids: en.wikipedia.org/wiki/Asteroid_impact_avoidance About fims - Amageddon is a bit of silly fun. Deep Impact tries to more realistic, but its still Hollywood. Neither are targeted at kids (both are rated for people aged 12+ in my country) You should look up the "Torino Scale" that rates the risk from and asteroid from 0 to 10. Currently every asteroid that we have discovered has a Torino Scale of 0 = no risk. $\endgroup$
    – James K
    Sep 2, 2017 at 20:20
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    $\begingroup$ Your question implied "My child was scared by video showing asteroid impact" So I'm not sure that watching a movie in which a comet impacts and kills millions of people is a good idea. It was rated PG-13 for intense disaster related elements. However, that is your parental decision. The infographic you show is about the names of different minor solar system bodies. I don't know of any infographic about the likelihood, effects and avoidance of asteroid impact, I certainly don't know any suitable for a Spanish 9-year-old. These are separate questions, which should be asked separately. $\endgroup$
    – James K
    Sep 3, 2017 at 8:00

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