Chance of a Space Rock Hitting the Earth

Scientific Calculation

Ever since that space rock hit a small lake near a town in Russia in 2012, everyone seems to be wildly speculating about whatever possible danger we might be in regarding some city being wiped out by such a collision with a space rock.

None of the people who are doing such speculation seems to have any background in science, or at least, they certainly are not using any scientific methods to back up their speculations.

It turns out that there are solid scientific methods which might show us accurate data regarding what our odds are of having such a space rock impact damage any specific city. Why doesn't someone use them?

People seem to assume that the few historical incidents that we know of are what give us the evidence we need. A major asteroid impact in Siberia in 1908 and the recent smaller incident in 2012 seems to represent the totality of what they then base their assumptions on. Most such people note that nearly 3/4 of the Earth's surface is covered by Oceans, so that they allow that additional space rocks probably hit the oceans.

But the main point is that no actual scientist would consider TWO EVENTS to represent a large enough pool of data to try to make any useful Statistical Analysis.

But it turns out that the NASA and the Russian space program left some Seismometers on the Moon's surface. Their actual reason for doing that was to try to detect if there are any Moonquakes like the many Earthquakes we have here. Those Seismometers on the Moon have been listening for more than 40 years already.

As it turns out the Moon apparently has a cold Core, and so the Moon is now believed to be essentially all solid rock, and not fluid like the Core and the Mantle of the Earth are. So actual Moonquakes turn out to be rather rare.

But when a Meteorite / Asteroid hits the Moon's surface, the impact causes shock waves to pass through the body of the Moon, much like Earthquake P- and S-wave shock waves do here inside the Earth. Since they left several Seismometers on the Moon, the exact timing of when the shock waves get to each Seismometer allows us to use standard and simple calculations to figure out exactly where the object his and the mass of the object which hit the Moon.

As a result, we have detected more than 5,000 meteorites or asteroids which have hit the Moon during the past 45 years or so. In general, that is a little over a hundred which hit the Moon every year, and they generally range from a few kilograms up to about a thousand kilograms (or about a ton).

With that many known impacts on the Moon, it is easy to do the Stastical Analysis regarding calculating probabilities.

We also know that the Earth is much larger than the Moon, being about 14 times the exposed target area for an incoming meteorite. This results in some interesting solid scientific facts. The Earth (statistically) must get hit by around 1,500 significant objects every year, but about 1,100 of them (statistically) hit the oceans and are lost. But that still results in about 400 objects (statistically) hit land areas of the Earth every year. And we even know what fraction of them (statistically) are on that larger end of the size scale.

When such objects hit Antarctica or the jungles of Africa or South America, or the remote forest of Siberia or northern Canada, probably no one ever sees them land. (By the way, in case you are thinking ahead, the Earth is so "active" that Seismometers here constantly are making wiggly lines in recording thousands of minor earthquakes, and so the very minor shock waves from a meteorite hitting the earth is generally lost in the "noise" of all those small earthquakes here. The Moon is so cold and "dead" that shock waves from even small meteorite impacts are generally recorded.)

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But we could add up the total Urban area of cities on Earth, and divide that by the total "exposed land area" of the Earth (57.470 * 106, to calculate what percentage of the Earth is represented by the New York City Metropolitan Area. For example, we see that we have an area of 3450 square miles. Simple division gives 0.006% of the area. We might want to count all significant space rocks, that is objects larger than 1 kg. We could then analyze the Moon's Seismomter data to discover that the land area of the Earth gets hit by a total of 400 space rocks every year which are at least 1 kg. Still following?

Well, if those were the numbers, then we might conclude that our 0.006% of the NYC area on Earth likely gets hit "about once every 42 years or so". The Chicago Metropolitan Area gets hit once every 62 years or so. For the Los Angeles Metropolitan Area, around once every 84 years or so.

Not that such knowledge would really do us much good, as such objects are too small for us to detect very far from the Earth in space. What about doing some statistical analysis of the Moon data to only considering larger objects? It might provide some valid scientific reassurance that their city (0.006% of the Earth's area), would likely then only get hit by such a larger size of meteorite every 33,000 years (or whatever). In other words, we are really pretty safe now.


There is another matter that I am amazed that NASA seems never to have had any curiosity about. They merrily talk about "sending men to Mars" and billions of dollars and decades of future plans are invested on that idea. In my opinion, other than for political reasons, its hard to see the value of spending billions of dollars for the 1960s program which got about a dozen men be able to briefly walk around on the Moon by the Apollo program. I am tempted to think that nearly all the accomplishments could have been achieved by robots on unmanned spacecraft.

However, because of the political reasons, I doubt that politicians in America are going to be dissuaded from trying to send a few men to walk on Mars.

But no one seems to have the slightest concern regarding the safety of the astronauts during such trips. Consider that the Apollo took about three days to get to the Moon, while a somewhat larger spacecraft for men to get to Mars will take more than a year each way.

My concern is this: We do detect micrometeorites that have made tiny holes in the unmanned spacecraft which orbit near the Earth. I do not see any admission that the International Space Station has had puncture holes in it, but it seems certain that during the years that it has orbited the Earth, such holes must have been created. But in a two-year trip to Mars and then back, what are the odds that the spacecraft will get punctured?

I don't see that anyone has even ever thought about that concern. But the reasoning above, regarding the seismometers that have been on the Moon's surface for several decades, and standard statistical analysis regarding smaller impacting particles which might not be sensed by a seismometer, seem likely that we could know what the odds are that a spacecraft of a particular cross-sectional area is likely to be hit by various sized particles in any given interval of time.

If that existing data suggests that such a spacecraft is likely to be punctured "ten times" during such a two year voyage, then either we should abandon all ideas of trying to send men to Mars, or we better provide them with a "tire repair kit" to be able to try to fix such holes in their spacecraft. How dare we endanger astronauts on such a potentially dangerous mission, without having studied existing data from the Moon's seismometers?


This presentation was first placed on the Internet in January 2013.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago