# How often do meteorites hit the Earth?

I heard of one that occurred in 2008 in the Nubian Desert in Sudan, how often does it happen? Is there a catalogue of all hits or are they very rare?

Related link (This is what inspired my interest not defining what I'm looking for) https://en.wikipedia.org/wiki/2008_TC3

At the time of asking I had not thought about the distinction between hitting the atmosphere and hitting the Earth.

• Are you referring to the individual asteroid that exploded in this case or the ~600 meteorites it formed?
– Dean
Feb 25 '16 at 11:26
• Slightly off topic but may be interesting to you - NASA page on reports of significant fireballs etc: neo.jpl.nasa.gov/fireballs (note it's not a complete list, as their notes say.) (Also not all of these necessarily reach Earth's surface.)
– Andy
Feb 25 '16 at 14:47
• @Dean good point, i think i'm really in the one that hit the upper atmosphere but I had not thought about it that far. Feb 28 '16 at 11:33
• amsmeteors.org/members/imo_view/… also tracks fireballs worldwide, but not just ones that hit the ground. However, their news section occasionally includes meteorite hits: amsmeteors.org/2011/08/…
– user21
Mar 1 '16 at 2:59

There is definitely not a catalogue of all meteorites hitting Earth. For instance, the ones falling in desert areas and in the ocean aren't found, and even the ones falling in more populous regions are easily mistaken for normal rocks.

## Meteorite rate

Meteorites come in all sizes, from sand grains to dinosaur-annihilating rocks. The size distribution of these rocks (as well as most other things in space) follow an approximate power law, meaning that the smaller the meteorites you consider, the more often they hit Earth.

Estimating the total number of meteorites hitting Earth is difficult, but one study (Bland et al. 1996), looking at the composite of the earth on three arid site and considering the weathering over time, finds that between 36 and 116 meteorites over 10 g hit Earth every year per $10^6\,\mathrm{km}^2$. Multiplying by the surface area of Earth $A_\oplus$, that's roughly 20,000 to 60,000 meteorites hitting Earth per year.

## Meteor rate

UPDATE: You want to know the rate of incoming meteors, i.e. all rocks that hit Earth's atmosphere. By definition, a meteorite is a meteor that lands on the surface of Earth. Before it enters the atmosphere and becomes a meteor, you can call it asteroid, meteoroid, rock, pebble, or dust grain, depending on its size and your mood.

Taylor et al. (1996) have monitored incoming meteors with the AMOR radar in New Zealand and detected in one year 350,000 trails from incoming meteors of size 10–100 $\mu$m vaporizing in ~100 km's height. I don't know exactly how large an area AMOR monitors, but from simple trigonometry, and as an order-of-magnitude estimate, if the events happen in $h \sim 100\,\mathrm{km}$'s height, the radar can "see" a distance $d=\sqrt{h^2+2R_\oplus h} \sim 1100\,\mathrm{km}$, and thus covers an area $A\sim4\times10^6\,\mathrm{km}^2$. Thus, again multiplying by $A_\oplus$, this amounts to the order of 40 to 50 million meteors per year.

## Killer meteor rate and the Sudan asteroid

The meteors discussed above is probably not what you had in mind. As mentioned, the bulk of these "rocks" are actually more like sand grains. So what is the rate of devastating killer-rocks from space? Brown et al. (2002) fit a power law to several different data sets (LINEAR, Spacewatch, NEAT and others), and find that the number $N$ of meteors above a diameter $D$ in meters is roughly $$N(>D) \simeq 37 D^{-2.7}.$$ The following plot is from their paper, with the long black line showing their fit, and the short black lines indicating the uncertainty:

You mention the asteroid 2008 TC3, which had a diameter of 4.1 m. According to the fit, such an event happens roughly $1/(37[4.1\,\mathrm{m}]^{-2.7})\sim$ once per year.

You can also see that the largest meteor hitting Earth in a century would be $(37\times100\,\mathrm{years})^{1/2.7}\sim20\,\mathrm{m}$, equivalent to $\sim500\,\mathrm{kiloton\,TNT}$. Similarly, hundred-meter-sized objects ($\sim1\,\mathrm{megaton\,TNT}$) should hit us roughly once per 10,000 years. By extrapolation of their fit beyond observational data, objects of $D>1\,\mathrm{km}$ — or 100 gigaton TNT — should hit Earth on average once every ~3.4 million years.

• By "hitting Earth" I assume you mean lands on the ground (or in the water) and leaves an artefact? Feb 26 '16 at 22:13
• @Octopus: Yes, as opposed to burning up in the atmosphere.
– pela
Feb 27 '16 at 12:44
• @Octopus: I edited the answer to include a discussion on the difference between the rates of meteors and meteorites.
– pela
Feb 29 '16 at 23:10