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## Meteor Watch Day

Today (the 30th of June) is the annual “meteor watch day”, and because of this I am on live TV this evening talking about meteors. I haven’t been able to find out why today (the last day of June) is designated as “meteor watch day”, but as the day seems to be American in origin I’d normally suspect that the greeting card industry were behind it! But, in this case, I cannot see too many cards saying “happy meteor watch day” being sold, so maybe on this occasion it is not an invented holiday by the US greeting card industry 😉

## What is a meteor?

I have discussed meteors several times before, for example here and here. But, just so this background information is all in one place, I’ll repeat myself. A meteor is simply a bit of (natural) space debris which enters into the Earth’s atmosphere. Before it enters the Earth’s atmosphere it is called a “meteoroid” (or, a really big one would be called an “asteroid”), but upon entering the atmosphere a meteoroid becomes a meteor.

The reason a meteor appears bright is because it is burning up in the Earth’s atmosphere due to friction. Most meteors are very small, no larger than grains of sand, but about once a day something the size of a basketball enters the atmosphere, and about once a week something about the size of a car. Larger and larger meteoroids are less and less common, so for example something the size of the meteor which exploded over Chelyabinsk in southern Russia last year (which is thought to have had a size of 17-20 metres as it entered the Earth’s atmosphere) probably enters the Earth’s atmosphere once every 50 or so years. The larger the meteor, the longer it will take to burn up, so most meteors (the sand-grain sized ones) burn up in less than a second.

A “meteorite” is a meteor which makes it to the ground, or at least of which fragments make it to the ground. So, for example, the meteor which exploded in the atmosphere over Chelyabinsk in February of last year (2013) led to several fragments landing (as I discussed <a href="“>here), and these fragments are meteorites.

Not surprisingly, it is only the larger meteors which make it to the ground as meteorites, although precisely what size a meteor needs to be to make it to the ground as a meteorite depends on a number of things including the composition of the meteor, the angle at which it enters the Earth’s atmosphere and the speed at which it enters the Earth’s atmosphere.

As I’ve also mentioned before, there is now strong evidence that it was the impact of an asteroid (remember that is just the name we give to large meteoroids) which wiped out the dinosaurs and most of life on Earth some 70 million years ago. Although the statistics are very paltry and therefore unreliable, we believe the Earth gets hit by an asteroid large enough to cause a mass-extinction event about once every 150-250 million years. Although undoubtedly rare, such events are cataclysmic to life on Earth, and so it is not wasted money to spend time and resources looking for such large “near Earth” objects.

## Meteor showers

There are many meteor showers during the course of each year, they occur when the Earth passes through some debris in its annual orbit about the Sun. This debris is often the material blown off of comets. Some of the better known meteor showers are

Some well-known meteor showers
Name of shower Month in which it occurs
Lyrids late April
Perseids mid August
Orionids late October
Leonids mid November
Geminids mid December

The naming convention is quite simple, they are named after the constellation from where the meteors appear to emanate. So, for example, the Perseids meteor shower in mid-August appears to radiate from the Perseus constellation, as this figure shows.

The Perseids meteor shower in mid-August appears to radiate from the Perseus constellation

The best way to see a meteor shower is not to use a telescope or binoculars, but rather to just look up with your eyes. Although when traced back they appear to emanate from a particular part of the sky, they can appear anywhere and using a telescope or binoculars will restrict your field of view, leading to your possibly missing a meteor.

Lying on one’s back on the ground (or a rug or mat preferably; or reclining in e.g. a deck chair are very effective ways to view a meteor shower. But, be warned that if you are observing one in the winter months it will probably get pretty cold, so have some warm clothes and blankets with you, and a warm drink.

Also, meteor showers are best viewed after midnight. This is because after midnight your particular part of the Earth is facing in the direction of the Earth’s motion about the Sun, and so the meteoroids enter the atmosphere at a steeper angle with a higher speed, and are more numerous than earlier in the night. The image below shows a wonderful image of the Geminids meteor shower, taken in 2012.

A wonderful image of the Geminids meteor shower, which occurs each December.

## Fireballs

If you are very lucky you may see an extremely bright meteor, which is called a “fireball”. I have never seen one, but I have a story of how much luck is involved in seeing one. One November, when I was working at Yerkes Observatory, we arranged a public viewing of the Leonids meteor shower. Several of us working at the Observatory were out for a few hours with members of the public, and we saw several dozen small meteors. But nothing spectacular. My boss had been in his office working all evening, not taking part in our public observing. He lived in George Ellery Hale’s old house (the Observatory’s “Director’s House”), which was all of 2 minutes walk from the Observatory, and at about 9pm he popped home for a break from his work. As he walked the 150 metres or so to the house, he saw a great big fireball streaking across the sky, and the rest of us all missed it as we were inside warming up with a hot drink!

A fireball streaking across the sky

The Perseids meteor shower is in mid-August, and is one of the most popular meteor showers as it occurs during the Northern Hemisphere summer when many people are on their summer holidays and away from city lights. If you get a chance to observe it this year, I highly recommend it, but bear in mind that you do need to be in a dark place. Most meteors are quite faint, and you will miss all but the brightest ones if you are in a city.

Which is the most spectacular meteor shower you have seen? Have you ever seen a fireball?

## Has Armageddon arrived?

Yesterday an extraordinary coincidence happened. As I blogged about here, a 50m long lump of rock (asteroid 2012 DA14) was scheduled to zip past Earth yesterday evening (15th of February) at some 12 kilometres per second at a distance of “only” 28,000 km. Although this sounds like a lot, it is closer than any asteroid that size has come to Earth to our knowledge for many decades.

But yesterday I awoke to the news that another lump of rock had exploded in the air over an area in the Ural mountains in southern Russia near Kazakhstan. Initial reports were that about 500 people were injured, this figure had risen to closer to 1,000 by the end of the afternoon.

Of course some people wondered whether these two events were connected. Well, I can assure you they were not. Earth is constantly being hit by particles from space, as we whizz around the Sun at 30 kilometres per second. Every day some 100 metric tonnes (a tonne is 1,000 kgs) of material enters the Earth’s atmosphere. Most of these are sand-grain sized particles, with the number of larger and larger objects becoming fewer and fewer. It probably follows what we call a power law size distribution, in much the same way as the interstellar dust grains I studied for my PhD do. In fact, at the very small level the particles are interstellar dust grains, it’s only when they become larger that we start referring to them as asteroids or lumps of rock.

What this power-law size distribution means is that larger lumps are rarer and rarer. NASA has calculated the size of the lump which exploded over Russia yesterday to be about 15m, and the damage gives an indication of what something this size entering the Earth’s atmosphere can do. To my memory, this is the first time I know of a meteor explosion causing human casualties. The 50m sized asteroid which thankfully missed Earth last night would have caused far more damage, enough to wipe out a large city. We believe 50m sized asteroids hit the Earth about once every 1,200 years, larger ones even less often. And, asteorids a few kilometres in size, large enough to cause widespread devastation and even lead to global catastrophies seem to happen as rarely as once every few hundred millon years.

The Earth will be hit by an asteorid large enough to cause global devastation, of that I have little doubt. But the chances of it happening when humanity still exists is very remote in my opinion. And, as long as we are vigilant whilst we still exist as a species, there is much that can be done to deflect such large asteorids which are on a collision course for Earth. Unlike the portrayals in movies, probably what we do not want to do is send a missle to destroy a threatening asteroid. Rather, we want to gently nudge its orbit into one which will miss the Earth.

Gently nudging an asteroid so it will miss Earth can be done in a number of different ways, but possibly the most elegant is just to paint one side of it white. This will cause the Sun’s light to reflect off of that side more than the rest of the asteroid, and we can use the fact that sunlight has momentum to push the asteroid using sunlight!

## The MRN size distribution

The most popular interstellar dust size-distribution used by astrophysicists is one due to Mathis, Rumpl and Nordsieck (MRN), which they published in 1977. It suggests that the number of dust grains of various sizes follows the following distribution – $n(a) = n_{0}a^{-3.5}$ where $n_{0}$ is the number at some size $a_{0}$. So, for example, if you want to compare the number of dust grains which have a size of 1mm compared to the number which have a size of 5mm then the ratio is $\frac{1}{5}^{-3.5} = 280$ or that 1mm sized dust grains are 280 times more numerous than 5mm sized dust grains. It is unlikely that the distribution of lumps of rock follows exactly the same size distribution, but if it did the number of 15m lumps compared to 50m lumps would be $\frac{15}{50}^{-3.5} = 68$, so 68 times more 15m lumps than 50m lumps.

## Will this asteroid wipe us out?

An asteroid named 2012 DA14 is going to come hurtling towards the Earth on Friday (15th of February). I shall be talking about this on the BBC tomorrow (Thursday 14th of February). Will it hit the Earth and wipe out the human race? No, it is going to just miss us, and even if it did hit the Earth directy it would not wipe us out, it is too small at less than 50m in size. But, is being wiped out by an asteroid something that should concern us?

There are many asteroids large enough to wipe out most of the life on Earth

## A history of being hit

The Earth is very old, 4,600 million years old to be precise. We know this figure very precisely from radioactive dating of the Uranium in the Earth. We human beings have not been around for very long by comparison. If you were to represent the Earth’s history as a 24-hour period, the 40,000 year history of modern civilised humans would represent about the last 20 seconds!

Over the course of Earth’s very long history is has been hit by asteroids on a fairly regular basis. It may surprise you to know that every day over 100 tonnes (a tonne is 1,000 kilograms) of space material enters the Earth’s atmosphere. Most of this 100 tonnes is made up of small grain of sand sized material. But every day basketball sized objects hit the Earth, but they will burn up in our atmosphere. Once every couple of weeks a car sized object hits the Earth, but these too will burn up in our atmosphere.

Something the size of asteroid 2012 DA14, 50m or so metres in size, is thought to hit the Earth about once every 1,200 years. The last object of this size to hit the Earth was in 1908, when a similarly sized asteroid exploded in the Earth’s atmosphere and flattened trees in Tungusta, Eastern Siberia in an area covering some 800 square miles.

If asteroid 2012 DA14 were to hit the Earth it would explode with the force of about 2.4 million tonnes of dynamite. It would not wipe out life on Earth, but would cause some serious damage over a fairly wide area. But the closest 2012 DA14 will come to Earth is about 17,000 miles, where it will hurtle past us at a speed of about 8 miles per second. It will be a near miss, not a hit.

## Mass extinction and asteroid impacts

There are 5 recognised mass extinction events that have happened on Earth in the last 540 million years. A mass extinction event is generally defined as an event where over 50% of life on Earth at that time is extinguished. These 5 mass extinction events are

1. 65 million years ago (the event associated with the extinction of the non-avian dinosaurs)
2. 200 million years ago
3. 251 million years ago
4. 360-375 million years ago
5. 440-450 million years ago

There now seems to be fairly overwhelming evidence that the mass extinction event 65 million years ago was due to the impact of an asteroid about 10 km across. An impact crater under the Yucatan peninsula was identified in 1980, and a layer of clay at the geological boundary of this time shows high levels of iridium, which would have been deposited by the asteroid’s explosion.

What is less well determined is the causes of the other mass extinction events. But certainly a strong candidate for each one of them is the Earth being hit by a sufficiently large object. We know such large asteroids are out there, and in the game of Solar System billiards going on it seems almost inevitable that every now and then one of these large ones will hit Earth.

So, really the question should not be will Earth be hit by another asteroid large enough to wipe out most of the life on the planet, but when will this happen? Based on the rather limited statistics we have, the answer would be sometime in the next 150 million years or so, but unlikely to be any time soon. 150 million years is 3 thousand times longer than civilised humanity’s current 50,000 years of existence. So I wouldn’t worry about it too much.

This 50 metre long asteroid will pass closer than the orbit of communication satelliites.