Posts Tagged ‘Dinosaur extinction’

Yesterday (Monday the 26th) I was on the BBC talking about Asteroid 2004 BL86, a fairly large asteroid which has just flown by us. It was at its closest to us at about 16:10 GMT/UT, and was expected to be just over 3 times further away than the Moon when at its closest. Thankfully it missed us, because such a large asteroid would cause wide-spread devastation were it to hit the Earth.

As the name of the asteroid implies, it was discovered in 2004 by a programme known as LINEAR, the Lincoln Near Earth Asteroid Research, named after the Lincoln laboratory at MIT, who are one of the partners along with NASA and the US Air Force. 2004 BL86 is estimated to be between 400 metres and 1km in size, considerably larger than the asteroid which exploded above Chelyabinsk in southern Russia in February 2013. That asteroid has been estimated to have been about 17 metres in size, and injured about 1,000 people in a sparsely populated part of Russia when it exploded in the atmosphere. Just imagine the devastation a 400m+ asteroid would cause, with a mass over twenty times larger its energy would also be over twenty times more (assuming the same speed)!

An artist's impression of Asteroid 2004 BL86 passing the Earth.

An artist’s impression of Asteroid 2004 BL86 passing the Earth.

The orbital parameters of asteroid 2004 BL86 have been sufficiently well determined that we know that it will almost certainly never hit the Earth. The only way in which it could would be if its orbit were somehow significantly altered, but this is unlikely given its large size as it is much harder to change the direction of a large asteroid than a smaller one. It will not visit the Earth’s vicinity again for over 200 years, and in fact it is the largest known asteroid to come this close to us until asteroid 1999 AN10 passes us in 2027.

Most asteroids are found in the asteroid belt, which lies between the orbit of Mars and Jupiter. They are thought to be the small building blocks of the planets, but which were prevented from coalescing to form a planet because of Jupiter’s gravitational disruption.

The asteroid belt lies between the orbits of Mars and Jupiter. Further out is the Kuiper belt, the reservoir of short-period comets.

The asteroid belt lies between the orbits of Mars and Jupiter. Further out is the Kuiper belt, the reservoir of short-period comets.

However, there are other asteroids which are not in the main asteroid belt, and some of these have orbits which bring them close to Earth. Every now and again we get hit by one, but thankfully it is very very rare for Earth to be bit by a large asteroid. Although the statistics are very sparse, we think that something the size of the Chelyabinsk asteroid hits the Earth about once a century, and something the size of 2004 BL86 hitting the Earth would be much rarer, maybe once every few tens of thousands of years.

Where to see asteroid 2004 BL86

Unfortunately, this asteroid is not bright enough to be visible to the naked eye, but it is visible through a small telescope or large binoculars. It will be about 9th magnitude at its brightest, and the chart below, taken from Sky & Telescope magazine, shows its path through the sky against the background stars. Note: the times on this chart are Eastern Standard Time, so add 5 hours for GMT/UT. At the time of its closest approach to Earth it was in the constellation Hydra, and could be seen about mid-way between Jupiter and Sirius. Just before midnight GMT/UT on the 26th it passed into Cancer, and passed close to the Beehive Cluster at about 6am GMT/UT earlier this morning.

Where to find asteroid 2004 BL86 in the sky over the nights of the 26th and 27th of January.

Where to find asteroid 2004 BL86 in the sky over the nights of the 26th and 27th of January.

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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 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.

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


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

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?

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