UPDATE on the big UK fireball

By Phil Plait | September 26, 2012 7:00 am

There’s been a bit more news on that amazingly bright and weird fireball seen moving across the skies of northern UK last week.

Marco Langbroek is a paleolithic archaeologist in Amsterdam, and also an amateur satellite tracker – though with modern tech, the term "amateur" is arguable. Anyway, he’s been looking at the track and velocity of the meteor using eyewitness accounts (and the video taken), and thinks he can rule out the cause being the re-entry of human-made debris from a spacecraft. In fact, he thinks the meteoroid (the term for the actual object responsible for the light show) was an Aten asteroid: part of a class of rocks that orbit the Sun on paths that tend to keep them inside Earth’s orbit*.

The key issues here are the slow speed it moved across the sky, and the fact it moved east-to-west. That last part is really important: very few satellites orbit retrograde, or in that direction. Most orbit either prograde – west-to-east, the same direction the Earth spins and also the same direction it orbits the Sun – or in polar orbits (north/south). So right away that makes it unlikely the meteor was from a spacecraft.

However, what has me scratching my head is the slow speed of the meteor. A rock orbiting the Sun retrograde means its velocity will add to the Earth’s, making it move faster as it burns up, not slower. It’s like two cars in a head-on collision; if each is moving 100 km/hr then the resulting collision speed is 200 km/hr relative to either car. You get slower relative collisions if they’re moving in the same direction; they’ll merely bump at low speed relative to one another.

We see this with meteors; the Leonid meteor shower, for example, is made up of tiny particles that move almost in the opposite direction of the Earth, and when they burn up in our atmosphere they move extremely rapidly across the sky. The collision speeds can be 70 kilometers per second!

So why was this meteor over the UK moving so slowly if it were an Aten? Marco thinks he has the answer to that. If the asteroid happened to be at aphelion – the top of its orbit, when it’s farthest from the Sun, also when moving most slowly and in a direction nearly parallel with that of the Earth – it would all add up. The backwards direction and the slow motion would be a natural consequence of this. [UPDATE: I made an error here: the asteroid can orbit the Sun prograde! When it’s at the top of its orbit, it can be moving slower than Earth does around the Sun, so when we look at it it appears to move east-to-west. It’s like passing a slower car in a faster one; to the driver of the passing car, the slower one appears to be moving backwards when in reality they are both moving in the same direction. I hope that clears up any misunderstanding!]

I’ll note that as far as I have thought about this, I agree with Marco. It’s not conclusive yet, though, but it’s compelling.

Meteors like this are rare. One that gets this bright, is seen by so many people, and drops bits of itself as it burns up are rare enough (the Peekskill meteor in 1992 is the best example of this), but one moving retrograde is even weirder. If Marco is right then I hope even more people submit their observations, pictures, and videos to the International Meteor Organization website. Those observations can help scientists determine the orbit of the object more accurately, and help pin down exactly what the heck this crazy object was.

Image credit: Craig Anderson

* Technically, an Aten asteroid has a semi-major axis less than one Astronomical Unit. Orbits are elliptical, and the semi-major axis is the half-diameter of the orbit along the long axis. Despite this, an Aten can cross the Earth’s orbit if its orbit is elongated (eccentric) enough.

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CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (17)

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  1. I don’t think Langbroek is suggesting that the *heliocentric* orbit is retrograde – just that the apparent orbit wrt Earth is retrograde. All you need is for the Earth to be overtaking the meteoroid in its (prograde) orbit. An Aten at aphelion would be going slower than Earth so it fits the bill.

  2. Nathan Z

    I understand what you are talking about because I play Kerbal Space Program. Before a few weeks ago terms like “retrograde” and “aphelion” would have gone right over my head!

    VERY COOL ideas on the orbit of this thing! :)

  3. MSL’s approach to Mars would be similar: an object moving slowly at the aphelion of a elliptical [Hohmann transfer] orbit and being “run over” by the faster moving planet behind it. I have to say I find it difficult to visualize what happens as the object falls into the planet’s gravitational field which would slow it down in a heliocentric frame of reference but speed it up relative to the the planet.

  4. SMM


    Off topic but I wanted to ask a question. You mention two cars traveling at 100 km/hr in opposite directions hit head on would have a resulting collision speed of 200 km /hr. I think that’s sort of true, but the resulting impact would still be at 100 km /hr. Channeling Netwon each force has an opposite force (I push on a wall, the wall pushes back). I’d think the resulting forces would cancel each other out, and would be the equivilant of a car hitting a wall at 100 km /hr, not 200 km /hr. It could be semantics, and I could totally be wrong (it’s been 20 years since my favorite HS class), but I think I misunderstood your analogy.


  5. I have zero idea how fast 70 km/sec is. I also have zero idea how fast 156,585 miles per hour is.

    The Sun’s photosphere temperature is about 5,778 K. That’s about 9,941 Fahrenheit or 5,504 Celsius. None of these numbers has the slightest impact.

    As an American, i use Fahrenheit (though i can’t spell it) for the temperature i might feel outside. I’m stuck with it for baking stuff in the oven. For everything else, i use Kelvin. But i don’t have a standard for absurd speeds. For absurdly high speed, percent of the speed of light makes sense. In this case, 0.023% C. It’s a scale that goes from zero to one hundred, with no other allowed values. Albert said so. ‘Wicked fast’ works too.

    It’s something sorta like this, though. Two muffins were baking in an oven at 450 degrees Kelvin. One says to the other, “It’s hot in here.” The other says, “Wow! A talking muffin!”

  6. Please note: the resulting orbit is “retrograde” only for an earth-orbiting, earth-centered orbit (i.e, a satellite orbit).

    The heliocentric orbit resulting from this entry direction (east-west) is a normal PROGRADE heliocentric orbit.

    I realise that is confusing: the key is that earth-centered orbits and heliocentric orbits have a different frame of reference! In an Earth-centered frame of reference an east-west moving object *must* be retrograde (it moves against the direction of the earth rotation). For a heliocentric orbit, an east-west movement can (and does in this case) match a normal prograde orbit.

    The Aten-like orbital solution is surprisingly robust, even allowing for considerable deviation of entry direction, entry angle and speed from the nominal values. That was the cool thing about it: even granted that the current trajectory (and certainly speed) solutions have a considerable margin, within that margin all the resulting orbits have an Aten character. You can vary by over 20 degrees on either side of the nominal azimuth of entry, by over 40 degrees in entry angle (much larger than what is realistic for this long grazing 1100 km trajectory!) and in speed between 12 and 30 km/s and still it will result in an Aten character orbit.

  7. TSC

    Phil, as others have said, the slow relative speed makes sense if the asteroid were in a prograde orbit around the sun. It would be like two cars travelling in the same direction down a highway at slightly different speeds. Though they’re both travelling very fast, their speed relative to each other is slow.

  8. ” if each is moving 100 km/hr then the resulting collision speed is 200 km/hr relative to either car”

    Euh… didn´t the Mythbusters do an episode were they smashed a car with 60 miles an hour into a wall? And then another (same make and model) with 120.
    And then to see if two cars headon, each going 60 would make the same damage as a 120 into a wall.
    And didn´t they find out that each car after the 60 headon looked just like an 60 into wall afterwards.
    Might be that I have the 60´s and 120´s wrong, but that´s because you do things in miles and we in proper km´s in the subtitles. But that´s not the point, point is that it simply doesn´t add up.
    But maybe that´s due to crumple zones, which meteorites don not posses. Earth does in a way due to it´s atmosphere.

    So maybe the Mythbusters should go up there and throw some stones. I´m sure they wouldn´t mind.

  9. Josh Andrews

    @SkyGazer The Mythbusters experiment doesn’t debunk relative speeds adding because although they combine their speed they also share the damage between the two cars.

    Marco’s solution seems inprobable. Given that it had a 1 a.u. orbit which the earth caught up with while it was at perihelion?

  10. Cairnos

    Oh, come on. It’s clearly a UFO that ran into a US secret chemtail spraying plane. It’s sadly obvious how all these ‘astronomers’ are part of some vague, ill-defined conspiracy being paid off by ‘Big Alien’ at the behest of Obama to somehow support global warming hysteria for some reason which probably involves taking our guns away.

    Back in reality, pity things like this always seem to happen on the opposite side of the darn planet from me.

  11. Michael Wallis

    A note on retrograde satellites ….

    Israel has launched a few satellites using domestically produced launch vehicles. Because of their proximity to, and political differences with, their neighbours, launching eastward was not reasonable. They launch them northwestward, out over the Mediterranean Sea.

    Identifying the ground track of the object would prove valuable as the inclination of intersection might yield a path backwards for the UK, over Belgium, towards the Middle East.

  12. tommy

    i believe this traveled north east to south west if you look at the second video on this site http://www.rte.ie/news/2012/0922/met…r-ireland.html and then compare it to google maps http://maps.google.ie/maps?hl=en&ll=…12,308.57,,0,3

  13. Darrell

    The behavior of this fireball appears to have some similarity with this one observed in 1860:


  14. @ #12 Michael Wallis:

    The Israeli objects are launched in a 144 degree inclined orbit, which *never* brings them over the UK or Belgium.
    This was 100% *not* a satellite (the object was too fast any way, in addition to the unlikely retrograde earth-orbit character)

  15. I find it odd that no-one has yet speculated on how bad this could have been. Sure, it may have been a tiny lump of rock skimming the atmosphere, but nobody saw it coming. What if it were a 50 metre nickel-iron asteroid? Or 100-200 metre M type? What kind of damage would it do if it actually hit us?

    Sure, if it hit the land it’d be pretty unpleasant, especially in an area of seismic instability, but 2/3rds of the Earth’s surface is water, or ocean, so there’s effectively a 2:3 chance of an ocean splash. In a big enough basin, this is going to produce a huge tsunami…


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