A meteor and asteroid: 1 in 100 million odds
updated 12:26 PM EST, Sat February 16, 2013
Asteroid 2012 DA14 made a
record-close pass -- 17,100 miles -- by Earth on February 15. Most
asteroids are made of rocks, but some are metal. They orbit mostly
between Jupiter and Mars in the main asteroid belt. Scientists estimate
there are tens of thousands of asteroids and when they get close to our
planet, they are called near-Earth objects.
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All about asteroids
STORY HIGHLIGHTS
- Meg Urry: Friday was an extremely unusual day, astronomically speaking
- Urry: The probability that a meteor hits and an asteroid passes by is improbable
- She says the chance of the two events happening on the same day is about 1 in 100 million
- Urry: Even though we think they could be connected, the two rare events are not connected
Editor's note: Meg Urry
is the Israel Munson professor of physics and astronomy and chairwoman
of the department of physics at Yale University, where she is the
director of the Yale Center for Astronomy and Astrophysics.
(CNN) -- Friday was an extremely unusual day,
astronomically speaking. Just as scientists were gearing up to witness
an asteroid's closest ever approach to Earth in recorded history, a
sizeable meteor exploded over Russia, causing thousands of injuries and
major damage to buildings.
The asteroid, named DA14,
came within 17,000 miles or so, as close as a telecommunication
satellite in geosynchronous orbit. DA14 is quite a bit smaller than
YU55, the asteroid that passed Earth in November 2011, but DA14 came
more than 10 times closer.
These two rare events
occurred the same day. Your inner mathematician and your inner prophet
of the end times think they should be connected. But scientists say they
are not. What gives?
Meg Urry
First, some facts.
Meteors are rocky bodies that enter the Earth's atmosphere. Some are
leftover debris out of which planets like Earth are formed, while others
are the remnants of shattered comets and asteroids. As long as their
orbit intersects the Earth's orbit, these rocks can in principle impact
the Earth.
Actually, this happens
all the time, although usually the impacts occur in unpopulated regions
since most of Earth is uninhabited. In fact, most meteors fall into the
ocean simply because water covers two-thirds of the planet.
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So we don't witness most
meteor impacts. If one landed in New York City or Moscow, people would
definitely notice. Fortunately, the odds are very much against hitting a
densely populated region.
The meteor that fell Friday near Chelyabinsk, Russia,
was pretty big, maybe 50 feet across. In 1908, a slightly larger meteor
-- perhaps three times larger in diameter, or 27 times larger in mass
-- flattened a thousand square miles of forest near Tunguska, Russia,
downing some 80 million trees.
Photos: Meteor explodes over Russia
NASA scientists estimate
that meteors as large as Friday's might hit the Earth every decade or
two, while Tunguska-like events are estimated to occur once every 1,000
years.
The close fly-by of an
asteroid like DA14, like the Tunguska meteor, is a once-in-100-years
event. Asteroids are large, irregular, rocky bodies orbiting the Sun
roughly between Mars and Jupiter. Many have impacted the Earth over its
4.5 billion-year history --as they have hit the moon, Mars and other
planets -- leaving craters behind.
A particularly large
asteroid -- roughly 300 times larger across than DA14 (and 30 million
times its volume, and far more rare) -- created a planetary extinction
event that did in dinosaurs 65 million years ago, allowing mammals to
rise to their present-day prominence.
Using NASA's WISE
infrared satellite, astronomers estimate there are about 5,000 known
meteors that can impact the Earth with sizes of about 100 feet or larger
-- that is, larger than the Chelyabinsk meteor. Smaller ones are
fainter and thus harder to find.
It makes sense that
smaller asteroids pass Earth more frequently and, on average, closer.
That's because in nature, small things are more common than big things.
So asteroids like YU55 are more rare than DA14, which in turn is more
rare than the Chelyabinsk meteor. Because there are more DA14s filling
interplanetary space than YU55s, a 50-foot asteroid can be found in a
smaller volume of space, on average, and thus closer to Earth, than a
150-foot one.
Now let's talk about coincidence. Mathematicians frame this issue in
terms of probability -- that is, the likelihood that something will
happen. A rare thing is unlikely, so we say it has a low probability of
occurring.
Two rare events
happening at approximately the same time is much more unlikely. Here is
how to think of it mathematically: If the events are not associated, the
probability of this coincidence comes from multiplying the individual
probabilities.
For example, the
probability that your birthday is on a given date -- say, January 1 --
is 1/365. That is, of every 365 readers of this article, roughly one
will have a birthday on January 1.
Now, the probability
that the next reader's birthday is also on January 1 is 1/365 times
1/365, or about 1 in 130,000. If that many people read the article, such
a coincidence could happen. Of course, it's much more likely that two
non-consecutive readers will have a birthday on January 1. And it's very
likely that lots of readers have the same birthday as other readers.
(In fact, in any group of 23 or more people, it is more than 50% likely
that two will share a birthday, but calculating that probability is a
bit more complicated.)
Back to the meteor and
the asteroid. Both events happening within one day makes us think they
could be connected. That instinct comes from doing the math -- if it is
improbable, then we think it cannot be a coincidence.
But the facts don't
support this conclusion. First of all, in the time between the two
events, the Earth moved roughly 300,000 miles, meaning the asteroid and
the meteor were in completely different places. Moreover, they traveled
in completely different directions, so they couldn't have been
associated.
So there is no way the
meteor and the asteroid are connected. It has to be a coincidence that
the two events happened on the same day. Yet this would seem to be at
odds with our instinct that two very rare things would not happen at the
same time.
How can we reconcile
these two opposite thoughts: the impossibility of an association based
on the physics of trajectories, and the improbability of coincidence
(lack of association) that the math suggests?
The answer is that we
need to rethink the probability calculation. If asteroids as big as DA14
pass close to Earth once every decade or two, and meteors as large as
the Chelyabinsk one impact once every 100 years (a similar meteor having
caused the Tunguska event in 1908), the chance of both events happening
on any one day are indeed very small: 1 in 3,650 days times 1 in 36,500
days, or about 1 in 100 million -- not odds you would bet against.
But think again: The
Earth has been around for 4.5 billion years -- which is 1.6 trillion
days. So the chance that these two events would happen on a day sometime
in the earth's history is actually larger than we first thought -- it
ought to have happened about 12,000 times already.
Of course, during most
of that 4.5 billion year history, the earth was not populated by
intelligent life -- human beings who might have noticed the two events
happening on the same day.
So what is the
probability that the meteor hits and the asteroid passes Earth on the
same day when someone could record it on video? That's probably been
possible for about 50 years, or only about five years if we have to do
it on a smartphone or dashboard camera. That's 1,825 days, which means
the chance of someone filming the event is only about one in 70,000 --
and that's if people blanketed the Earth. Given how sparsely the Earth
is populated, we should correct this number downward by a (large!)
geographical factor. It's also unlikely that this event would happen
within 3,000 miles of the Tunguska impact.
What to think? Our rough
calculation says a large meteor impact on the same day as closest
passage of the DA14 asteroid is really improbable. But it did happen.
Something in our assumptions could be wrong. For example, the frequency
of meteor impacts could be much larger and our estimates too low because
we don't notice most of them.
Then again, maybe sometimes, long odds just pay off.
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