Skip to main content

A meteor and asteroid: 1 in 100 million odds

By Meg Urry, Special to CNN
February 19, 2013 -- Updated 0116 GMT (0916 HKT)
NASA scientists used Earth-based radar to produce these sharp views of the asteroid designated<a href='http://www.nasa.gov/jpl/asteroid/giant-telescopes-pair-up-to-image-near-earth-asteroid/index.html#.U5nrgii4SEK' target='_blank'> "2014 HQ124"</a> on Sunday, June 8. NASA scientists used Earth-based radar to produce these sharp views of the asteroid designated "2014 HQ124" on Sunday, June 8.
HIDE CAPTION
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
All about asteroids
<<
<
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
>
>>
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?

11 meteor tweets we wish we'd thought of

Meg Urry
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.

Become a fan of CNNOpinion
Stay up to date on the latest opinion, analysis and conversations through social media. Join us at Facebook/CNNOpinion and follow us @CNNOpinion on Twitter. We welcome your ideas and comments.



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.

A large chunk of a meteor that exploded over Russia is found in a lake on Friday, February 15. A large chunk of a meteor that exploded over Russia is found in a lake on Friday, February 15.
Meteor explodes over Russia
HIDE CAPTION
<<
<
1
2
3
4
5
6
7
8
>
>>
Photos: Meteor explodes over Russia Photos: Meteor explodes over Russia
Meteor fragment leaves huge hole in lake
Russian meteor was like 'a rocket bomb'
Videos capture exploding meteor in sky

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-1,000-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.

Opinion: Meteor shows why it is crucial to keep an eye on the sky

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.

Five things to know about asteroids and meteors

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.

Russian scientists track down fragments of Urals meteor

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.

Follow us on Twitter @CNNOpinion

Join us on Facebook/CNNOpinion

The opinions expressed in this commentary are solely those of Meg Urry.

ADVERTISEMENT
Part of complete coverage on
August 1, 2014 -- Updated 1812 GMT (0212 HKT)
By now it should be painfully obvious that this latest round of the Israeli-Palestinian crisis in Gaza is fundamentally different than its predecessors.
August 1, 2014 -- Updated 2124 GMT (0524 HKT)
Sally Kohn says like the Occupy Wall Street protesters, Market Basket workers are asking for shared prosperity.
July 31, 2014 -- Updated 2331 GMT (0731 HKT)
President Obama will convene an Africa summit Monday at the White House, and Laurie Garrett asks why the largest Ebola epidemic ever recorded is not on the agenda.
August 1, 2014 -- Updated 1803 GMT (0203 HKT)
Seventy years ago, Anne Frank made her final entry in her diary -- a work, says Francine Prose, that provides a crucial link to history for young people.
July 31, 2014 -- Updated 2350 GMT (0750 HKT)
Van Jones says "student" debt should be called "education debt" because entire families are paying the cost.
July 30, 2014 -- Updated 1941 GMT (0341 HKT)
Stuart Gitlow says pot is addictive and those who smoke it can experience long-term psychiatric disease.
July 31, 2014 -- Updated 2300 GMT (0700 HKT)
Marc Randazza: ESPN commentator fell victim to "PC" police for suggesting something outside accepted narrative.
July 31, 2014 -- Updated 1845 GMT (0245 HKT)
Mark O'Mara says working parents often end up being arrested after leaving kids alone.
July 30, 2014 -- Updated 2031 GMT (0431 HKT)
Shanin Specter says we need to strengthen laws that punish auto companies for selling defective cars.
July 30, 2014 -- Updated 1645 GMT (0045 HKT)
Gabby Giffords and Katie Ray-Jones say "Between 2001 and 2012, more women were shot to death by an intimate partner in our country than the total number of American troops killed in the Iraq and Afghanistan wars combined."
July 30, 2014 -- Updated 1158 GMT (1958 HKT)
Vijay Das says Medicare is a success story that could provide health care for everybody, not just seniors
July 30, 2014 -- Updated 1743 GMT (0143 HKT)
S.E. Cupp says the entrepreneur and Dallas Mavericks owner thinks for himself and refuses to be confined to an ideological box.
July 30, 2014 -- Updated 1311 GMT (2111 HKT)
A Christian group's anger over the trailer for "Black Jesus," an upcoming TV show, seems out of place, Jay Parini says
July 30, 2014 -- Updated 2028 GMT (0428 HKT)
LZ Granderson says the cyber-standing ovation given to Robyn Lawley, an Australian plus-size model who posted unretouched photos, shows how crazy Americans' notions of beauty have become
July 30, 2014 -- Updated 1939 GMT (0339 HKT)
Carol Dweck and Rachel Simmons: Girls tend to have a "fixed mindset" but they should have a "growth mindset."
ADVERTISEMENT