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
July 24, 2014 -- Updated 1349 GMT (2149 HKT)
Patrick Cronin and Kelley Sayler say the world is seeing nonstate groups such as Ukraine's rebels wielding more power to do harm than ever before
July 23, 2014 -- Updated 2205 GMT (0605 HKT)
Ukraine ambassador Olexander Motsyk places blame for the MH17 tragedy squarely at the door of Russia
July 24, 2014 -- Updated 1142 GMT (1942 HKT)
Mark Kramer says Russia and its proxies have a history of shooting down civilian aircraft, often with few repercussions
July 24, 2014 -- Updated 1853 GMT (0253 HKT)
Les Abend says, with rockets flying over Tel Aviv and missiles shooting down MH17 over Ukraine, a commercial pilot's pre-flight checklist just got much more complicated
July 24, 2014 -- Updated 1317 GMT (2117 HKT)
Mark Kramer says Russia and its proxies have a history of shooting down civilian aircraft, often with few repercussions
July 24, 2014 -- Updated 1637 GMT (0037 HKT)
Gerard Jacobs says grieving families and nations need the comfort of traditional rituals to honor the remains of loved ones, particularly in a mass disaster
July 24, 2014 -- Updated 1413 GMT (2213 HKT)
The idea is difficult to stomach, but John Sutter writes that eating dog is morally equivalent to eating pig, another intelligent animal. If Americans oppose it, they should question their own eating habits as well.
July 23, 2014 -- Updated 1630 GMT (0030 HKT)
Bill van Esveld says under the laws of war, civilians who do not join in the fight are always to be protected. An International Criminal Court could rule on whether Israeli airstrikes and Hamas rocketing are war crimes.
July 23, 2014 -- Updated 1205 GMT (2005 HKT)
LZ Granderson says Ronald Reagan went horseback riding and took a vacation after the Korean Air Crash of 1983. So why does the GOP keep airbrushing history to bash Obama?
July 23, 2014 -- Updated 1338 GMT (2138 HKT)
Aaron Miller says Kerry needs the cooperation of Hamas, Israel, Egypt and others if he is to succeed in his peacemaking efforts
July 23, 2014 -- Updated 1251 GMT (2051 HKT)
Errol Louis says the tragic death of Eric Garner at the hands of the NYPD has its roots in the "broken windows" police strategy from the crime-ridden '80s.
July 23, 2014 -- Updated 1408 GMT (2208 HKT)
Gordon Brown says the kidnapped Nigerian girls have been in captivity for 100 days, but the world has not forgotten them.
July 23, 2014 -- Updated 1127 GMT (1927 HKT)
Ruben Navarrette says Texas Gov. Rick Perry is right to immediately send 1,000 National Guard troops to the U.S.-Mexico border in response to the border children crisis.
July 22, 2014 -- Updated 1356 GMT (2156 HKT)
Ukraine's president says the downing of MH17 was a terrorist act, but Richard Barrett says it would be considered terrorism only if it was intentional
July 22, 2014 -- Updated 2015 GMT (0415 HKT)
Robert McIntyre says the loophole that lets firms avoid taxes should be closed
July 22, 2014 -- Updated 1535 GMT (2335 HKT)
Jeronimo Saldana and Malik Burnett say Gov. Perry's plan to send National Guard to the border won't solve the escalating immigration problem.
July 22, 2014 -- Updated 1742 GMT (0142 HKT)
Sally Kohn: The world's fish and waters are polluted and under threat. Be very careful what fish you eat
July 22, 2014 -- Updated 1242 GMT (2042 HKT)
Les Abend says threat information that pilots respond to is only as good as the intelligence from air traffic controllers. And none of it is a match for a radar-guided missile
July 21, 2014 -- Updated 1235 GMT (2035 HKT)
Frida Ghitis: Anger over MH17 is growing against pro-Russia separatists. It's time for the Dutch government to lead, she writes
July 21, 2014 -- Updated 1227 GMT (2027 HKT)
Julian Zelizer says President Obama called inequality the "defining challenge" of our time but hasn't followed through.
July 21, 2014 -- Updated 1157 GMT (1957 HKT)
Gene Seymour says the 'Rockford Files' actor worked the persona of the principled coward, charming audiences on big and small screen for generations
July 21, 2014 -- Updated 1417 GMT (2217 HKT)
Daniel Treisman says that when the Russian leader tied his fate to the Ukraine separatists, he set the stage for his current risky predicament
July 18, 2014 -- Updated 1642 GMT (0042 HKT)
Andrew Kuchins says urgent diplomacy -- not sanctions -- is needed to de-escalate the conflict in Ukraine that helped lead to the downing of an airliner there.
July 19, 2014 -- Updated 0150 GMT (0950 HKT)
Jim Hall and Peter Goelz say there should be an immediate and thorough investigation into what happened to MH17.
July 18, 2014 -- Updated 1507 GMT (2307 HKT)
Pilot Bill Palmer says main defense commercial jets have against missiles is to avoid flying over conflict areas.
July 20, 2014 -- Updated 1755 GMT (0155 HKT)
Valerie Jarrett says that working women should not be discriminated against because they are pregnant.
July 21, 2014 -- Updated 1953 GMT (0353 HKT)
David Wheeler says the next time you get a difficult customer representative, think about recording the call.
July 18, 2014 -- Updated 1933 GMT (0333 HKT)
Newt Gingrich says the more dangerous the world becomes the more Obama hides in a fantasy world.
July 18, 2014 -- Updated 1011 GMT (1811 HKT)
Michael Desch: It's hard to see why anyone, including Russia and its local allies, would have intentionally targeted the Malaysian Airlines flight
July 17, 2014 -- Updated 1914 GMT (0314 HKT)
LZ Granderson says we must remember our visceral horror at the news of children killed in an airstrike on a Gaza beach next time our politicians talk of war
July 17, 2014 -- Updated 1206 GMT (2006 HKT)
Sally Kohn says now the House GOP wants to sue Obama for not implementing a law fast enough, a law they voted down 50 times, all reason has left the room.
July 17, 2014 -- Updated 1214 GMT (2014 HKT)
A street sign for Wall Street
Sens. Elizabeth Warren, John McCain and others want to scale back the "too big to fail" banks that put us at risk of another financial collapse.
July 17, 2014 -- Updated 2016 GMT (0416 HKT)
Newt Gingrich writes an open letter to Robert McDonald, the nominee to head the Veterans Administration.
July 18, 2014 -- Updated 1601 GMT (0001 HKT)
Paul Begala says Dick Cheney has caused an inordinate amount of damage yet continues in a relentless effort to revise the history of his failures.
July 18, 2014 -- Updated 1404 GMT (2204 HKT)
Kids who takes cell phones to bed are not sleeping, says Mel Robbins. Make them park their phones with the parents at night.
July 25, 2014 -- Updated 1237 GMT (2037 HKT)
Buzz Aldrin looked at planet Earth as he stood on talcum-like lunar dust 45 years ago. He thinks the next frontier should be Mars.
July 16, 2014 -- Updated 1804 GMT (0204 HKT)
Mark Zeller never thought my Afghan translator would save his life by killing two Taliban fighters who were about to kill him. The Taliban retaliated by placing him on the top of its kill list.
July 17, 2014 -- Updated 1518 GMT (2318 HKT)
Jeff Yang says an all-white cast of Asian characters in cartoonish costumes is racially offensive.
July 17, 2014 -- Updated 0124 GMT (0924 HKT)
Gary Ginsberg says the late John F. Kennedy Jr.'s reaction to an event in 1995 summed up his character
July 16, 2014 -- Updated 1641 GMT (0041 HKT)
Meg Urry says most falling space debris lands on the planet harmlessly and with no witnesses.
ADVERTISEMENT