Impact! Tracking Near-Earth Asteroids
Probability and 2004 MN4: A New Drama
Bloggers, our modern-day town criers, seemed to follow the celestial soap opera of asteroid 2004 MN4 with a level of urgency topped only by astronomers themselves. Slashdot, an Internet site summarizing science and tech news, posted the first whisperings about the object on Friday, December 24, 2004, at 1:30 PM:
ASTEROID IMPACT ODDS:
HOW DO THEY STACK UP?
1:100, 000,000 - Odds of an asteroid impact causing global disaster in any given year (1)
1:95,980,407 - Odds of dying from a venomous snake or lizard bite in any given year (2)
1:4,362,746 - Odds of dying from a lightning strike in any given year (2)
1:3,529,526 - Odds of winning Lotto from a two-draw slip (3)
1:2,598,960 - Odds of drawing a royal flush in spades in a five-card poker draw (4)
1:100,000 - Odds of an asteroid impact causing regional disaster in any given year (5)
1:91,149 - Odds of dying in a fire in any given year (2)
1:541 - Odds of having triplets if you give birth (6)
1:10 - Odds that astronomers are actually aware of an asteroid larger than ½ mile in diameter (5)
1:6 - Odds of a sunny day in Seattle (7)
1 ESA, 2 NSC, 3 MISSOURI LOTTERY, 4 WWW.INDEPTHINFO.COM, 5 NASA, 6 CDC; 7 KOMOTV
“Introducing Asteroid 2004 MN4: from the hopefully-you'll-hear-nothing-more-about-it dept…Numerous readers wrote in with bits about a potential asteroid collision: ‘…asteroid 2004 MN4 is currently listed as having a 1/233 chance of hitting the Earth…If it strikes the Earth it will release an energy of 1,900 Megatons of TNT….’ So, in summary, there's a 1-in-233 chance of the worst disaster in recorded history happening on April 13, 2029, and a 232-in-233 chance of nothing happening. Have a nice day!”
As for the worst disaster in history part—too early to tell. If it struck land, the 320 m diameter rock could pummel a crater “only” 3 to 5 km wide (more of a regional than a global catastrophe). If it struck ocean, it could kick-start a tsunami to rival the Indonesian one that occurred just two days after this post appeared. Still, in the astronomy business, odds as low as 1 in 233 are, well, odd. And the ensuing plot twists only got more nail-biting. As posted on Slashdot:
December 24, 22:14 PM: Update: A 1 in 62 chance.
December 25, 6:31 PM: “2004 MN4, Even Higher Probability” 1 in 45.
December 27, 3:56 PM: “2004 MN4 Asteroid Odds Inching Up Again.” Now 1 in 37: the highest risk ever reported in space rock history.
December 27, 8:44 PM: “2004 MN4 Probably Won't Kill Us.” The odds had sunk like a stone, to 1 in 56,000.
Like any good drama, this one is still unfolding. While the rock will miss us in 2029, in February scientists had a new prediction: it may have a second chance at Earth collision, in 2034.
Asteroid orbit paths don’t veer wildly in a matter of days or months. So why did 2004 MN4’s chances of impact do just that? A closer look at each episode of the saga reveals how astronomers calculate orbits, how impacts are predicted, and just how much you should bite your nails.
Episode 1: Discovered!
The asteroid’s debut was unremarkable. It was first noticed in June 2004 the way all near-Earth objects (NEOs) are: through a telescope. To qualify as an NEO, an object must be an asteroid (a chunk of rock or iron) or a comet (an icy chunk of rock) that passes within 200 million km of the Sun at some point on its loop around it. (Earth orbits the Sun at 150 million km away.) Three astronomers searching for a different asteroid with the University of Arizona’s Kitt Peak telescope found this one instead. They took photos of it low in the western sky six times in two days.
In each black-and-white frame, stars glow brightly, yet remain still from shot to shot. Any starlike point moving among them is likely an asteroid or comet. For each image, the team identified the object's coordinates in space, then extrapolated a range of possible orbit paths called solutions.
Episode 2: Lost … or Hidden?
For six months, 2004 MN4 was too faint and too hard to find to show up on any observer’s telescope.
Episode 3: Reemergence
On December 18, 2004, an Australian astronomer spotted what he thought was a new NEO. He emailed his findings to the Minor Planet Center, a group at the Harvard-Smithsonian Center for Astrophysics which collects, catalogs, and disseminates data on tens of thousands of asteroid observations per day by astronomers around the globe.
Astronomers at the Minor Planet Center identified the Australia object as 2004 MN4. In an instant, its observation arc went from short (two days) to substantial (six months). The farther apart an asteroid’s observations are spaced, the more accurate the estimates of its complete orbit. The Minor Planet Center was able to predict that the margin of error for 2004 MN4’s orbit would cross Earth’s orbit. The object’s impact probability—the likelihood that it would actually hit us—was also calculated: a 1 in 2,500 chance of collision on Friday, April 13(!), 25 years hence. Less than half a percent of the 3,000 known NEOs have ever achieved such a high impact probability. 2004 MN4 became an overnight celebrity.
