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:
“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.
Episode 4: Conflict
“Semipro” astronomer Roy Tucker spent the holidays tracking 2004 MN4. Tucker had a personal stake in the outcome: he was one of the three observers who had discovered the object at Kitt Peak. Tucker is an instrumentation engineer for the University of Arizona by day, and takes 30,000 image sets of starfields a year by night, mostly via $13,000 worth of observational equipment sheltered in a white wooden pop-top shack in his backyard.
For three nights, Tucker contributed his data to the Minor Planet Center’s accruing pile from other worldwide observers. Usually, further observations of an object quickly eliminate its impact probability. But the odds, surprisingly, kept rising daily. “It was kind of creepy,” laughs Tucker. “I wondered if I should quit sending in observations.” By December 26, the asteroid reached the highest-ever Earth-impact risk: a likelihood of 1 in 37. Tucker says he wasn’t worried. “But now that I think about it, if someone told me there was a 1 in 37 chance that my plane would crash, I probably wouldn’t get on it.”
Episode 5: Pre-covery
On December 27, two astronomers from the Tucson observatory Spacewatch hunted for 2004 MN4 not in the sky, but in their file cabinets. They found a set of photos from March 2004 with a dot so dim it had been overlooked. It was 2004 MN4. The asteroid’s observation arc was now nine months long. Since 2004 MN4’s orbital period is just over 10 months, the nine months of data was more than enough to recalculate its orbit solution more accurately. As a result, the object’s probability of Earth impact plummeted from 1 in 37 to 1 in 56,000. The tension over, most fans of 2004 MN4 moved on to the next big rock.
Episode 6: A Close Shave
Certain asteroids are interesting enough, large enough, and likely to pass close enough to Earth to warrant a few hours of coveted observation time at one of the world’s two radar telescope observatories. So far, about 300 asteroids, both NEOs and those in the main asteroid belt between Jupiter and Mars, have been analyzed with radar. Radar can yield a range of details far more precise than can optical equipment, including the object’s speed, distance, size, shape, spin, and surface properties. In January 2005, astronomers imaged the barely-radar-detectable 2004 MN4 at Puerto Rico’s Arecibo Observatory. The data revealed that the March optical observationsthe ones that inspired so much hubbubwere off-target.
Jon Giorgini, an analyst in the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory, calculated the new radar-based orbit solution. “The solution actually moved much closer to Earth than it had been before,” he says. The data revealed that we’re in for a much closer shave in 2029 than the optical data from had predicted2004 MN4 will pass a mere 36,400 km away from us, one-tenth the distance to the Moon. (In fact, the radar solution shifted the orbit to a path that the 10-month optical observations were 99.99995 percent certain it wouldn’t go!)
But the good news is twofold: the radar solution is so precise that astronomers are indubitably clear that it will miss Earth in 2029. (The impact probability went from 1 in 56,000 to an infinitesimal numberostensibly zero.) The other bonus is that the object will be visible to naked eye that year, a bright, fast point of light careening through the constellation Cancer. No other known asteroid has been visible that way.
Episode 7: Fear the Future?
Asteroid 2004 MN4’s close brush with Earth in 2029 means that our planet’s gravity will perturb its path in a way astronomers cannot predict perfectly at the moment. Unlike the stable, utterly predictable orbits of planets, asteroid orbits are chaotic. This means that a small influence on an asteroid’s path at one moment could hugely offset its trajectory many years in the future. “The uncertainty region, the region of space where it could statistically be, gets spread out as a result of its near-Earth encounter in 2029,” says Giorgini.
Current predictions are that 2004 MN4 could come close by Earth in 2034, or perhaps in 2035 through 2038, or in later years this century. Determining exactly how close, however, will have to wait until 2012, the next time 2004 MN4 is in radar range. “If we get radar data in 2012, we should be able to predict its position out to 2070 reliably,” says Giorgini. “It might be possible to exclude encounters in the 2030s sooner than that, but not the later encounters. Not until we get those measurements.” Seven years to find out this story’s next chapter? Now that’s some suspense.
NASA/JPL Near Earth Object Program
Understanding Risk Pages
Jon Giorgini explains how to decipher the an impact risk summary for a near-Earth asteroid.
More About This Resource...
Supplement a study of astronomy with a classroom activity drawn from this Science Bulletin essay.
- Ask students what they know about asteroids. What are they? What happens when they collide with Earth?
- Have them read the essay (either online or a printed copy).
- Working individually or in small groups, have students research Asteroid MN4 and what has been said about it and the odds of its impact by scientists in the years since this article was published.
SubtopicTools and Methods