There was a time, way back in the 1990s, when the news media would get really frothy every time a newfound asteroid had even a slight chance of hitting Earth. I remember clearly the saga of 1997 XF11 (now a numbered asteroid, 35396), which for a few uncertain weeks after its discovery had a 1-in-900 chance of striking Earth in 2028. But once some archival images were found and the orbit recomputed, the impact probability went to zero.
In the aftermath of this false alarm and others since, I would have thought that we in the news media (and I use that term advisedly) had learned enough to temper our reporting on such matters.
This past week the obscure Spanish Foundation for Science and Technology posted a story about a research paper co-authored by Spanish scientist Maria Eugenia Sansaturio (University of Valladolid). The work involved the numbered-but-unnamed asteroid 101955, which was initially designated 1999 RQ36 when discovered by the LINEAR survey telescope. This asteroid's cumulative risk of striking Earth, the report announced, is about 1 in 1000 (0.1%) between now and the year 2200 — with the most likely target date coming in 2182. Given that it's a third of a mile (0.56 km) across, this object would surely do global harm if it hit.
So why am I not worried, and why shouldn't you be? For one thing, even though the paper (whose lead author is University of Pisa dynamicist Andrea Milani) appeared in the respected journal Icarus, it was published without fanfare last October — it's hardly breaking news. Second, you need to read the paper to understand the authors' intent.
Predicting doomsday might be easy for prophets, but it's a lot harder for solar-system dynamicists. When a threatening asteroid is discovered, usually its orbit is known too poorly, even after an intense observing effort, to compute credible impact probabilities more than a few decades out.
In that respect, asteroid 101955 is an unusual case. Thanks to hundreds of careful optical measurements and radar observations made during two close passes, astronomers know the size of its orbit more accurately (to within 20 feet) than that of any other asteroid. As the orbital uncertainty drops, the predictive confidence goes up.
But the point of the article was to explore the validity of impact probabilities made centuries (not just decades) in advance. To do that with any confidence, dynamicists must have more than just a precise orbit in hand. They also need to know the object's shape and surface characteristics in order to calculate the Yarkovsky effect, a subtle interaction caused by sunlight falling on a rotating space rock. Milani's team estimates that this force might be altering the semimajor axis of the asteroid's orbit by more than a mile (2 km) per decade.
Using a best guess of the effect's accumulated nudging over time, the researchers discovered that in 2060 this particular asteroid slips past Earth about a half million miles away. The problem, they explain, is if it goes by just so, passing though a dynamical "keyhole" less 1 km wide, then its chances of striking Earth in 2182 go up dramatically. All this assumes the authors have computed the Yarkovsky effect correctly.
So what should we do? Sit tight, and keep an eye on NASA's impact-risk page. Asteroid 101955 next comes our way next year, passing about 16 million miles from us (as a 20th-magnitude blip) on September 11, 2011. Radar observations made around that time should settle to what degree, if any, the Yarkovsky effect is in play. Chances are excellent — at least 999 in 1,000 — that in time a future impact with Earth will be ruled out.
Finally, there's a effort under way to see this asteroid up close and personal — even bring a sample of it back to Earth — in the not-too-distant future. Because it's got an extremely dark and presumably carbon-rich surface, 101955 (or 1999 RQ36, if you insist) has likely remained unaltered since its formation 4½ billion years ago. This would-be Rosetta Stone of early solar-system history is the destination of a proposed mission called OSIRIS-REx.
If picked over two competing proposals by NASA managers next summer, OSIRIS-REx would launch in 2016, land on this dark, worrisome asteroid in 2020, and return to Earth in 2023 with up to 4½ pounds (2 kg) of its surface material.