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Planetary News: Asteroids and Comets (2002)

A Close Call for Earth in 878 Years

by Amir Alexander
12 April 2002

A Spring Day

March 16, 2880 will most likely be an ordinary spring day like any other. At least, we are 99.6734% certain it will be. That is because according to a paper by Jon D. Giorgini of the Jet Propulsion Laboratory and several collaborators published in the April 5 issue of Science, there is a 0.3266% chance that on that day the Earth will be hit by a chunk of space rock around one kilometer (0.6 miles) in diameter traveling at over 14 kilometers (9 miles) per second.

Although the Earth's chances of skipping this unwanted rendezvous appear good, an actual impact would be a serious matter. If the asteroid strikes land, it would likely produce a crater of approximately 22 kilometers across, and cause extensive devastation within a radius of 300 kilometers. If it landed in the ocean, it would launch a 25 meter high tsunami 1000 kilometers (600 miles) from the impact site. So even though it would not bring about the extensive species extinction caused by its much larger predecessor 67 million years ago, it is certainly an experience that we Earthlings would like to avoid - if at all possible.

Lost and Found

The asteroid in question is a chunk of space rock on a 2.2 year long elliptical orbit, that brings it within 77 million miles of the Sun - well inside the Earth's orbit. It was observed on December 31, 2000 by the Lowell Observatory in Arizona and designated 2000YK66. This "discovery" however did not last long, since within two hours astronomers had established that they had seen this one before. 2000YK66 was, in fact, already known as asteroid 1950 DA, discovered at the Lick Observatory near San Diego on February 23, 1950 and tracked for 17 days before being lost to view for half a century. Further research revealed that the asteroid was also present (though not noted) in astronomical photographs from the 1980s.

The 50 years that elapsed between 1950 DA's original discovery and its reappearance was the first thing that made this asteroid stand out from other space rocks. The positional measurements over such an extended length of time made it possible for Giorgini and his associates to calculate the orbit of this asteroid with an unprecedented degree of precision.

Other factors made 1950 DA special as well. The plane of its orbit is inclined to the plane of the ecliptic, where all planets (except Pluto) and most asteroids reside. This means that it is comparatively unlikely to encounter any large space bodies that could potentially disrupt its orbit. This makes 1950 DA's orbit more stable and predictable than most. Finally, Giorgini and his associates had at their disposal high precision astronomical radar technology, far in advance of anything available to their predecessors in 1950. Using the 310 meter (1000 foot) dish at Arecibo in Puerto Rico, and the 70 meter (230 foot) radio telescope at Goldstone, California, they were able to measure the trajectory of the asteroid with remarkable accuracy, and even get a glimpse of its rough contours.

Why Asteroids Go Where They Do

All these factors - the long span of time for which observations were available, the inclined orbit, and the radar measurements - suggested to Giorgini and his colleagues that it would be possible to predict 1950 DA's future course with far greater accuracy than is possible for most asteroids.

The researchers were well aware, however, that even with these advantages predicting the future position of an asteroid is a tricky business. Over time even tiny and seemingly negligible effects can accumulate and change an asteroid's orbit. For example, one must take into account the uneven distribution of mass in the Sun, which affects the asteroid's speed when approaching the Sun or receding from it. Solar winds, which push objects towards the outer reaches of the Solar System, affect the orbit, as does the slight but cumulative loss in the Solar mass over time. Our imperfect knowledge of the precise mass of the major and minor planets which an asteroid will encounter on its way also makes predictions difficult. Even the effects of light, reflected off the rocky surface and pushing it in the opposite direction, has to be considered. This miniscule tug, known as the "Yarkovsky effect," could make a difference of hundreds of thousands of miles in the asteroid's position a few centuries down the roads.

All of these factors (and some others as well) create considerable uncertainty as to the future position of any space object, especially a small one like 1950 DA. And the further away the target date is, the greater the uncertainty, as the tiny effects accumulate over time. Even so, Giorgini and his associates decided to do their best to estimate the future course of the asteroid, while taking the uncertainties into account. They found that on March 16 of the year 2880, asteroid 1950 DA will cross the Earth's orbit at a point exceedingly close to the position of the Earth itself. So close, in fact, that there was a 1 in 300 chance that the asteroid will slam into the Earth.

A Game of Chance

As Giorgini and his colleagues were well aware, however, such calculations are fraught with uncertainties. It is practically impossible to take into account all the factors affecting an asteroid's orbit, and tracking their influence over centuries. The smaller influences are therefore rounded to their estimated values. While this is unavoidable, it runs the risk of overlooking some seemingly minor effects, which may ultimately make all the difference between a global catastrophe and a friendly star party when the asteroid crosses Earth's orbit.

The researchers therefore decided to supplement their mathematical analysis of the 1950 DA's course with a "Monte Carlo" study. In a Monte Carlo, scientists select a large number of scenarios, each representing a possible combination of the factors affecting the asteroid's orbit. One scenario, for example, might assume a high value for the effects of Solar wind and a low value for the mass of the planet Mars, while another point would assume the reverse. Of the 10,000 scenarios Giorgini's group selected, more covered the likelier combinations, and fewer represented the more extreme and unlikely values.

For each of the 10,000 scenarios, the researchers then ran a computer simulation showing the exact course of the asteroid given that specific combination of factors. Here there are no shortcuts or rounded values - the course plotted is the precise one for each given set of values. Giorgini and his colleagues then checked to see where the asteroid would end up in 2880 according to each scenario. What they found confirmed their original mathematical calculation. In 99.67% of the cases, the asteroid passed by the Earth without incident; but in 0.33% percent of the possible scenarios, 1950 DA slammed into the Earth.

The Monte Carlo study also revealed important but hidden features of 1950 DA. Its orbit, it was discovered, is remarkably steady over time. For most asteroids, the cumulative effects of encounters with other bodies, Solar wind, etc., shift their orbit slowly but relentlessly over time. In contrast, 1950 DA's orbit seems to adjust itself continuously. This means that in a scenario where the asteroid is relatively fast, and "pulling ahead" in its orbit relative to its average course around the Sun, it later on "slows down" and is pulled back towards its "normal" position. Similarly, if it is relatively slow and falls behind, it then accelerates back towards its average position. This phenomenon of a self-adjusting orbit is known as "resonance." Were it not for this unique feature, it would have been impossible to predict the 1950 DA's position so far into the future.

The Monte Carlo also showed that the most significant factor in determining whether the asteroid will impact the Earth or not is the Yarkovsky effect - the gentle push elicited by light and heat radiating off the Sun-facing side of the space rock. What influence this effect has on the orbit of 1950 DA, however, strongly depends on the asteroids axis of rotation. This, however, is not known, as two different possibilities fit the radar data equally well, and each of these possible axes is known only within a rather wide angular range. The Yarkovsky effect also depends on the shape and texture of the asteroid - whether rocky or dust covered, smooth or jagged. None of these factors is known, leaving the Yarkovsky effect as the great unknown factor determining the future course of the asteroid.

So while the chances of a collision with the Earth are slim, 1950 DA is a rock to keep an eye on. 878 years provide plenty of time for that, and perhaps even to divert the asteroid's orbit if necessary. All just to make sure that March 16, 2880, will indeed be just an ordinary spring day.