First the sky was falling, and then it wasn�t.
The recent, astronomical miscalculation over the path of asteroid 1997 XF 11 put astronomers in the unseemly position of having to publicly refute data � publicized as fact � that ordinarily would have been mere fodder for academic discussion. Initial reports that the asteroid would come within 30,000 miles of Earth on 26 October 2028 were later corrected by NASA, which predicts it will travel about 600,000 miles from Earth - not such a close shave after all.
As the dust settles from this flare-up, one thing becomes clear: Despite technological advances in observation equipment, astronomers are still left with performing sleight of hand to come up with their predictions.
"One needs to feel one�s way along observations [of an asteriod]," explained Peter Shelus, research scientist with the McDonald Observatory at the University of Texas in Austin. Shelus said astronomers look carefully at the results from their software and then extrapolate from there. But getting to this final point is a bit fuzzy.
"How you make extrapolations is very difficult to say. It�s like telling someone how you painted the Mona Lisa � [something about it] just felt right," Shelus said.
For the most part, astronomers do what feels right for them. And what is right for Tom Gehrels is to lose sleep just before, during, and after each new moon, when the night sky is best for viewing bodies such as stars, meteor showers, and asteroids. Instead of relying on automated observations, Gehrels and his colleagues at the Arizona Spacewatch Project split an 18-day viewing period, with each scientist spending six straight days in the observatory watching as the data come in across the computer monitors.
"[Staying up] is not a problem. It�s very exciting to look at screens full with stars, bright stars, nebulae, and satellites," said Gehrels, professor of planetary sciences at the University of Arizona.
For Gehrels, the six contiguous sleepless nights are a test of mental mettle, an exercise of will, and, if nothing else, a period of deep meditation. And all with a purpose � without keeping vigil, he might miss something of the celestial smorgasbord. The information received is fed to the computer from a highly sensitive CCD camera which can, on a good night, capture 700 sightings. Most of them do turn out to be asteroids, Gehrels said. But it is this "hands-on" approach that gives Gehrels a good feel for how data behave when something is an asteroid and when something isn�t.
Once an object is determined to be an asteroid, finding where it�s been and where it's going is where multiple observations become necessary. For this, Gehrels and his astronomer brethren must look up the history of the orbit of a body and run a comparison. "We�re freezing an orbit at a certain time," said Paul Chodas, planetary scientist at NASA�s Jet Propulsion Laboratory. "It doesn�t tell us how it will move. You�re left with a 1/3600th of a degree of error at this one certain time."
While this margin of error seems minute at any given moment, the errors accumulate over time. So a look 30 years into the future would be very uncertain, since the margin of error would grow to be quite large, Chodas said.
All predictions have the same core built upon Newton�s Laws of Motion and Universal Gravitation. The Laws of Motion take into account the movements of the planets and other objects, including asteroids, when gravity or other forces act upon them. Universal Gravitation describes the level of attraction as a force between two bodies � an asteroid and Earth, for example � that results from the product of their masses divided by the square of the distance between them.
But Newton�s laws run into limits in distant parts of the universe because they assume gravity remains as a constant force. These are areas where there are strong gravitational fields, so gravity is not constant. To account for these inconsistencies that may act upon an asteroid, astronomers use Einstein�s theory of relativity which assumes that the speed of light traveling between two frames of reference is the same for observers in both spots. Because an observer uses light in one frame of reference to calculate the position and velocity of a body in another frame of reference, this will change the way the observer sees the object or body in another point of reference.
In the case of 1997 XF 11, astronomer Brian Marsden used observations made by Arizona Spacewatch, McDonald Observatory, and an observatory in Japan, giving him several frames of reference from which to determine the position and velocity of the asteroid. With several observations, made as the asteroid traveled through different frames of reference, Marsden came up with a preliminary calculation that put the asteroid within 30,000 miles of the Earth in 30 years.
But, as Chodas pointed out, this calculation was the result of 88 days of observations in 1997, a relatively short period of time. Observations over a longer period of time would give the data points to fill in the gaps and give astronomers a better idea of XF 11�s path.
To derive a more detailed picture of 1997 XF 11�s orbit, JPL researchers were able to pull up observations they made of the same asteroid back in 1990. When put together with the existing XF 11 data, JPL scientists had a seven-year study of XF 11�s behavior, one that showed more clearly that the asteroid would not come close to Earth in 30 years.
This more complete picture of XF 11, and the confusion over its path that was publicized, has inspired a new cooperative effort among asteroid gazers. Hammered out in a meeting late last week, scientists from around the country, including Chodas and Marsden, will form a committee to use its combined expertise to assess the threat of asteroids.
The mixing of different calculations and observations will make the picture of an asteroid even clearer, a worthwhile goal that won't interfere with the sometimes competitive nature of astronomers, Chodas noted. "There is a competitive element [among astronomers] to find bodies first, but this new pact will help us coordinate our efforts more so we can agree on predictions."
