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How To Evaluate Asteroid/Comet Warnings

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    NHNE News List Current Members: 657 Subscribe/unsubscribe/archive info at the bottom of this message. ... CAVEAT IMPACTOR NASA Science News Friday, July 26,
    Message 1 of 1 , Jul 26, 2002
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      NASA Science News
      Friday, July 26, 2002


      An asteroid with almost no chance of hitting Earth made big headlines this

      July 26, 2002: I slid a dollar bill across the counter, and the cashier
      handed back a lottery ticket. The odds for winning: 1-in-250,000. A long
      shot, but you never know.

      Walking out of the store, ticket in hand, I glance at a newspaper. "Tony
      Phillips wins the lottery!" the headline declared. Gosh, I thought, that
      seems premature ... not to mention weird.

      Indeed, it's fiction. For one thing, I never buy lottery tickets. But
      mainly, no one would write such a headline based on such slender odds.

      Yet that's what happened this week, in real life, to an asteroid.

      On July 9, 2002, MIT astronomers discovered 2002 NT7, a 2 km-wide space rock
      in a curious orbit. Unlike most asteroids, which circle the Sun in the plane
      of the planets, 2002 NT7 follows a path that is tilted 42 degrees. It spends
      most of its time far above or below the rest of the solar system. Every 2.29
      years, however, the asteroid plunges through the inner solar system not far
      from Earth's orbit.

      After a week of follow-up observations, researchers did some calculations.
      There was a chance, they concluded, that 2002 NT7 might hit our planet on
      February 1, 2019. The odds of impact: 1-in-250,000.

      "Space Rock 'on Collision Course'," a headline declared days later.
      "Asteroid Could Wipe Out a Continent in 2019," another one warned. Really.

      "In fact," says Don Yeomans, the manager of NASA's Near-Earth Object Program
      at JPL, "the threat is minimal. One-in-250,000 is a very small number."

      The odds are not only low, but also uncertain. Yeomans explains: "We've been
      tracking 2002 NT7 for a very short time--only 17 days so far," Meanwhile,
      the asteroid takes 2.29 years to orbit the Sun. Predictions based on such a
      small fraction of an orbit are seldom trustworthy.

      It's becoming a familiar routine: Astronomers discover a near-Earth
      asteroid. With only meager data at hand, they can't rule out a collision in
      the distant future. Headlines trumpet the danger. Finally, the alarm
      subsides when more data lead to a better orbit--one that rules out an

      "As far as the public is concerned," says Jon Giorgini of JPL's Solar System
      Dynamics Group, "it just isn't worth getting worked up about an object with
      a couple weeks of data showing a possible Earth encounter many years from
      now. Additional measurements will shrink the uncertainty by a large
      amount--and Earth will (almost certainly) fall out of the risk zone."

      Already this is happening for 2002 NT7. The calculated probability of a
      collision with Earth is shrinking as astronomers add new data each day. "I
      suspect it will take only a few more weeks (or maybe months) to completely
      rule out an impact in 2019," says Yeomans.

      Giorgini explains further: "When we calculate an asteroid's position (based
      on measurements made at a telescope), the result isn't a single point in
      space. Instead, it's a volume of space where the asteroid could be with some
      probability. We deal with probabilities, not absolute answers, because the
      measurements contain errors." For example, optical data can be corrupted by
      twinkling and refraction in Earth's atmosphere. (Radar is better, notes
      Giorgini, but no radar data have yet been obtained for 2002 NT7.)

      "When you project this initial probability region years into the future, it
      naturally expands. For a newly discovered object with only a few days
      tracking, the uncertainty region can easily grow to cover a big part of the
      inner solar system. Because Earth is in the inner solar system, and can
      potentially cut through this volume of smeared out probability, we end up
      with finite impact probabilities."

      "A finite probability, however, is not really a prediction of
      impact," he cautions, "but a statement that one is possible." Of
      course, many things are possible. Like the newspaper headline "Tony Phillips
      wins the Lottery!" But most of them do not happen.

      JPL lists asteroids like 2002 NT7 on their Internet "risk page" not to raise
      an alarm, says Yeomans, but to alert astronomers when new discoveries merit
      attention. "It's important that we continue tracking these asteroids to
      refine their orbits," he says. The more observers, the better.

      What's an ordinary person to do?

      The next time you see a headline "Killer asteroid threatens Earth!" ask
      yourself two questions: Have we known about this space rock for more than a
      week or so? (If not, check again in a month. It probably won't be considered
      a killer then.) And what are the odds of impact?

      If you're more likely to win the lottery, there's probably nothing to worry

      Editor's note: Big asteroids have hit Earth before and it's only a matter of
      time before one threatens us again. Will it be years, decades, millions of
      years? No one knows. The point of this article is not that we are safe from
      asteroid strikes. We are not safe. Rather, we hope to give readers some of
      the information they might need to evaluate popular reports of impending


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