WASHINGTON -- In the cosmic equivalent of a bullet whizzing by Earth's ear, a half-mile-wide asteroid looks as though it will come closer to smashing into our planet than any other space rock astronomers have tracked.
It won't hit Earth. But its arrival in 28 years will be a visible reminder that space can be a dangerous place.
After computing a new path for the dangerous rock, NASA scientists and other astronomers determined this week that the asteroid will come close to -- but definitely not hit -- Earth on Aug. 7, 2027.
The asteroid could swing as close as 19,000 miles to Earth's surface. That's only one-twelfth the distance between here and the moon.
``It is indeed very interesting,'' said Paul Chodas, a research scientist in NASA's Near Earth Object tracking office. ``We're taking it seriously.''
Because this is an asteroid that was discovered only in January, astronomers aren't certain how close the call will be. While 19,000 miles is the closest it can come in 2027, it could miss by as much as 600,000 miles. The most likely miss will be by a scant 32,000 miles, Chodas said.
Astronomers are certain it won't hit Earth because its current path and the pull of gravity put it on a track that cannot intersect with Earth's orbit, Chodas said.
But the newest tracks put the asteroid, called 1999 AN10, on top of the list of close shaves compiled by Harvard University's Minor Planet Center. This rock will probably come nearly 15 times closer to Earth than the next known near miss, an asteroid that will approach Earth in 2086.
If it comes as close as 19,000 or even 32,000 miles, the rock should be visible with the naked eye, Chodas said. It would look like a moderately bright star.
``We're not used to thinking about things being that close,'' said Clark Chapman, a planetary scientist at the Southwest Research Institute in Colorado. ``They've been coming that close for a long time, we just haven't known about it.''
As researchers spend more time and money looking for asteroids and comets that come too close for comfort, there will probably be one or two in the next few decades that will even get closer than 1999 AN10, he said.
There is a very slight chance -- 1 in 10 million -- that when 1999 AN10 gets close to Earth in 2027, our planet's gravity will shift the asteroid's path in such a way that it will come within 5,000 miles in 2034 and hit us in 2039, Chodas said.
But that's as likely as shooting an arrow through the keyhole of your front door that then travels through the front-door keyhole of the house across the street and out through its back-door keyhole, said Brian Marsden, director of the Minor Planet Center.
タイトル： New Analyses Identify a Small Possibility that Asteroid 1999 AN10 Could Collide with Earth in 2044 or 2046
New Analyses Identify a Small Possibility that Asteroid 1999 AN10 Could Collide with Earth in 2044 or 2046 New orbital analyses for the kilometer-sized asteroid 1999 AN10 have revealed a remote chance that this object might collide with the Earth inthe year 2044 or 2046. Although this asteroid will be monitored in the future, it is not thought to be a serious hazard to Earth at this time.
Researchers Andrea Milani, Steven Chesley and Giovanni Valsecchi in Italy, as well the undersigned at JPL, have identified these new impacting possibilities by using new observational data, and by projecting the asteroid's motion somewhat farther into the future than before. New measurements of 1999 AN10, made over the last week and a half by amateur astronomer Frank Zoltowksi in Australia, have allowed astronomers to make significantly more precise orbital calculations, and the revised predictions indicate that the asteroid could approach the Earth particularly closely on August 7, 2027. The orbital motions of the Earth and the asteroid do not permit a collision in 2027, but the close approach will certainly change the asteroid's orbital path. During the past week, researchers have focused on the range of possible paths the asteroid could follow after 2027.
The accompanying diagram shows the uncertainty in the predicted close approach in 2027. The asteroid must pass through the plane of the diagram somewhere within an extremely skinny uncertainty ellipse, which appears simply as a line segment. New measurements taken over the weekend have moved the center of the ellipse (the most likely point of passage) out to a distance of about 200,000 km from the Earth, significantly farther than last week's estimate. The predicted point of passage may continue to bounce around within the ellipse as new data are added, but it cannot decrease below a minimum of about 37,000 km from the Earth's center.
During the 2027 close approach, Earth's gravity will change the asteroid's orbit by an amount which depends on the precise location of the point of passage through the uncertainty ellipse. In particular, a range of post-2027 orbital periods are possible: a passage on the left side of the Earth in this diagram will decrease the orbital period; a passage on the right side will increase the period. If the asteroid passes through certain narrow "keyholes" in the uncertainty ellipse, its changed orbit will bring it back for another Earth close approach in a later year. The 2039 impacting scenario identified by Milani et al. last month actually required passage through two keyholes, one in the 2027 ellipse, and one in the 2034 ellipse, which explains why it was so unlikely (about one chance in a billion using last month's orbital estimate). This week's new observations have now moved the uncertainty ellipse completely off the 2034 keyhole, which indicates that this impacting scenario is no longer possible.
The newly identified impacting possibilities for August 6, 2044 and August 7, 2046 each require passage through only a single keyhole in the 2027 ellipse, and the probabilities of impact for these cases are correspondingly larger, on the order of one chance in 500,000 for 2044, and one chance in five million for 2046. These odds of collision are larger than those for any other object, but they are still less than one hundredth the chance of an undiscovered asteroid of equivalent size striking the Earth sometime before 2044.
In summary, asteroid 1999 AN10 does not pose a serious impact hazard at this time, but observers should continue to monitor its motion over the next few decades, and the chance of impact should be carefully reassessed whenever new data become available.
Paul W. Chodas
Near Earth Object Program Office
Jet Propulsion Laboratory
May 26, 1999
Sky & Telescope's News Bulletein-May 28に本件の要約が掲載されております。
タイトル：Close-Call Asteroid Coming
CLOSE-CALL ASTEROID COMING
Astronomers admitted this week that there is a small but real possibility that an asteroid could hit the Earth in less than 50 years. The asteroid of the hour -- designated 1999 AN10 -- had been put on the list of "potentially hazardous objects" soon after its discovery earlier this year. A team of Italian astronomers determined that there was a one-in-a-billion chance of impact in 2039 and their results were prematurely announced by outside individuals.
While the object became a topic of discussion in astronomical circles, it didn't cause a widespread media frenzy as with minor planet 1997 XF11 a year ago.
Once the object cleared the Sun and could be observed again, additional positions and analysis eliminated the 2039 threat, but revealed that 1999 AN10 would skirt the Earth on August 7, 2027, perhaps as close as 30,000 kilometers above Earth's surface. The situation changed with yet more positions of the object, and on May 26th, Paul W. Chodas (Jet Propulsion Laboratory) announced that analysis by several dynamicists revealed that while the asteroid could not hit us on 2027, the uncertainty of where the object would be when it came by the planet allowed "keyholes" that could send it on a collision course in 2044 or 2046. The odds of a collision are now roughly one in 500,000 -- slightly more likely than a chance hit from an undiscovered asteroid over the next 40 years.
Further observations of 1999 AN10 will help refine its orbit and shrink the error ellipse. Thus even this danger could evaporate.