2004 MN4小行星2029年将肉眼可见！与我们擦身而过（距离只有三万公里）！

Asteroid 2004 MN4: A Really Near Miss!
By Alan M. MacRobert



 
 
February 4, 2005 | If you plan to be alive on April 13, 2029, you can look forward to an asteroid-watching party across three continents like nothing the world has ever seen.
The near-Earth asteroid 2004 MN4 made headlines for a couple of days last December when astronomers estimated that it had a 1-in-38 chance of hitting Earth in 2029. The threat quickly passed when old images were found that pinned down the asteroid's orbit well enough to guarantee that it will miss our planet. Now, extremely precise radar observations made on January 27th, 29th, and 30th have refined its orbit even further. The asteroid is still certain to miss Earth, but it will be a squeaker indeed — and the event will present a once-in-a-millennium naked-eye asteroid show.

Paul Chodas, Steve Chesley, Jon Giorgini and Don Yeomans of NASA's Near Earth Object Program calculate that the asteroid will pass 4.7 Earth radii (30,000 kilometers, or 18,600 miles) from Earth's surface. This is half the distance predicted by the previous orbit. The object should brighten to magnitude 3.3 (similar to the faintest star in the Big Dipper) while moving northwest across Sextans and Cancer at a speed up to 42° per hour. Europe, Africa, and Asia will have the ringside seats. With a newly estimated diameter of 320 meters, 2004 MN4 will appear up to 2 arcseconds wide, making it barely resolvable in amateur telescopes.

The Americas will get their best view hours earlier, while the asteroid is still on its way in and only 7th magnitude. The next night it will be much fainter than this and near our line of sight to the Sun.

The radar observations were made by Lance Benner, Mike Nolan, Steve Ostro, and Jon Giorgini using the giant Arecibo dish in Puerto Rico. It's a good thing the refined orbit is still a miss: the impact energy would be about 850 megatons, 15 times more powerful than the largest hydrogen bomb ever tested and about 60 times more powerful than the Tunguska explosion over Siberia in 1908. According to Chodas and his colleagues, "On average, one would expect a similarly close Earth approach by an asteroid of this size only every 1,300 years or so."

Mark your calendar, and if you're on the wrong side of the world, plan on a vacation to join the party.

Further information is on NASA's Near Earth Object Program page.

NASA的网站也是我最喜欢的网页之一，不仅可以巩固我的英文知识，还使我大开了眼界呢，一句话，强