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. As a result, we’ll get 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.
When they realized how close the asteroid would pass, astronomers at first thought that Earth's tidal force might pull it apart into a string of rubble before our very eyes. This is what happened to Comet Shoemaker-Levy 9 when it dipped very close to Jupiter, one orbit before the resulting string of comet chunks impacted Jupiter in 1994. However, calculations by Derek Richardson and Kevin Walsh (University of Maryland) make a breakup seem very unlikely. Even if 2004 MN4 is a loose "rubble pile" with no internal strength, it should hold together after all.
But just barely. "My best estimate," says Daniel D. Durda (Southwest Research Institute) "is that it might have its rotation spun up a bit, and if it's a rubble pile it might have some internal readjustments. If you were standing on it you might notice rumblings, asteroid-quakes. This will be an outstanding opportunity for a spacecraft mission to go and sit on its surface and hear it creak and groan as it goes by. We won't have to ping it to sound its interior — Mother Nature will do it for us."
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 orbit is still a miss: the impact energy in 2029 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.