The odds of impact in the year 2029 by a recently discovered asteroid are unlikely to change much in the next few weeks, astronomers said Monday.

Last Thursday, Dec. 23, scientists announced that a space rock named 2004 MN4 had about a 1-in-300 chance of striking Earth on April 13, 2029. On Friday, the risk was upgraded as more observations rolled in. The asteroid was given an unprecedented risk rating of 4 on the Torino Scale, which means it warrants careful monitoring. The odds at various times were put at 1-in-63 and 1-in-45.

As of Monday, the chances of an impact on April 13, 2029 stood at about 1-in-40, or 2.6 percent.

Experts point out that means a 97.4 percent chance the giant boulder will miss, and they stress that the odds are likely to go down to zero, eventually, when more detailed observations of its path are made. Significant revision -- if it comes -- probably won't happen soon.

"Slow changes should be expected for the next days and weeks, but probably nothing too dramatic unless the Arecibo [Observatory] radar weighs in late January or early February," Don Yeomans, as asteroid expert at NASA's Jet Propulsion Laboratory, told SPACE.com on Monday.

2004 MN4 is about a quarter mile (400 meters) wide, large enough to cause considerable local or regional damage were it to hit the planet. It is larger than the asteroid that carved Meteor Crater in Arizona thousands of years ago, and much bigger than one that exploded in the air above Siberia in 1908, flattening thousands of square miles of forest.

This latest potentially threatening asteroid was discovered in June and spotted again this month. It circles the Sun, but unlike most asteroids that reside in a belt between Mars and Jupiter, the 323-day orbit of 2004 MN4 lies mostly within the orbit of Earth.

JPL scientists, and a separate research group in Italy, continually refine the projected orbit as new observations are provided by observatories around the world. Given the limited sightings, they can predict only a general area of space through which 2004 MN4 will pass when it makes what will be, at the least, a close flyby of Earth on April 13, 2029.

The odds of an impact by the newly discovered object took astronomers by surprise.

"I hadn't expected to see such a high probability of impact -- recognizing that there are still 39 chances in 40 that it won't impact -- by such a large object in my lifetime," said Clark Chapman of the Southwest Research Institute.

Chapman studies asteroids and is a founding member of the B612 group, which promotes the idea of deflecting an asteroid if one is ever found to be on a collision course with our planet.

The lack of media hype surrounding 2004 MN4 has been another pleasant surprise.

In a handful of memorable instances going back about six years, scientists announced space rocks with long odds of Earth collisions, and media outlets made big headlines of the small data. Sometimes the scientists put the data out to the press in a manner that some considered inflammatory, and other times they were criticized for keeping it largely out of public view.

In each case, the impact odds evaporated within hours or days as the orbits were pinned down. But headlines were not typically rewritten, and scientists have grown to worry about alarming the public unnecessarily.

With 2004 MN4, Yeomans and his JPL colleagues quietly issued an informational statement on the JPL web site last Thursday and updated it Friday.

"We decided long ago that keeping the public and media in the dark was a very bad idea, so we try to maintain a sober assessment on the web site as often as necessary," Yeomans said. "So far the media has shown very good sense on this one. I hope this is due to our maturing ideas on near-Earth objects."

Meanwhile, 2004 MN4 is remarkable given current asteroid search technology and the limited funding provided, mostly by NASA. Never has a space rock been found -- after several days of observation -- to have such high odds of collision on such a near-term future date.

"I think 2004 MN4 stands as highly unusual in the context of present survey programs," said Alan Harris, senior research scientist at the Space Science Institute in La Canada, Calif. "That may change when we have new surveys."

As more detailed searches come online, more asteroids will be found, Harris said via email, but orbit forecasts are expected to be more accurate. Overall, he figures situations like that with 2004 MN4 will occur "somewhat more often, but not nearly in proportion to the increased discovery rate."

For now, scientists have plenty to worry about with 2004 MN4.

Harris said Arecibo observations, if they come in late January or early February, could "dramatically improve the determination of the orbit, which would change the impact probability estimate greatly one way or the other, most likely -- but not certainly -- downward."

• Catastrophe Calculator
• Initial Story about Asteroid 2004 MN4
• Asteroid 2004 MN4 Underscores Planetary Defense Mission