The world can exhale a collective sigh of relief. A newfound asteroid tagged with the highest warning level ever issued will not strike Earth, scientists said Monday.
The giant space rock, named 2004 MN4, was said on Dec. 23 to have an outside shot at hitting the planet on April 13, 2029. The odds climbed as high as 1-in-37, or 2.7 percent, on Monday, Dec. 27.
Researchers had flagged the object as one to monitor very carefully. It was the first asteroid to be ranked 4 on the Torino Scale, a Richter-like measure for potentially threatening space rocks. The asteroid is about a quarter mile (400 meters) wide, large enough to cause considerable local or regional damage were it to hit the planet.
All along, scientists said additional observations would likely reduce the chance of impact to zero for the April 13 scenario, but they did not expect any significant new data to allow such a downgrading for days or weeks.
Instead, old observations provided the data necessary to rule out an impact.
Several groups were looking for the asteroid in past observations. Jeff Larsen and Anne Descour of the Spacewatch Observatory near Tucson, Arizona, found very faint images of asteroid 2004 MN4 on archival images dating to March 15 this year. Astronomers already had observations in June and from this month.
"An Earth impact on April 13, 2029 can now be ruled out," read a statement issued Monday evening by asteroid experts Don Yeomans, Steve Chesley and Paul Chodas at NASA's Jet Propulsion Laboratory.
It is not the first time a potentially threatening asteroid has been theoretically defused by looking into the past, pointed out Clark Chapman of the Southwest Research Institute. Most famously, a space rock catalogued as 1997 XF11 was said, in 1998, to be on a collision course before archived data showed it would pass harmlessly.
"Past observations can greatly extend the time baseline and strongly influence knowledge of the orbit," Chapman told SPACE.com. "At some level, we are 'lucky' that these earlier sightings were made since 2004 MN4 is usually too faint to be detected by near-Earth-object search telescopes."
The difficulty in predicting a precise path earlier in the game owes to knowing only a small section of an asteroid's orbit around the Sun. New observations -- or old ones -- make the known path longer and allow a better prediction of the full path, as well as where an asteroid will be years from now.
Orbits change slightly with time because of gravitational tugs by the Sun and planets, among other factors.
2004 MN4 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.
Scientists cannot say that the asteroid will never hit Earth, but there are no serious threats in the foreseeable future. "No subsequent Earth encounters in the 21st century are of any concern," the NASA statement read.
Editor's note:Now obsolete are stories about 2004 MN4 early on Dec. 27 and the initial one on Dec. 24. A related story Dec. 27 discusses how this asteroid illustrates the need for a planetary defense strategy.