[meteorite-list] Whew! Asteroid Won't Hit Earth in 2029, Scientists Now Say

[Ron Baalke]

http://space.com/scienceastronomy/asteroid_update_B_041227.html

Whew! Asteroid Won't Hit Earth in 2029, Scientists Now Say
By Robert Roy Britt 
space.com
27 December 2004

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.