From Wikipedia, the free encyclopedia
2007
WD5 is a 50 m (160 ft)
diameter Apollo class[3] Near-Earth
object and a Mars-crosser
asteroid discovered on November 20, 2007 by Andrea Boattini
of the Catalina Sky Survey. Early
observations of 2007 WD5 caused excitement amongst the
scientific community when it was estimated as having as high as a 1
in 25 chance of colliding with Mars on January 30, 2008.[4]
However, by January 9, 2008 additional observations allowed NASA's
Near Earth Object Program (NEOP) to reduce the uncertainty region
resulting in only a 1-in-10,000 chance of impact.[5]
2007 WD5 most
likely passed Mars at a distance of 6.5 Mars radii. Due to this
relatively small distance and the uncertainty level of the prior
observations, the gravitational effects of Mars on its trajectory
are unknown and, according to Steven Chesley of NASA's Jet Propulsion Laboratory
Near Earth Object program, 2007
WD5 is currently considered 'lost' (see lost asteroids).[6]
Discovery
The asteroid was discovered on November 20, 2007 by Andrea
Boattini[7]
of the NASA-funded Catalina
Sky Survey on Mount Lemmon, near Tucson, Arizona, USA, using a 1.5m telescope.[2] It
was discovered in the constellation Taurus at an apparent
magnitude of +20. This is about 400,000 times fainter than most
people can see with the naked eye on a dark night far from city
lights.[7]
It was discovered nineteen days after passing near the Earth. By
the time it arrived at Mars it had an apparent magnitude of roughly
+26 and appeared over 100x fainter than at the time of
discovery.[8]
January 2008
Mars encounter - chance of impact
Timeline of observations
and events
- November 1, 2007: Nineteen days before its discovery, this
small asteroid passed within 7.5 million km (5 million miles or
0.0476 AU) of the Earth.[2]
- December 21, 2007: 2007
WD5 was approximately half way between Earth and
Mars traveling at 27,900 mph (44,900 km/h).
It was estimated by NASA's Near Earth Object Program (NEOP) to have
a 1-in-25 chance of colliding with Mars on January 30, 2008 at approximately 10:55
UT.[2] It
was thought it would pass about 50,000 km (0.00034AU) from
Mars.
- December 28, 2007: NASA
scientists at the Near-Earth Object program office at JPL announced they had found
2007 WD5 in 3
precovery images from
November 8, 2007. The refined orbit placed the odds of a Mars
impact at 1-in-25. The uncertainty region was reduced from 1
million km to roughly 400,000 km.[4]
The best fit trajectory had the asteroid passing within
21,000 km of Mars and only 16,000 km from the moon Deimos.[9]
The pre-discovery observations were located by Andy Puckett in the
archive of the Sloan Digital Sky Survey II at
the Apache Point Observatory.
- January 2, 2008: NASA
scientists revised the probability of an impact with Mars to
1-in-28 after more observations were reported by Bill Ryan with the
2.4 meter telescope at New Mexico Tech's Magdalena Ridge
Observatory. The uncertainty region was reduced to roughly
200,000 km and still intersected Mars, but the most likely
path moved a little farther away from the planet.[10]
- January 8, 2008: NASA
scientists revised the probability of an impact with Mars to
1-in-40 after refinements to the Sloan precovery observations and
observations with the 3.5 meter telescope at the Calar
Alto Observatory in Spain. The uncertainty region was reduced
by a factor of 3.[11]
- January 9, 2008: Following several new observations NASA
reduced the uncertainty region and effectively ruled out a Mars
collision. The chance of collision became only 1-in-10,000
(0.01%).[5]
The best estimate was that around 12:00 UTC the asteroid passed about
26,000 km (0.00017AU)[12]
from the planet's center (about 7 Mars radii from the surface).
Analyses show there is no possibility of an impact with either Mars
or Earth in the next century.
This trend of increasing probability of impact followed by a
dramatic decrease is typical as uncertainties are gradually
reduced.[5]
In December 2004, a similar trend was observed with 99942 Apophis where
the predicted probability of impact with Earth in 2029 at one point
reached as high as 2.7%.
NASA Animation showing the motion of the uncertainty region of 2007
WD5 as it approaches Mars. The thin white line is the orbit of
Mars. The blue line traces the motion of the center of the
uncertainty region, which is the most likely position of the
asteroid.
Orbital data as known on December 21, 2007
Estimates of resulting
impact
Track of asteroid
2007
WD5 over Mars (NASA/JPL)
If the asteroid had collided with Mars, it would have hit with a
velocity of about 13.5 km/s (8.4 miles per second), and would
have produced an explosion equivalent to about 3 megatons of
TNT.[2] Due
to the thin atmosphere of Mars, it was predicted
that the asteroid would have reached the surface intact and blasted
out a crater approximately 0.8 km (0.5 mi) in
diameter.[13]
A crater this size would be equal to the size of the Meteor Crater in Arizona, USA. NASA officials say if it
had hit Mars, it would have done so north of the location of the Opportunity
rover.[7]
2007 WD5 is
roughly the size of the cometary object that caused the Tunguska event in
1908, in remote central Siberia, Russia. Due to the Earth's greater gravity an
impact with the power of Tunguska is expected to occur once every
few hundred years.[14]
Since Mars has only 1/10th the mass to attract objects, these types
of impacts occur roughly every one thousand years on Mars.[1]
Future
encounters
In July 2003, the asteroid passed within 0.012AU of Mars.[12]
The exact fate of 2007
WD5 following the January 2008 Mars encounter is
unknown although it is most likely that it safely passed the planet
at a distance of 6.5 Mars radii. Mars, unlike Jupiter, is not big
enough to eject the asteroid from the solar system, however, the
gravitation effect from the encounter on the asteroid's trajectory
is uncertain and the asteroid is currently considered 'lost'.[6]
Assuming 2007
WD5 passed Mars safely, its low inclination to
the Ecliptic of only 2.3
degrees and high eccentricity of 0.6 could cause it
to swing close to Mars or Earth for years or decades into the
future.
See also
References
- ^ a
b
"Astronomers Monitor Asteroid
to Pass Near Mars". NASA/JPL. 2007-12-21. http://www.jpl.nasa.gov/news/news.cfm?release=2007-152. Retrieved
2007-12-22.
- ^ a
b
c
d
e
Steve Chesley and Paul Chodas (December
21, 2007). "Recently Discovered Asteroid
Could Hit Mars in January". NASA/JPL Near-Earth Object Program
Office. http://neo.jpl.nasa.gov/news/news151.html. Retrieved
2007-12-21.
- ^
"JPL Small-Body Database
Browser: 2007WD5". http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2007wd5. Retrieved
2008-01-12.
- ^ a
b
Don Yeomans, Paul Chodas and Steve
Chesley (December 28, 2007). "Mars Impact Probability
Increases to 4 Percent". NASA/JPL Near-Earth Object Program
Office. http://neo.jpl.nasa.gov/news/news153.html. Retrieved
2007-12-28.
- ^ a
b
c
Steve Chesley, Paul Chodas and Don
Yeomans (January 9, 2008). "2007 WD5 Mars Collision
Effectively Ruled Out - Impact Odds now 1 in 10,000". NASA/JPL
Near-Earth Object Program Office. http://neo.jpl.nasa.gov/news/news156.html. Retrieved
2008-01-09.
- ^ a
b
Lakdawalla, Emily (February 4, 2008). "WD5 most likely missed Mars,
but we may never know". http://www.planetary.org/blog/article/00001316/. Retrieved
2008-02-24.
- ^ a
b
c
d
Lori Stiles, University Communications
(December 21, 2007). "Catalina Sky Survey Discovers
Space Rock That Could Hit Mars". The University of Arizona. http://uanews.org/node/17415. Retrieved
2007-12-23.
- ^
Horizons Brightness Difference between 11-20-07 and 01-30-08: (5th
root of 100) ^ (@marsJan30th APmag 25.9 - DiscoveryNov20th APmag
20.2) = 190x
- ^ "Horizons Archive Mars/Earth
2003/2008". http://home.comcast.net/~kpheider/2007WD5.txt. Retrieved
2007-12-23.
(Soln.date:
2007-Dec-23)
- ^ Don Yeomans, Paul Chodas and Steve Chesley
(January 2, 2008). "New Observations Slightly
Decrease Mars Impact Probability". NASA/JPL Near-Earth Object
Program Office. http://neo.jpl.nasa.gov/news/news154.html. Retrieved
2008-01-02.
- ^ Steve Chesley, Paul Chodas and Don Yeomans
(January 8, 2008). "Mars Impact Seems Less
Likely". NASA/JPL Near-Earth Object Program Office. http://neo.jpl.nasa.gov/news/news155.html. Retrieved
2008-01-08.
- ^ a
b
"Horizons Output Mars/Earth
2003/2008". http://home.comcast.net/~kpheider/2007WD5-0109.txt. Retrieved
2008-01-09.
(Soln.date:
2008-Jan-09)
- ^ Johnson Jr., John (2007-12-21). "Asteroid on track for
possible Mars hit". Science (Los Angeles
Times). http://www.latimes.com/news/science/la-sci-mars21dec21,0,6729483.story?coll=la-home-center. Retrieved
2007-12-21.
- ^ David Morrison (2007-12-21). "Tunguska Revision, and a
Possible NEA Impact on Mars". Asteroid and Comet Impact Hazards
(NASA). http://impact.arc.nasa.gov/news_detail.cfm?ID=179. Retrieved
2008-01-03.
External
links
Wikis
Facts
Map
Search
