|Discovered by||Mount Lemmon Survey|
|Discovery date||20 November 2007|
|NEO · Apollo |
|Orbital characteristics |
|Epoch 13 January 2016 (JD 2457400.5)|
|Uncertainty parameter 5  · 0 |
|Aphelion||3.9289 AU (587.76 Gm)|
|Perihelion||0.991120 AU (148.2694 Gm)|
|2.4600 AU (368.01 Gm)|
|3.86 yr (1409.3 d)|
Average orbital speed
|12.5 km/s (27,900 mph)|
|0° 15m 19.62s /day|
|Earth MOID||0.0260299 AU (3.89402 Gm)|
|Dimensions||50 m (160 ft)|
2007 WD5 is an Apollo asteroid some 50 m (160 ft) in diameter and a Mars-crosser asteroid first observed on 20 November 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 30 January 2008. However, by 9 January 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. 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).
The asteroid was discovered on 20 November 2007 by Andrea Boattini of the NASA-funded Catalina Sky Survey on Mount Lemmon, near Tucson, Arizona, United States, using a 1.5-meter telescope. 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. It was discovered nineteen days after passing near Earth. By the time it arrived at Mars it had an apparent magnitude of roughly +26 and therefore appeared over 100 times fainter than at the time of discovery.
This trend of increasing probability of impact followed by a dramatic decrease is typical as uncertainties are gradually reduced. 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%.
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. 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. A crater this size would be equal to the size of the Meteor Crater in Arizona, United States. NASA officials say if it had hit Mars, it would have done so north of the location of the Opportunity rover.
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. Since Mars has only 1/10 the mass to attract objects, these types of impacts occur roughly every one thousand years on Mars.
In July 2003, the asteroid passed within 0.012 AU of Mars. The exact fate of 2007 WD5 following the January 2008 Mars encounter is unknown although it likely passed Mars 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'. 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.