2007 WD₅ is anApollo asteroid some 50 m (160 ft) in diameter^[2] and aMars-crosser asteroid first observed on 20 November 2007, byAndrea Boattini of theCatalina Sky Survey. Early observations of2007 WD₅ caused excitement amongst the scientific community when it was estimated as having as high as a 1 in 25 chance ofcolliding withMars on 30 January 2008.^[6] 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.^[7]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 ofNASA'sJet Propulsion Laboratory Near-Earth Object program,2007 WD₅ is currently considered 'lost' (seelost asteroids).^[8]

The asteroid was discovered on 20 November 2007 by Andrea Boattini^[9] of theNASA-fundedCatalina Sky Survey onMount Lemmon, nearTucson,Arizona, United States, using a 1.5-meter telescope.^[4] It was discovered in theconstellationTaurus at anapparent magnitude of +20. This is about 400,000 times fainter than most people can see with thenaked eye on a dark night far fromcity lights.^[9] 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.^[10]

• 1 November 2007: Nineteen days before its discovery, this small asteroid passed within 7.5 million km (5 million miles or 0.0476AU) of the Earth.^[4]
• 20 November 2007: The asteroid was first discovered by Andrea Boattini of theCatalina Sky Survey.^[9]
• 21 December 2007:2007 WD₅ was approximately halfway 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-75 chance ofcolliding withMars on 30 January 2008 at approximately 10:55 UT.^[4] It was thought it would pass about 50,000 km (0.00034AU) from Mars.
• 28 December 2007:NASA scientists at the Near-Earth Object program office atJPL announced they had found2007 WD₅ in 3precovery images from 8 November 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.^[6] The best fit trajectory had the asteroid passing within 21,000 km of Mars and only 16,000 km from the moonDeimos.^[11] The pre-discovery observations were located by Andy Puckett in the archive of theSloan Digital Sky Survey II at theApache Point Observatory.
• 2 January 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'sMagdalena 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.^[12]
• 8 January 2008: NASA scientists revised the probability of an impact with Mars to 1-in-40 after refinements to the analysis of the Sloan precovery observations and observations with the 3.5 meter telescope at theCalar Alto Observatory in Spain. The uncertainty region was reduced by a factor of 3.^[13]
• 9 January 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%).^[7] The best estimate was that around 12:00UTC the asteroid passed about 26,000 km (0.00017AU)^[14] 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.^[7] In December 2004, a similar trend was observed with99942 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 3megatons of TNT.^[4] Due to the thinatmosphere 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.^[15] A crater this size would be equal to the size of theMeteor Crater inArizona, United States. NASA officials say if it had hit Mars, it would have done so north of the location of theOpportunity rover.^[9]

2007 WD₅ is roughly the size of thecometary object that caused theTunguska event in 1908, in remote centralSiberia, Russia. Due to the Earth's greater gravity, an impact with the power of Tunguska is expected to occur once every few hundred years.^[16] Since Mars has only 1/10 the mass to attract objects, these types of impacts occur roughly every one thousand years on Mars.^[3]

In July 2003, the asteroid passed within 0.012 AU of Mars.^[14] The exact fate of2007 WD₅ following the January 2008 Mars encounter is unknown although it likely passed Mars at a distance of 6.5 Mars radii. Mars, unlikeJupiter, 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'.^[8] Assuming2007 WD₅ passed Mars safely, its low inclination to theecliptic of only 2.3 degrees and higheccentricity of 0.6 could cause it to swing close to Mars or Earth for years or decades into the future.^{[citation needed]}

• C/2013 A1 - Comet that passed extremely close to Mars on 19 October 2014
• Comet Shoemaker–Levy 9 - Jupiter impact 1994
• Impact events on Mars

• CNEOS – Center for Near Earth Object Studies,JPL–NASA
• A Meteoroid Hits the Moon (25 cm wide) on 2 May 2006
• Gravity Simulator Diagram of Mars Passage (based on 23 December orbital elements)
• Computing Crater Size from Projectile Diameter (H. Jay Melosh and Ross A. Beyer)
• Solar System Collisions (Send an asteroid or comet hurtling toward your favorite planet!)
• Science Friday Podcast with Don Yeomans (2007-12-28)
• Possible Asteroid Strike on Mars: Scientific Paydirt (2008-01-03)
• 2007 WD5 atNeoDyS-2, Near Earth Objects—Dynamic Site
  • Ephemerides · Observation prediction · Orbital info · MOID · Proper elements · Observational info · Close approaches · Physical info · Orbit animation
• 2007 WD5 atESA–space situational awareness
  • Ephemerides · Observations · Orbit · Physical properties · Summary
• 2007 WD5 at theJPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit viewer · Orbit parameters · Physical parameters

• Apollo asteroids
• Minor planet object articles (unnumbered)
• Discoveries by Andrea Boattini
• Mars impact events
• Lost minor planets
• Astronomical objects discovered in 2007
• Predicted impact events

• CS1: unfit URL
• Articles with short description
• Short description is different from Wikidata
• Use dmy dates from January 2020
• All articles with unsourced statements
• Articles with unsourced statements from October 2019