A minor planet is "lost" when today's observers cannot find it, because its location is too uncertain to target observations. This happens if the orbital elements of a minor planet are not known accurately enough, typically because the observation arc for the object is too short, or too few observations were made before the object became unobservable (e.g. too faint due to increasing distance, or too close to the Sun to view at night).

By some definitions thousands, if not tens of thousands, of mostly small observed minor planets are lost.^[2] Some lost minor planets discovered in decades past cannot be found because the available observational data is insufficient for reliable orbit determination. With limited information astronomers cannot know where to look for the object at future dates.

Lost objects are sometimes recovered when serendipitously re-observed by a later astronomical survey. If the orbital elements of the newly found object are sufficiently close to those of the earlier lost object, the two may be equated. This can be established by calculating backwards the "new" object's orbit (once it is firmly known) and checking past positions against those previously recorded for the lost object. This usually greatly extends the object's arc length, thus fixing the orbit much more precisely. The back-orbit calculations are especially tricky for lost comets because their orbits can be affected by non-gravitational forces, such as emission of jets of gas from the comet nucleus. Many previously lost asteroids (a type of minor planet) were rediscovered in the 1980s and 1990s, but many minor planets are still lost.^[3]

This is a small selection of some early lost or notable asteroids with their discovery and rediscovery dates. (A more detailed description for some of these minor planets can be found in the following sections.) The true number of lost asteroids may be over 150,000.^[2] There are also about 30,000 unnumbered bodies with a condition code of U = 9, indicating the highest possible uncertainty of their orbit determination. Many of these bodies have been observed years if not decades ago and must be considered lost.^{[5][lower-alpha 1]} There are also more than a thousand near-Earth objects (NEOs) with an observation arc of one or two days only.^[6]

The number of asteroids that were only observed once and not re-observed grew throughout the 19th and 20th centuries, but improved telescopes, searches, and detection techniques led to resolution of most of these cases between 1970 and 2000. There are earlier examples also, such as 132 Aethra, which was lost between 1873 and 1922.^[7]

Recently lost minor planets

• 2007 WD5 is a 50 m (160 ft) Apollo-class NEO and a Mars-crosser discovered on 20 November 2007, by Andrea Boattini of the Catalina Sky Survey.^[27] 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.^[28] 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.^[29] 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 JPL-Near Earth Object program, 2007 WD₅ is currently considered "lost".^[28][30] The best fit trajectory had the asteroid passing within 21,000 km of Mars and only 16,000 km from its moon Deimos.^[29][31]
• 2010 AU118 is a kilometer-sized Amor-class NEO and Mars-crosser discovered on 27 May 2010, by the Wide-field Infrared Survey Explorer (WISE) spacecraft. The asteroid was only observed 19 times during 13–15 January 2010, and has not since been observed.^[32] Virtual clones of the asteroid that fit the uncertainty region in the known trajectory, showed a 1 in 770 million chance that the asteroid could have impacted the Earth on 2020 October 20.^[33] However, NEODyS lists the nominal 20 October 2020 Earth distance as 3 AU (450,000,000 km; 280,000,000 mi).^[34]
• In 2007, the object 2007 RR₉ was found to be the near-Earth asteroid 6344 P–L, lost since 1960. It is a potentially hazardous object and probably a dormant comet, although it was not visibly outgassing at that time.^[13]

• List of unnumbered minor planets § 1900–1959
• Lost comet
• Precovery