What’s so big about “small objects”? If an asteroid’s orbit brings it to within 0.05 astronomical units (AU) of Earth's orbit, it is categorized as “potentially hazardous”
unless it has an absolute magnitude
H greater than 22.0, which corresponds to a diameter on the order of 135
meters/yards. Larger H means less bright, thus smaller size. And 0.05 AU is about 19.5 times the distance between Earth and Moon (0.00256 AU). To be discovered and tracked, such objects usually must come close (a few are Earth’s nearest neighbors, coming closer than the Moon). They are exposed pieces of distant asteroid populations, and they have within their own population tomorrow’s meteors. And their discovery and follow-up represents today’s best amateur and professional asteroid observing work.
Diameter & Earth MOID: In the following observation summary
table, the stated diameters are rough best estimates from a standard but
very inexact H-to-size
formula using
H (absolute magnitude, or brightness) from the JPL
NEO Orbital Elements page, source also for Earth MOID (minimum orbital intersection distance). Current Minor Planet Center H is also given, along with the original H from each object's discovery MPEC.
Other sources: Planetary MOIDs are from Lowell Observatory. Priorities and visibilities are from the European Spaceguard Central Node (
SCN). And flyby distances and times are from the JPL
Close Approach Table. See also the Sormano Observatory
SAEL (H>22.0 and Earth MOID<0.015 AU), and NEODyS
listings have yet another H calculation.
