The CNEOS database of close approaches lists some close approaches a full orbit or more before the discovery of the object, derived by orbit calculation. The list below only includes close approaches that are evidenced by observations, thus the pre-discovery close approaches would only be included if the object was found by precovery, but there was no such close approach in 2008.
This list and the relevant databases do not consider impacts on Earth as close approaches. The impact of 2008 TC3 was the first successfully predicted asteroid impact, while the impact of several small asteroids in Earth's atmosphere were observed visually or recorded by sensors designed to detect the detonation of nuclear devices as bolides.
Rows highlighted red indicate objects which were not discovered until after closest approach
Rows highlighted yellow indicate objects discovered less than 24 hours before closest approach
Rows highlighted green indicate objects discovered more than one week before closest approach
Rows highlighted turquoise indicate objects discovered more than 7 weeks before closest approach
Rows highlighted blue indicate objects discovered more than one year before closest approach (i.e. objects successfully cataloged on a previous orbit, rather than being detected during final approach)
This table visualizes the warning times of the close approaches listed in the above table, depending on the size of the asteroid. The sizes of each pie chart show the relative sizes of the asteroids to scale. For comparison, the approximate size of a person is also shown. This is based around the absolute magnitude of each asteroid, an approximate measure of size based on brightness.
Absolute magnitude H ≥ 30 (smallest)
Silhouette of man standing and facing forward
(size of a person for comparison)
Absolute magnitude 30 > H ≥ 29
Absolute magnitude 29 > H ≥ 28
Absolute magnitude 28 > H ≥ 27
Absolute magnitude 27 > H ≥ 26
Absolute magnitude 26 > H ≥ 25
None
Absolute magnitude 25 > H (largest)
After closest approach: 0 (0.0%)
< 24 hours before: 0 (0.0%)
up to 7 days before: 1 (100.0%)
> one week before: 0 (0.0%)
> 7 weeks before: 0 (0.0%)
> one year before: 0 (0.0%)
Notes
^Distance from the center of Earth to the center of the object. See the NASA/JPL Solar System Dynamics Glossary: Geocentric. Earth has a radius of approximately 6,400 km.
Additional examples
An example list of near-Earth asteroids that passed more than 1 lunar distance (384,400 km or 0.00256 AU) from Earth in 2008.
2007 TU24 (~250 meters in diameter) passed 1.44 LD (554,000 km) from Earth on January 29, 2008.
2008 CE22 (~18 meters in diameter) passed between 0.9997 and 1.0023 LD (384,300 to 385,300 km) from Earth on February 6, 2008.
(450894) 2008 BT18 (~650 meters in diameter) passed 5.9 LD (2.3 million km) from Earth on July 14, 2008.
2008 XK (~12 meters in diameter) may have passed as close as 0.23 LD (89,300 km) from Earth on December 5, 2008, but the nominal orbit suggests it passed nearer to 1.46 LD (560,500 km) from Earth instead.
