Contents: Near Earth Objects | Asteroids

Near-Earth Objects:
NASA's Near-Earth Objects Program studies Asteroids and Comets which have orbits which bring them into the inner solar system. As of Dec., 2003 there were currently 611 known Potentially Hazardous Asteroids (PHAs).
Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid's potential to make threatening close approaches to the Earth. Specifically, all asteroids with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU * or less and an absolute magnitude ‡ (H) of 22.0 or less are considered PHAs. In other words, asteroids that can't get any closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs.

As of Sept., 2007 there were 884 known PHAs.

* AU = Astronomical Unit - Approximate distanced from the earth to the sun
  150,000,000 km (93,000,000 mi.)
Magnitude is a measure of brightness

The most potentially dangerous asteroid is 1950 DA. About 1 km in diameter and projected to have a 1 in 300 chance of hitting the earth in 2880.

Small Asteroids come close relatively often:
On August 18, 2002 Asteroid 2002 NY40 came within 1.5 LD's †of the earth. Flybys like this happen every 50 years or so," says Don Yeomans, the manager of NASA's Near-Earth Object Program office at JPL.

A small near-Earth asteroid (NEA), discovered in March 2004 by the NASA-funded LINEAR asteroid survey, will make the closest approach to Earth ever recorded. There is no danger of a collision with the Earth during this encounter.

† LD = Lunar Distance - distance between Earth and the Moon - 384,401 km (238,856 mi.)

The object, designated 2004 FH, is roughly 30 meters (100 feet) in diameter and will pass just 43,000 km (26,500 miles, or about 3.4 Earth diameters) above the Earth's surface in 2004.

On average, objects about the size of 2004 FH pass within this distance roughly once every two years, but most of these small objects pass by undetected. This particular close approach is unusual only in the sense that scientists know about it. The fact that an object as small as asteroid 2004 FH has been discovered now is mostly a matter of perseverance by the LINEAR team, which is funded by NASA to search for larger kilometer-sized NEAs, but also routinely detect much smaller objects.

May-July 2004 Earth-asteroid encounters
ASTEROID

 DATE (UT)

 MISS DISTANCE

 MAG.
2001 US16

May 8

11 LD

 15
2004 HC39

May 12

13 LD

 17
2004 FJ11

May 24

12 LD

 18
1998 SF36

June 26

5 LD

 13
1999 MN

July 14

7 LD

 16
Notes: LD is a "Lunar Distance." 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals 0.00256 AU. MAG is the visual magnitude of the asteroid on the date of closest approach.
Source: SpaceWeather.com/


Large Asteroids:
Most of these are in the asteroid belt between mars and jupiter and in no danger of hitting earth.

As of 2004 there were 26 known asteroids larger than 200 km in diameter.
Ceres is the largest by far at 933 km in diameter. Most are in the asteroid belt, but some are closer in or farther out.

Chiron was discovered in 1977. It lies between Saturn and Uranus and occasionally passes within the orbit of Saturn. At first thought to be a comet or asteroid it is now considered a "planetoid" or small planet-like body. It is 170 km in diameter.

No.   Name   Distance (km) Radius (km) Mass   Discoverer   Date
----  --------    ------    ------  -----  ----------  -----
2062 Aten         144514       0.5   ?      Helin       1976
3554 Amun         145710       ?     ?      Shoemaker   1986
1566 Icarus       161269       0.7   ?      Baade       1949
 433 Eros         172800      33x13x13      Witt        1898
1862 Apollo       220061       0.7   ?      Reinmuth    1932
2212 Hephaistos   323884       4.4   ?      Chernykh    1978
 951 Gaspra       330000       8     ?      Neujmin     1916
   4 Vesta        353400     265  3.0e20    Olbers      1807
   3 Juno         399400     123     ?      Harding     1804
  15 Eunomia      395500     136  8.3e18    De Gasparis 1851
   1 Ceres        413900     466  8.7e20    Piazzi      1801
   2 Pallas       414500     261  3.18e20   Olbers      1802
 243 Ida          428000      35     ?      ?           1880?
  52 Europa       463300     156     ?      Goldschmidt 1858
  10 Hygiea       470300     215  9.3e19    De Gasparis 1849
 511 Davida       475400     168     ?      Dugan       1903
 911 Agamemnon    778100      88     ?      Reinmuth    1919
2060 Chiron      2051900      85     ?      Kowal       1977
Source: Asteroids at seds.lpl.arizona.edu

Kuiper Belt Objects (KBOs)

Name               Orbit  Diameter
Quaoar (2002 LM60) 42 AU  1250 km 800 mi
Orcus  (2004 DW)   45 AU 
2005 FY9                         700-1000 mi
2003 UB313       > 100   3000 km 1,850 mi
See Planets: Planetoids at NASA.

See:
Potentially Hazardous Asteroids at JPL's Near Earth Object (NEO) website, neo.jpl.nasa.gov
Torino Impact Hazard Scale
Asteroid Comet Impact Hazards Website at NASA Asteroids at the Astronomical Events page
Asteroids
Satellite Orbits


last updated 1 Sep 2007