What are Near Earth Asteroids? Asteroids NEO, PHA or Main Belt? Friend or Foe?

Asteroids, or minor planets, are small and often irregularly shaped celestial bodies. Most asteroids orbit the Sun in what is called the "asteroid belt" between the orbits of Mars and Jupiter. Gravity and other forces cause the asteroids in the main belt to move closer to Sun, until they begin to cross the orbits of Mars, Earth, Venus and Mercury. Once they cross the Earth's orbit, they are considered Near-Earth Asteroids or NEAs.

NEA Information

Astra's Asteroid Catalog

How Asteroids were Discovered

In the 1700s, the astronomer Titius and the mathematician Bode realized that there was a relationship between the distances from the Sun to the planets. This relationship is called the Titius-Bode law even though it really isn't a physical law. In the numerical sequence, it was predicted that there should be another planet located between Mars and Jupiter. Because of this "law" many astronomers began to search for a new planet in that area of the sky. On January 1, 1801, Giuseppe Piazzi discovered Ceres. The newly discovered object was soon named after the Roman goddess of the harvest. Fifteen months later, Heinrich Olbers discovered the second asteroid, Pallas.

There was a great deal of uncertainty as to the true nature of these newly discovered objects. It seemed they were neither planets nor were they comets. The observational astronomer and telescope maker, William Herschel carried out a detailed study of the newly found objects in 1802. During his observations he realized these were not comets, nor were they planets. Due to their star-like appearance and shape, he called them "asteroids" or "star" + "shape" and suggested that they be placed into a new category.

Slowly, astronomers added more new objects, each of these were smaller than Ceres. By 1845, the fifth asteroid was found and one by one others followed. Eventually it was realized that there was not one single planet between Jupiter and Mars, but a "belt" that we now call the main asteroid belt. The presences of Jupiter made it gravitationally impossible for a large planet to form between the rocky planet Mars and the great gas giant of our solar system. Astronomers discovered new steroids until 1898, the discovery of Eros marked the first asteroid that was identified as an Earth-crossing asteroid and not a member of the main asteroid belt.

When is an Asteroid an NEA or PHA?

An asteroid is called a Near Earth Asteroid (NEA) when its orbit brings it within 1.3 AU (Astronomical Unit) of the Sun. This will also bring it within 0.3 AU of the Earth's orbit. The largest known NEA is 1036 Ganymed (1924 TD) that is estimated to be over 30 km in diameter based on astronomical observations. Please Note: 30 km is the asteroid largest measurement--asteroids are irregularly shaped objects so the term "diameter" can be misleading. The first NEA to be discovered was 433 Eros in 1898.

Potentially Hazardous Asteroid (PHA)

An NEA is considered a Potentially Hazardous Asteroid (PHA) when its orbit comes within 0.05 AU (7.5 million km) of the Earth's orbit. This is referred to as the Earth Minimum Orbit Intersection Distance (MOID).The largest known PHA is 4179 Toutatis (1989 AC),estimated to be 9 km in diameter.

Assessing the Impact Risk of Asteroids

After observations by astronomers establish the orbit of newly discovered asteroids, their orbital paths are evaluated using the Torino scale and the Palmero Scale. TheTorino Scale is used to communicate to the public the risk level from 0 - 10 (with 10 as the most dangerous). This scale predicts the impact energy of the object and the likelihood of an object striking the Earth or impact probability. The Palmero Scale is used to make fine grained determinations of likelihood and also determine the level an object should be monitored.

Characteristics of Asteroids

Asteroids are classified according to two criteria: one is the orbit, sometimes called dynamical classification. The other classification is based on the asteroid's surface composition as well as can be determined by spectral classification.

EROs - Easily Retrievable Objects has recently been suggested as objects that can be gravitationally captured in orbits around the L1 and L2 libration points of the Sun-Earth system with a delta-vee threshold of 500 m/s.

In recent years, there are other classifications for Near Earth Asteroids.

Small Earth Approachers (SEA) - for objects less that 50 meters in size and orbit .95 AU to 1.05 AU in semi-major axis, eccentricity between 0-.01 degrees and inclination between 0-10 degrees. In short, very Earth-like in their orbit.

Easily Retrievable Objects (EROs) - has recently been suggested as objects that can be gravitationally captured in orbits around the L1 and L2 libration points of the Sun-Earth system with a delta-vee threshold of 500 m/s. This new designation was suggested by D. Yárnoz, J.Sánchez, C. McInnes, 2013 referenced in the linked asteroid retrieval mission papers available on this website.

Asteroids orbits are affected by collisions, close flybys of large planets or other objects, collisions and pressure from the Sun.

Other Sources of Information on NEAs or NEO

Asteroid Classification I - Dynamics by J.L. Galache on March 5, 2011, posted on the MPCDaily Minor Planet log. This is a tour of all asteroid classifications, beyond the scope of this website.

When did the asteroids become minor planets? by James L. Hilton of the Naval Observatory

Description of Asteroids as of May 20, 2004Astronomer Gérard FAURE began tracking asteroids in 1975 keeping his own database, published on the web. After 2004, the number of discoveries overwhelmed the author.

Asteroid (and Comet) Groups - Petr Scheirich, 2005. Wow and thank you, Petr! Petr has done some amazing work animating the asteroid data up to 2005. Hopefully these pages will be updated with new objects.

Spaceguard Foundation NEO page This is a great place to start for more information on NEOs in general.

Asteroid families and groups - from Project Pluto

Rules for Naming Asteroids - from The Planetary Society.


Near Earth Asteroids (NEAs)
Acronym Definitions

  • AU - Astronomical Unit
  • MPC - Minor Planet Center
  • NEO - Near Earth Objects
  • NEA - Near Earth Asteroid
  • PHO - Potentially Hazardous Object
  • PHA - Potentially Hazardous Asteroid
  • MOID - Earth Minimum Orbit Intersection Distance

Solar System Dynamics Glossary - Use this NASA site for help with orbital elements and other terms

Available Papers that Discuss Asteroids and NEAs

Click on the link to navigate to the On-line papers that discuss Near Earth Asteroids:

Near Earth Asteroid Information

Sites of Interest