NEO Background Information
An introduction to the wide world of NEOs
Near Earth Objects
A Near Earth Object, or NEO, is any asteroid or comet that comes close to Earth. Near, in this case, is when the orbit of the object allows it to approach closer than about 45 million kilometres (0.3 Astronomical Units, AU) to the earth’s orbit. In comparison, the average distance between the earth and the sun is about 150 million kilometres (1 AU).
Almost all NEOs are asteroids. Currently there are 9948 NEOs catalogued by the JPL NEO Program (as of 18 May 2013), of which 9854 are asteroids and only 94 are comets.
Most NEOs originate in the main belt, or asteroid belt, a region between the orbits of Mars and Jupiter, where there are a very large number of asteroids (577148 according to the Minor Planet Center as of 21 May 2013). These asteroids are remnants of the formation of the solar system. Collisions sometimes occur in the asteroid belt, and it is from these collisions that asteroids can be diverted to an inner orbit, where they could become NEOs.
(Image Credit: ESA 2010 MPS for OSIRIS Team MPS/UPD LAM/IAA/RSSD/INTA/UPM/DASP/IDA)
Potentially Hazardous Asteroids (PHAs)
A subcategory of NEOs are Potentially Hazardous Asteroids, or PHAs. As their name suggests, these are asteroids that have the potential to be a hazard to Earth. PHAs are defined by how close their orbit comes to that of the Earth, and also their intrinsic brightness, which is an indicator of their size. In technical terms the absolute magnitude of a PHA is numerically less (brighter) than 22 and the Earth Minimum Orbit Intersection Distance (MOID) is less than 0.05 AU or 7.5 million km. An absolute magnitude of 22 corresponds to a diameter of at least 70 m, depending on the object's albedo. With a "typical" albedo of 15% a PHA with an absolute magnitude of 22 would have a diameter of 140 m. The impact of an object of this size could cause very serious regional damage (worst case: the complete destruction of a large city or urban area). Amongst the PHAs so far discovered, the one with the brightest absolute magnitude (14.0) is (3122) Florence, according to the Minor Planet Center, which implies a diameter of about 5 km. If Florence were to impact the Earth the result would be an unimaginable global catastrophe.
Minimum Orbit Intersection Distance, or MOID, is the minimum distance between the two orbits (circles or ellipses) of two bodies - see the Glossary for more information.
Apparent magnitude is a measure for the apparent brightness of an asteroid, which can appear bright due to either its size, the reflectiveness of its surface, or both - see the Glossary for more information.
Of the current NEO population, 1311 are PHAs (source: JPL NEO Program on 3 June 2012). It is from this population of asteroids that, due to the gravitational pull from the Sun and Planets, an asteroid may find itself on a path to impact the Earth.
See this link for a list of NEOs that will pass within 0.1AU (about 15 million kilometres or 4 times the distance from Earth to the Moon) within the next 60 days.
For more, see the section on the threat from NEOs.
To find orbital details of NEOs, go to either of the following sites:
The animation below is taken from the Animations Page on the Minor Planet Center's website. In this animation, created on 17 April 2011, the motion of all objects in the inner region of the solar system are shown at one day intervals:
- The Sun is the yellow dot at the centre
- The orbits of Mercury, Venus, Earth and Mars shown as light blue ellipses, with the planets themselves as crossed circles (the Earth is the dark blue one)
- NEOs are indicated by red dots
- Main belt asteroids are indicated by green dots (note how these are mostly beyond Mars orbit)
Click on the animation to see it in full size, or click here to see more animations at the Minor Planet Center website, including out to Jupiter orbit, as well as from the viewpoint of the Earth.
Discovering More and More NEOs
This graph here, from the E.A.R.N. (European Asteroid Research Node), shows the number of new NEOs discovered by year.
Note that the graph has to be split into before and after 1990, due to the large increase in new detections in the '90s. The United States Congress mandated that NASA find and catalogue 90% of NEOs 1km or larger by the year 2008, an initiative known as Spaceguard, following which NASA in particular invested significantly into asteroid search programs.
This explains the increase from around 40 new NEOs per year in the mid 1990s to around 900 per year for the last few years.
This image, taken from the JPL NEO Program, shows the number of NEAs discovered, broken down by size. The graph shows the number of NEAs smaller than 30m in diameter, between 30m and 100m, between 100m and 300m, between 300m and 1000m, and larger than 1000m (as at 17 April 2012). Note that the smaller NEOs are harder to detect because they are small, and so the observations, and hence the NEOs that we know about, are biased towards larger NEOs.
The graph below shows the currently known NEO population by size, as well as the predicted number of NEOs (taken from the NASA report Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters, available here).
The graph shows the effect of this so called observation bias. Almost all NEOs larger than 1km have been discovered, however there are still many more smaller NEOs to be found, as shown by the red line, which is an approximation calculated in the NASA NEO Science Definition Team Report available here.
Keeping Track of the NEOs
When new NEOs are discovered, they are submitted to the Minor Planet Center (MPC) in Cambridge, Massachusetts, USA. The MPC performs preliminary calculations to determine for example if the new discovery is in fact new.
The NEO information is then passed to both the JPL NEO Program in the USA and NEODyS in Europe. These two organisations then use their highly specialised, mostly automated computer processes to analyse the orbits in detail and to determine for example if there is an impact risk with the Earth.
Detailed information on the orbits of any NEO can be found on these two sites.