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). Among 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. 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, 1522 are PHAs (source: JPL NEO Program on 20th of February 2015). 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:

Or click here to see animations of NEOs in the solar system 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 JPL NEO Program, shows the number of new Near Earth Asteroids (NEAs) discovered by year.

Total number of NEAs discovered. (Source: JPL NEO program)

Total number of NEAs discovered. (Source: JPL NEO program)

Note that the graph shows a 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. After this the mission was extended further and NASA now has the goal to find 90% of all NEOs larger than 140m. This explains the increase from around 40 new NEAs per year in the mid 1990s to more than a 1000 per year for the last few years.  What can also be seen is that although the total number of NEAs keeps increasing, the number of large NEAs does not increase that fast anymore; Large NEAs are easier to find and most of them have thus already been discovered by now.



The graph above 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). 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 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.Prediction of the number of NEOs


Keeping Track of the NEOs

New NEO discoveries

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.


The ESA SSA-NEO Coordination Centre has started to publish a monthly newsletter summarising the most relevant data and events on asteroids and comets approaching the orbit of the Earth. The newsletter wishes to provide information useful to experts, journalists and the public at large.

[Text credit: ESA] Here is the direct link to the ESA SSA-NEO website with the link to the newsletter PDF: ESA Newsletter.