Quite obviously, we call “near-Earth objects” (NEOs) those asteroids and comets coming close enough to our Earth. Saying “enough” is not enough. We have to fix some thresholds. Asteroids are the dominant component in the big NEO family and we indicate them as NEAs, “near-Earth asteroids”.

Near-Earth objects: groups (credit: JPL/NASA)

Near-Earth objects: groups (credit: JPL/NASA)

The plot above shows the different NEOs groups, using some parameters: the semimajor axis is referred to the elliptic orbit of the body, while perihelion and apheion are the minimum and maximum distance of the object from the Sun, respectively. AU is the Astronomical Unit, the mean distance between the Sun and the Earth, which is of ~149.6 millions of km.

The table below, from JPL/Nasa, adds some details (a= semimajor axis, q=perihelion distance in AU, Q=aphelion distance in AU, P=orbital period in years)

Group Description Definition
NECs Near-Earth Comets q<1.3 AU, P<200 years
NEAs Near-Earth Asteroids q<1.3 AU
Atiras NEAs whose orbits are contained entirely with the orbit of the Earth
(named after asteroid 163693 Atira).
a<1.0 AU, Q<0.983 AU
Atens Earth-crossing NEAs with semi-major axes smaller than Earth’s
(named after asteroid 2062 Aten).
a<1.0 AU, Q>0.983 AU
Apollos Earth-crossing NEAs with semi-major axes larger than Earth’s
(named after asteroid 1862 Apollo).
a>1.0 AU, q<1.017 AU
Amors Earth-approaching NEAs with orbits exterior to Earth’s but interior
to Mars’ (named after asteroid 1221 Amor).
a>1.0 AU, 1.017<q<1.3 AU
PHAs Potentially Hazardous Asteriods: NEAs whose Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less and whose absolute magnitude (H) is 22.0 or brighter. MOID<=0.05 AU, H<=22.0

A quick general view is useful to better understand the NEA topic.

We know more than 600.000 asteroids, 50% of them have a well determined orbit), we recognize about 10.000 bodies approaching our planet within a given threshold (moving along orbits bringing them  between 0.983 and 1.3 Astronomical Units from the Sun; an Astronomical Unit equals to the mean distance of the Earth from the Sun, about 149.6 millions of km): they are called near-Earth Asteroids (NEAs). Among them, we call potentially hazardous asteroids (PHAs) those on orbits approaching the one of the Earth within 7,500,000 of km and with a diameter larger than about 100 meters. To date (11 Mar. 2014), we know almost 900 NEAs with a diameter larger than 1 km and a total of almost 1458 PHAs, 155 of them larger than 1 km.

The chart below shows the cumulative total known near-Earth asteroids versus time. The blue area shows all near-Earth asteroids while the red area shows only large near-Earth asteroids (those with diameters roughly one kilometer and larger).

The chart below shows the cumulative total known near-Earth asteroids versus time. The blue area shows all near-Earth asteroids while the red area shows only large near-Earth asteroids (those with diameters roughly one kilometer and larger).

As an obvious consequence of their orbits, NEAs can reach the proximity of Earth, sometimes posing a risk of collision and this is why we reserve a special attention to them. Handling these issues is a delicate matter, involving advanced mathematical and statistical considerations and we end with a impact probability and an impact risk for everyone of those objects. Of course, saying that an asteroid has a given probability to impact does not mean it will collide with us: probability is an expression of our uncertain knowledge of a given orbit and it evolves as the orbit itself becomes better and better determined.