Key Facts & Summary
- Near-Earth objects are usually less than 1.3 AU when they are at the perihelion point.
- There are well over 200.000 known near-Earth asteroids, and over a hundred short-period near-Earth comets.
- Such objects were created through collisions and had a significant role in the shape and forming of Earth.
- Some large asteroids have penetrated Earth’s atmosphere and caused various events such as tsunamis or huge craters.
- All NEOs have a low surface gravity, while many of them have Earth-like orbits, making them easy targets for spacecraft.
- Since 1998 many nations started to track and scan NEO’s, they are called the Spaceguard.
- More than ten near-Earth objects have already been visited by spacecraft.
- Private companies have drafted many preliminary plans for commercial asteroid mining. These objects could indeed be a great source of materials.
- NEAs are divided into groups based on their semi-major axis, perihelion, and aphelion distance.
- NEAs do not have the tail or coma of a comet.
- Many of them are eliminated by planetary perturbations, causing ejection from the Solar System or a collision with the Sun or a planet.
- Many NEAs survive in their orbits for millions of years. Though in astronomical terms, it is a short lifespan.
- Since these objects are shortlived, they are surely renewed.
- A small percentage of NEAs are extinct comets that have lost their volatile surface materials.
- Most NEAs are driven out of the asteroid belt by gravitational interactions with Jupiter.
- Many of these NEAs even have natural satellites.
There are more than 19.363 near-Earth asteroids currently known. More than 1.955 are potentially hazardous and have short lifespans. Such objects are continuously monitored since they pose different levels of threat.
Not only can these objects be a danger to us, but they can, on the other hand, provide us both with resources and knowledge of our little world.
Origin / Formation
Since NEAs have short lifespans, just a couple of millions of years, they surely must be renewed somehow. The leading states that NEAs are moved into the inner Solar System through orbital resonances with Jupiter.
This interaction perturbs the asteroid’s orbit, and it comes into the inner Solar System. The asteroid belt has some gaps, known as Kirkwood gaps, where these resonances occur.
New asteroids migrate into these resonances due to the Yarkovsky effect that provides a continuous supply of near-Earth asteroids. The asteroid belt is thus losing mass, totaling less than 6% over the past 3.5 billion years.
NEAs come in many different sizes; however, they tend not to be so big in comparison to other celestial objects. Very few of them have had their diameter measured accurately.
One of the largest, 3122 Florence, has a diameter of 4.5 km / 2.8 mi, and it even has two moons of around 100 – 300 / 330 – 980 ft across, as companions.
Predicting the exact number and sizes of NEAs has been a controversial topic for a long time. The latest reports from 2019 suggest that around 1.000 such objects have diameters of approximately 1 km / 0.6 mi, and more than 8.452 known NEAs are larger than 140 m / 459 ft in diameter.
One of the smallest NEA is 2008 TS26; it has an absolute magnitude of 33.2, which equates to a diameter of around 1 m / 3.3 ft. On the other hand, one of the largest NEAs, 1036 Ganymed, has an absolute magnitude of 9.45 and a diameter of around 38 km / 24 mi.
All NEAs are divided into groups based on their semi-major axis, perihelion, and aphelion distance. Four such classes exist:
Atiras or Apoheles – have orbits only inside that of Earth’s. Their aphelion distance is smaller than Earth’s perihelion distance, which is 0.9 AU. This implies that these asteroids have a semi-major axis less than 0.9 AU.
Atens – these asteroids have a semi-major axis less than 1 AU, and cross Earth’s orbit.
Apollos – This class is the opposite, having a semi-major axis of more than 1 AU and also cross Earth’s orbit.
Amors – They have orbits strictly outside that of Earth’s. Their perihelion distance is greater than Earth’s aphelion distance ( 1.017 AU), this implies that their semi-major axis is greater than 1.017 AU. Some of these asteroids even cross Mars’s orbit.
Some NEAs have a co-orbital configuration, which makes them have the same orbital period as our Earth. These co-orbital asteroids have unusual orbits that are relatively stable, and at the same time, it prevents them from getting close to Earth. Here are some examples:
Trojans – In the orbit of a planet, there are five points of equilibrium named the Lagrangian points, in which an asteroid would orbit the Sun in fixed formation with a planet. Two of these points, L4 and L5, are 60 degrees ahead and behind the planet along its orbit. In these points, asteroids could reside for millions of years, even if perturbed by other planets and non-gravitational forces.
Horseshoe Librators – Some asteroids run around both the L4 and L5 points, and if observed from Earth, their orbits resemble that of a horseshoe. Some may consist of annual loops that wander back and forth in a horseshoe-shaped area. Regardless of the case, the Sun is at the horseshoe’s center of gravity, Earth is in the gap of the horseshoe, and the L4 and L5 points are inside the ends of the horseshoe.
Quasi-satellites – They are asteroids on a regular elliptic orbit but with a higher eccentricity than Earth’s orbit. Thus they travel in a way synchronized with our planet. The asteroids orbit the Sun slower than Earth when they are further away, and faster than Earth when they are close to the Sun. When observed from Earth, they appear to be in retrograde motion.
Temporary Satellites – Some NEAs can change between solar orbits and distant Earth orbits, becoming gravitationally bound temporary satellites. These objects are typically caught when they pass the L1 or L2 Lagrangian points.
The smallest NEAs explode in the upper atmosphere with most or all of the solids vaporized, while larger NEAs usually hit the water surface, forming waves, or on a solid surface, forming impact craters.
These events are relatively common and usually not hazardous but very hard to predict. The asteroid impact prediction is quite a new domain which has successfully predicted future impacts, yet, these predictions were rare.
There are many missions planned for the visiting of near-Earth asteroids. Some of them were intended as early as 1971. Currently, a sample taken from an NEA is expected to be delivered in December 2020 by the JAXA’s Hayabusa mission.
Another spacecraft, OSIRIS-Rex, is also expected to return samples from some Trojans by 2023. There is also the Near-Earth Object Surveillance Mission or NEOSM – an effort to characterize the orbit of most of the potentially hazardous asteroids larger than 140 m / 460 ft. This mission is planned to be launched by 2025.
Did you know?
- The first NEA to be visited by a spacecraft was 433 Eros; this occurred in 2001.
- As of 2019, 36 Atiras, 1,510 Atens, 10,199 Apollos, and 8,583 Amors have been discovered and cataloged.
- More than 19,470 NEOs have been discovered. Only 107 (0.55%) of them are comets, whilst 19,363 (99.45%) are asteroids. There are 1,955 NEOs that are classified as potentially hazardous asteroids.
- 1566 Icarus, is an asteroid with an extremely eccentric orbit, the first to be observed by radar being a near-Earth object of the Apollo group.