A near-Earth object (NEO) is a Sol System object whose orbit brings it into proximity with the Earth. All NEOs have a closest approach to the Sun (perihelion) of less than 1.3 AU. They include a few thousand near-Earth asteroids (NEAs), near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the planet. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose to the Earth, and active mitigations are being researched.
Those NEOs that are asteroids (NEA) have orbits that lie partly between 0.983 and 1.3 astronomical units away from the Sun. When an NEA is detected it is submitted to the IAU's Minor Planet Center (located at the Harvard-Smithsonian Center for Astrophysics) for cataloging. Some near-Earth asteroids' orbits intersect that of Earth's so they pose a collision danger. The United States, European Union and other nations are currently scanning for NEOs in an effort called Spaceguard.
In the United States, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be catastrophic. As of August 2012, there had been 848 near-Earth asteroids larger than 1 km discovered, of which 154 are potentially hazardous asteroids (PHAs). It was estimated in 2006 that 20% of the mandated objects have not yet been found. As a result of NEOWISE in 2011, it is estimated that 93% of the NEAs larger than 1 km have been found and that only about 70 remain to be discovered.
Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth. Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU (7,500,000 km; 4,600,000 mi), or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%), are not considered PHOs. The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in lunar distances, and this usage has become a common unit of measure used by the news media in discussing these objects.
Some NEOs are of high interest because they can be physically explored with lower mission velocity even than the Moon, due to their combination of low velocity with respect to Earth (ΔV) and small gravity, so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation. This makes them an attractive target for exploration. As of 2012, three near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe,[ 25143 Itokawa, by the JAXA Hayabusa mission, and 4179 Toutatis, by CNSA's Chang'e 2 spacecraft.
1 History of human awareness of NEOs
1.1.1 Risk scales
1.1.2 Highly rated risks
1.1.3 List of current threats
1.2 History of NEO science and exploratory mission proposals
2 Number and classification of near-Earth objects
2.1 Near-Earth asteroids
2.2 Near-Earth comets
3 Impact rate
4 Historic impacts
4.1 1908 Tunguska event
4.2 1979 Vela Incident
4.3 2002 Eastern Mediterranean event
4.4 2008 Sudan event
4.5 2009 Indonesia event
4.6 2013 Chelyabinsk meteor
5 Close approaches
6 Future impacts
7 Projects to minimize the threat
8 See also
10 External links
History of human awareness of NEOs
Human perception of near-Earth objects as benign objects of fascination or killer objects with high risk to human society have ebbed and flowed in the short period of human history that NEOS have been scientifically observed.
More recently, a typical frame of reference for looking at NEOS has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society. "NEOs have been understood differently throughout history." Each time an NEO is observed, "a different risk was posed, and throughout time that risk perception has evolved. It is not just a matter of scientific knowledge."
Such perception of risk is thus "a product of religious belief, philosophic principles, scientific understanding, technological capabilities, and even economical resourcefulness."
There are two schemes for the scientific classification of impact hazards from NEOs:
the simple Torino Scale, and
the more complex Palermo Technical Impact Hazard Scale
The annual background frequency used in the Palermo scale for impacts of energy greater than E megatonnes is estimated as:
For instance, this formula implies that the expected value of the time from now until the next impact greater than 1 megatonne is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of E.
However, another paper published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:
This formula gives considerably lower rates for a given E. For instance, it gives the rate for bolides of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.
Highly rated risks
On 24 December 2004, minor planet 99942 Apophis (at the time known by its provisional designation 2004 MN4) was assigned a 4 on the Torino scale, the highest rating ever achieved. There was a 2.7% chance of Earth impact on 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but future potential impact solutions were still rated 1 on the Torino scale. The 2036 risk was lowered to a Torino rating of 0 in August 2006. The Palermo rating is −3.2.
The only known NEO with a Palermo scale value greater than zero is (29075) 1950 DA, which may pass very close to or collide with the Earth (probability ≤ 0.003) in the year 2880. Depending on the orientation of its axis of rotation, it will either miss the Earth by tens of millions of kilometers, or have a 1 in 300 chance of hitting the Earth. However, humanity has over 800 years to refine the orbit of (29075) 1950 DA, and to deflect it, if necessary.
List of current threats
NASA maintains a continuously updated Sentry Risk Table of the most significant NEO threats in the next 100 years. All or nearly all of the objects are highly likely to eventually drop off the list as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions. (The list does not include 1950 DA, because that will not strike for at least 800 years.)
History of NEO science and exploratory mission proposals
In a 2013 article in Wired Science, David Portree provides an overview of NEO science and proposed asteroidal missions, with an emphasis on the outcome of two conferences held in the 1970s. The International Astronomical Union minor planets workshop was held in Tucson, Arizona in March 1971 and a consensus "emerged that launching spacecraft to asteroids would be 'premature'." "In January 1978, NASA’s Office of Space Science held a workshop at the University of Chicago to "assess the state of asteroid studies and consider options for the future."
Of all of the near-Earth asteroids (NEA) that had been discovered by mid-1977, it was estimated that spacecraft could rendezvous with and return from only about one in 10 using less propulsive energy than is necessary to reach Mars. "Because even the most massive NEA—35 kilometres (22 mi)-wide 1036 Ganymed, discovered in 1924, has a very low surface gravity—landing and takeoff would need very little energy. This meant that a single spacecraft could sample multiple sites on any given NEA." Overall, it was estimated that about one percent of all NEAs might provide opportunities for human-crewed missions, or no more than about ten known NEAs. Therefore, unless the NEA discovery rate were "immediately increased five-fold, no opportunity to launch 'astronaut-scientists' to an NEA was likely to occur within a decade of the Chicago workshop."
Number and classification of near-Earth objects
NEAs with (H) < 16 discovered since 2008:
2008 EJ1 15.8
(243298) 2008 EN82 15.6
2009 NE 16.0
2009 UV18 16.0
2011 UL21 15.8
2012 SF51 15.5
2012 US136 15.7
2013 GJ35 15.7
Near-Earth objects are classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets create meteoroid streams that can generate meteor showers.
As of February 2013, 9,683 NEOs have been discovered: 93 near-Earth comets and 9,590 near-Earth Asteroids. Of those there are 751 Aten asteroids, 3,613 Amor asteroids, and 5,214 Apollo asteroids. There are 1,360 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 155 PHAs and 861 NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.
As of February 2013, there are 430 NEAs on the Sentry impact risk page at the NASA website. A significant number of these NEAs – 215 as of May 2010 – are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "yellow zone" (Torino Scale 2), meaning that none warrant the attention of general public. As of January 2013, only asteroid 2007 VK184 is listed as having a Torino Scale of 1. The JPL Small-Body Database lists 1,518 near Earth asteroids with an absolute magnitude (H) dimmer than 25 (roughly 50 meters in diameter).
Near-Earth asteroids smaller than ~1 meter are near-Earth meteoroids and are listed as asteroids on most asteroid tables. The smallest known near-Earth meteoroid is 2008 TS26 with an absolute magnitude of 33 and estimated size of only 1 meter.
Near Earth Asteroids by Size
Cumulative Discoveries of Near Earth Asteroids Known by Size, 1980–2013
There are significantly fewer near-Earth asteroids in the mid-size range than previously thought.
These are objects in a near-Earth orbit without the tail or coma of a comet. As of May 2012, 8,880 near-Earth asteroids are known, ranging in size from 1 meter up to ~32 kilometers (1036 Ganymed). The number of near-Earth asteroids over one kilometer in diameter is estimated to be about 981. The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.
NEAs survive in their orbits for just a few million years. They are eventually eliminated by planetary perturbations which cause ejection from the Solar System or a collision with the Sun or a planet. With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that asteroid-belt asteroids are moved into the inner Solar System through orbital resonances with Jupiter. The interaction with Jupiter through the resonance perturbs the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as Kirkwood gaps, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the Yarkovsky effect that provides a continuing supply of near-Earth asteroids.
A small number of NEOs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.
Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q):
The Atiras or Apohele asteroids have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, Q < 0.983 AU. (This implies that the asteroid's semi-major axis is also less than 0.983 AU.)
The Atens have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, a < 1.0 AU and Q > 0.983 AU.
The Apollos have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, a > 1.0 AU and q < 1.017 AU. (1.017 AU is Earth's aphelion distance.)
The Amors have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so q < 1.3 AU. In summary, 1.017 AU < q < 1.3 AU. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.
(Note: Some authors define the Atens group differently: they define it as being all the asteroids with a semi-major axis of less than 1 AU. That is, they consider the Atiras to be part of the Atens. Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.)
Because the Atens and all Apollos have orbits that cross Earth's orbit, they might impact the Earth. Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.