The Near-Earth Object Program
Potentially Hazardous Asteroids and Comets

The Near-Earth Object Program
Potentially Hazardous Asteroids and Comets

The rate of impacts of objects of at least 1 km in diameter is estimated as 2 per million years. Assuming that this rate will continue for the next billion years, there exist at least 2,000 objects of diameter greater than 1 km that will eventually hit Earth.

However, most of these are not yet considered potentially hazardous objects because they are currently orbiting between Mars and Jupiter.

Eventually they will change orbits and become NEOs. Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or hitting a planet.

The purpose of the Near-Earth Object Program is to detect, track and characterize potentially hazardous asteroids and comets that could approach the Earth.

Asteroids and comets have been crashing into the Earth for as long as there's been an Earth.

Soon after it's formation about 4.5 billion years ago our planet became a frequent target in a cosmic shooting gallery.

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 expected to produce severe to catastrophic effects.

As of October 2008, 982 of these mandated NEOs had been detected. It was estimated in 2006 that 20% of the mandated objects have not yet been found.

As of April 2011, 7,954 NEOs have been discovered: 87 near-Earth comets and 7,867 near-Earth Asteroids. Of those there are 647 Aten asteroids, 2,920 Amor asteroids, and 4,289 Apollo asteroids.

There are 1,215 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 148 PHAs and 824 NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.

The Apollo asteroid 2007 TU24 approached Earth on January 29, 2008 with a distance of 450,000 km, with an estimated size between 300–600 meters. It may be the closest asteroid to pass Earth until 2027.

Efforts are under way to use an existing telescope in Australia to cover the ~30% of the sky that has not yet been surveyed.

Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.

The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in Lunar Distances, and this usage has become the more usual unit of measure used by the press and mainstream media in discussing these objects.

A study showed that the United States and China are
the nations most vulnerable to a meteor strike. 

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.

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 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 2008, two near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe, and 25143 Itokawa, by the JAXA Hayabusa mission.

Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood.

The scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the solar system formation process some 4.6 billion years ago.

The giant outer planets (Jupiter, Saturn, Uranus, and Neptune) formed from an agglomeration of billions of comets and the left over bits and pieces from this formation process are the comets we see today.

Likewise, today's asteroids are the bits and pieces left over from the initial agglomeration of the inner planets that include Mercury, Venus, Earth, and Mars.

Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter.

As of May 2010, 84 near-Earth comets have been discovered. Although no impact of a comet in earth history has been conclusively confirmed, the Tunguska event may have been caused by a fragment of Comet Encke.

Cometary fragmenting may also be responsible for some impacts from near-Earth objects. These near-Earth objects were probably derived from the Kuiper belt, beyond the orbit of Neptune.

The scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the solar system formation process some 4.6 billion years ago.

As the primitive, leftover building blocks of the solar system formation process, comets and asteroids offer clues to the chemical mixture from which the planets formed some 4.6 billion years ago. If we wish to know the composition of the primordial mixture from which the planets formed, then we must determine the chemical constituents of the leftover debris from this formation process - the comets and asteroids.

Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. (89959) 2002 NT7 was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale, with approximately one in a million on a potential impact date of February 1st, 2019.

Asteroid 29075 1950 DA is a near Earth asteroid.

It is notable for having the highest known probability of impacting Earth (although this probability remains low).

Asteroid (29075) 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting of its orbit, and then rediscovered on December 31st, 2000.

The chance it will impact Earth on March 16th, 2880 during its close approach has been estimated as 1 in 300. This chance of impact for such a large object is roughly 50% greater than that for all other such objects combined between now and 2880. It has a diameter of about a kilometer (0.6 miles).

However, over the intervening time, its rotation will cause its orbit to change (by the Yarkovsky effect). Available radar and optical data suggest two possible pole directions; one trajectory misses the Earth by tens of millions of kilometers, while the other has an impact probability of roughly 1 in 300.

The energy released by a collision with an object the size of 1950 DA would cause major effects on the climate and biosphere which would be devastating to human civilization. The discovery of the potential impact has heightened interest in asteroid deflection strategies.

Close Approaches to Earth
  • On August 10, 1972 a meteor that became known as The Great Daylight 1972 Fireball was witnessed by many people moving north over the Rocky Mountains from the U.S. Southwest to Canada.

    It was an Earth-grazing meteoroid that passed within 57 kilometers (about 34 miles) of the Earth's surface. It was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera.

    Analysis of its appearance and trajectory showed it was a meteoroid about 3 metres (9.8 ft) to 14 metres (46 ft) in diameter.

    If it had not entered at such a grazing angle, this meteoroid would have lost all its velocity in the upper atmosphere, possibly ending in an airburst, and any remnant would have fallen at terminal velocity.
  • On March 23, 1989 the 300-meter (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometers (400,000 miles) passing through the exact position where the Earth was only 6 hours before.

    If the asteroid had impacted it would have created the largest explosion in recorded history, thousands of times more powerful than the Tsar Bomba, the most powerful nuclear bomb ever exploded by man.

    It attracted widespread attention as early calculations had its passage being as close as 64,000 km (40,000 miles) from the Earth, with large uncertainties that allowed for the possibility of it striking the Earth.
  • On March 18, 2004, LINEAR announced a 30-meter asteroid, 2004 FH, which would pass the Earth that day at only 42,600 km (26,500 miles), about one-tenth the distance to the Moon, and the closest miss ever noticed. They estimated that similar sized asteroids come as close about every two years.
  • On March 31, 2004, two weeks after 2004 FH, meteoroid 2004 FU162 set a new record for closest recorded approach, passing Earth only 6,500 km (4,000 miles) away (about one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU162 was detected only hours before its closest approach. If it had collided with Earth, it probably would have harmlessly disintegrated in the atmosphere.
  • On March 2, 2009, near-Earth asteroid 2009 DD45 flew by Earth at about 13:40 UT. The estimated distance from Earth was 72,000 km (44,740 miles), approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters (115 feet) wide.
  • On January 13, 2010 at 12:46 UT, near-Earth asteroid 2010 AL30 passed at about 122,000 km (76,000 mi). It was approximately 10–15 m (33–49 ft) wide. If 2010 AL30 had entered the Earth's atmosphere, it would have created an air burst equivalent to between 50 kT and 100 kT (kilotons of TNT). The Hiroshima "Little Boy" atom bomb had a yield between 13-18kT.

  • On June 28, 2011 an asteroid designated 2011 MD, estimated at 5–20 m (16–66 ft) in diameter, passed within 20,000 km (12,000 mi) of the Earth, passing over the Atlantic Ocean.

Historic Impacts
  • 1908 Tunguska Event - It is now commonly believed that on 30 June 1908 a stony asteroid exploded over Tunguska with the energy of the explosion of 10 megatons of TNT. The explosion occurred at a height of 8.5 kilometers. The object that caused the explosion has been estimated to have had a diameter of 45–70 meters.
  • 1979 Vela Incident - A 22 September 1979 event recorded as occurring near the junction of the South Atlantic and the Indian Ocean was possibly a low-yield nuclear test, but was also initially thought to have been caused by the possible impact of an extraterrestrial object.

    The event, which became known as the Vela Incident, was identified by a U.S. Vela defence satellite in Earth orbit. The event alarm triggered multi-year investigations by several organizations which could not conclusively determine if the explosion was of nuclear or non-nuclear origin.
  • 2002 Eastern Mediterranean Event - On 6 June 2002 an object with an estimated diameter of 10 meters collided with Earth. The collision occurred over the Mediterranean Sea, between Greece and Libya, at approximately 34°N 21°E and the object exploded in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a small nuclear weapon.
  • 2008 Sudan Event - On 6 October 2008, scientists calculated that a small near-Earth asteroid, 2008 TC3, just sighted that night, should impact the Earth on 7 October over Sudan, at 0246 UTC, 5:46 local time. The asteroid arrived as predicted. This is the first time that an asteroid impact on Earth has been accurately predicted.

    However, no reports of the actual impact have so far been published since it occurred in a very sparsely populated area. A systematic search for fragments found a total of 600 fragments, with a mass of 10.5 kilograms. The object is confirmed to have entered Earth's atmosphere as a meteor above northern Sudan at a velocity of 12.8 kilometers per second (29,000 mph).

The Universe: End of the Earth

This series takes a fascinating new look at a very old universe. Fifty years after man first ventured into outer space, we examine the greatest secrets of the heavens.

Each episode outlines how humans have explored the universe, and scrutinizes the discoveries they have made.

We look at hi-tech space telescopes which record the violent birth of stars, robotic rovers which glimpse the red surface of Mars, and sophisticated NASA probes which delve into the mysterious make-up of comets.

As the earth churns ominously with the effects of global warming, this is a revealing and prescient journey into the heavens.

From the planets to the stars and out to the edge of the unknown, history and science collide in this epic exploration of the Universe and its mysteries. In ‘The End of the Earth’, our experts discuss how the earth has become an extremely dangerous place to live within the wider universe.

According to many cosmologists, it is only a matter of time before a cosmic force will annihilate the planet. At this very moment, NASA’s top brass and other scientists are arming themselves with the latest technology to pre-empt an apocalyptic attack. We investigate a number of bizarre, unexplained and terrifying ‘end of earth’ scenarios.

We meet scientists from around the world who are racing against the clock to develop technology to detect and defend our planet from apocalyptic demons. The prime suspects: asteroids, comets, gamma ray bursts, the sun and the Big Rip - a mysterious phenomenon called ‘dark energy’ which could eventually rip apart everything in the universe.