What Is a Near-Earth Object?

Near-Earth objects (NEOs) are asteroids and comets whose orbits allow them to enter Earth’s neighborhood (defined as a perihelion distance of less than 1.3 Astronomical Units), according to the definition in the “George E. Brown Jr. Near-Earth Object Survey Act” (“NEO Survey Act”) passed by the U.S. Congress as Public Law No:109-155 on December 30, 2005. (1-3)

Meteorite Crater, Arizona. Source: http://news.nationalgeographic.com/news/2005/03/0310_050310_meteorcrater.html; accessed June 13, 2007

In the NEO Survey Act, “Congress directed that the National Aeronautics and Space Administration [NASA] initiate a Near-Earth Object (NEO) Survey program to detect, track, catalogue, and characterize objects larger than 140 meters [about 420 feet] in diameter, with a perihelion distance of less than 1.3 Astronomical Units from the Earth…The survey would warn of and inform attempts to mitigate the hazard.” (3,4)

The NEO Survey Act stated four reasons for its need (Statement of Need), as follows:

1. “Near-Earth objects pose a serious and credible threat to humankind, as many scientists believe that a major asteroid or comet was responsible for the mass extinction of the majority of the Earth’s species, including the dinosaurs, nearly 65,000,000 years ago.
2. Similar objects have struck the Earth or passed through the Earth’s atmosphere several times in the Earth’s history and pose a similar threat in the future.
3. Several such near-Earth objects have only been discovered within days of the objects’ closest approach to Earth, and recent discoveries of such large objects indicate that many large near-Earth objects remain undiscovered.
4. The efforts taken to date by NASA for detecting and characterizing the hazards of near-Earth objects are not sufficient to fully determine the threat posed by such objects to cause widespread destruction and loss of life.” (5)

Congress set a deadline for the Survey program to achieve 90 percent completion of its near-Earth object catalogue by 2020. (3)

I. Near Earth Object Groups: Asteroids and Comets

NASA workers have divided NEOs into two main groups: near-Earth comets (NECs) and near-Earth asteroids (NEAs).

Asteroid. Source: http://news.nationalgeographic.com/kids/2003/12/images/asteroid-big.jpg; accessed June 13, 2007.	Comet. Source: http://www.astro.cornell.edu/academics/courses/astro201/images/comet_west.gif; accessed June 13, 2007.

Asteroids and comets that have a potential to eventually impact the Earth are called “potentially hazardous objects”, or PHOs. “A PHO is an object in our solar system that passes within 0.05 AU (about 7.5 million kilometers) of Earth’s orbit and is large enough to pass through Earth’s atmosphere and cause significant damage on impact; that is, about 50 meters and larger. In [NASA’s Report to Congress, March 2007] the term PHO [is] used to indicate potential threats, with the understanding that those smaller than 1 kilometer (0.6 mile) are predominantly asteroids. Approximately 21% of the NEOs [near-Earth objects] of any given size class are expected to be potentially hazardous.” (6)

A. Near-Earth Asteroid Group

The near-Earth asteroid group has five subdivisions, including Atens, Apollos, Amors, Interior-Earth objects (IEOs) and potentially hazardous asteroids, also known as PHAs (not to be confused with PHOs, see above).

• Atens are near-Earth asteroids whose orbits cross Earth’s orbit with a period of less than one year. They spend most of their time inside Earth’s orbit. They comprise about 6% of known asteroids. (2)
• Apollos are near-Earth asteroids whose orbits cross Earth’s orbit with a period of more than one year. They comprise about 62% of known asteroids. (2)
• Amors are near-Earth asteroids whose orbits exist completely outside Earth’s orbit. They comprise about 32% of known asteroids. (2)
• Interior-Earth objects (IEOs) are asteroids whose orbits exist completely within Earth’s orbit. There are six (6) known asteroids in this subgroup. (2)
• Potentially hazardous asteroids (PHAs) are asteroids currently defined based on parameters that measure the asteroid’s potential to make threatening close approaches to the Earth. (7) “Specifically, all asteroids with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. [A MOID is also defined as the closest possible distance between the orbits of two objects. (7) ] In other words, asteroids that can't get any closer to the Earth (i.e., MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 miles) or are smaller than about 150 meters (500 feet) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs.” (8)

There are currently 868 known potentially-hazardous asteroids. NASA explains: “This ‘potential’' to make close Earth approaches does not mean a PHA will impact the Earth. It only means there is a possibility for such a threat. By monitoring these PHAs and updating their orbits as new observations become available, we can better predict the close-approach statistics and thus their Earth-impact threat.” (8)

B. Near-Earth Comet Group

NASA restricts the near-Earth comet group to include only short-period comets, that is, comets with an orbital period of less than 200 years, as described elsewhere. (9) Extinct comets may make up 5-15% of the NEO population. (10)

Comets have an enhanced signature (volatile vaporization near the Sun that we can see from Earth) when they come within 1.3 AU of the Sun. Thus, NASA scientists believe they have found the majority of short-period comets. (11) The total number of near-Earth comets is unknown, but it is estimated to be smaller than 1% of the near-Earth asteroids. (11)

II. Impact Frequencies and Typical Consequences

The chance that an NEO of 140 meters or larger will strike the Earth in any given year is about one time every 5,000 years. “The random nature of the hazard means that it is equally probably that a 140-meter object will hit the Earth in the next 50 years (~1%) or that the Earth will experience no impacts of that size in the next 23,000 years (0.9998 to the 23,000 power ~ 1%). The occurrence or absence of past events has no influence on the likelihood of future impacts.” (7)

Table 2: Impact Frequencies and Typical Consequences (7)

III. Mitigation Alternatives for NEO Deflection

A wide range of techniques existsto divert a threatening object, including “impulsive mitigation alternatives” (they act nearly instantaneously) or “slow push mitigation alternatives” (they act over an extended time). (12)

A. Impulsive mitigation techniques include

• Conventional explosive (surface) that detonates on impact with a PHO.
• Conventional explosive (subsurface) that drives into a PHO and then detonates.
• Nuclear explosive (standoff) that detonates on flyby via a proximity fuse.
• Nuclear explosive (surface) that makes impact and then detonates via a contact fuse.
• Nuclear explosive (delayed) that lands on the surface of a PHO and then detonates at the optimal time.
• Nuclear explosive (subsurface) that drives into a PHO and then detonates.
• Kinetic impact, described as “high velocity impact”. (13)

B. Slow-push mitigation techniques include

• “Focused Solar: Use a large mirror to focus solar energy on a spot, heat surface, ‘boil off’ material.
• Pulsed Laser: Rendezvous, position spacecraft near PHO and focus laser on surface, material ‘boiled off’ surface provides small force.
• Mass Driver: Rendezvous, land, attach, mine material and eject material from PHO at high velocity.
• Gravity Tractor: Rendezvous with PHO and fly in close proximity for extended period, gravitational attraction provides small force.
• Asteroid Tug: Rendezvous with PHO, attach to PHO, push.
• Enhanced Yarkovsky effect: Change albedo of a rotating PHO; radiation from sun-heated material will provide small force as body rotates.” (13)

In the impulsive category, the use of a nuclear device is the most effective means to deflect a PHO. In general, the slow push systems are at very low technology readiness levels and will require significant development efforts, says NASA. (13)

IV. Scenarios

NASA has developed six hypothetical scenarios to illustrate how NASA could apply the alternative deflection techniques if an NEO approaches Earth. NASA notes: “The inclusion of actual objects in these scenarios was chosen not because they represent actual impact threats, but because they are both publicly known and are representative of classes of potential threats.” (14)

Diagram of Apophis asteroid’s orbit in relation to Earth’s orbit. Source: http://www.msnbc.msn.com/id/12859900; accessed June 13, 2007.

The six scenarios are

1. The 330-meter asteroid, Apophis, before its close approach to Earth in 2029.
2. Apophis after its close approach and before the 2036 Earth encounter, assuming a predicted collision.
3. The 500-meter asteroid (VD17) that could be a threat in the year 2102.
4. A hypothetical 200-meter asteroid, representative of 100-meter-class asteroids.
5. A hypothetical asteroid larger than one kilometer in diameter.
6. A hypothetical long-period comet with a very short time (9-24 months) to impact.

V. Summary

Immanuel Velikovsky started something with his book Worlds in Collision (1950), which many leading astronomers viciously attacked for decades after its publication. (15) The science and effort now devoted to cataloguing and describing near-Earth objects will benefit defense against these things should they cross our orbit and further our understanding of primitive bodies (e.g., what processes led to the formation of these objects and how did primitive bodies make planets?). In addition, the NEOs may provide metal and other resources for use on Earth. Not too far in the future is a lucky astronaut’s ride on an NEO. NASA is currently developing a new launch system, the Ares I and V launch vehicles, and a new crew exploration vehicle, the Orion, for surmounting or otherwise getting near an NEO. (16)

Notes:

1. One AU is about 150 million kilometers, or the mean distance between the Sun and Earth.
2. National Aeronautics and Space Administration: “Near-Earth Object Survey and Deflection Analysis of Alternatives. Report to Congress.” March 2007, p. 5. Available online at: www.nasa.gov/pdf/171331main_NEO_report_march07.pdf; accessed June 13, 2007.
3. S. 1281 [109th]: National Aeronautics and Space Administration Authorization Act of 2005, Section 321. Available online at: http://www.govtrack.us/congress/billtext.xpd?bill=s109-1281; accessed June 13, 2007.
4. NASA: “2006 Near-Earth Object Survey and Deflection Study: Final Report.” December 2006, p. Errata-0-2. Available online at: http://www.b612foundation.org/papers/NASA-finalrpt.pdf; accessed June 13, 2007.
5. Ibid, p. 20.
6. National Aeronautics and Space Administration: “Near-Earth Object Survey and Deflection Analysis of Alternatives. Report to Congress.” March 2007, p. 7. Available online at: www.nasa.gov/pdf/171331main_NEO_report_march07.pdf; accessed June 13, 2007.
7. NASA: “2006 Near-Earth Object Survey and Deflection Study: Final Report.” December 2006, p. 25. Available online at: http://www.b612foundation.org/papers/NASA-finalrpt.pdf; accessed June 13, 2007
8. NASA: “Near Earth Object Program: NEO Groups”, available at http://neo.jpl.nasa.gov/neo/groups.html; accessed June 13, 2007.
9. See SEMP Biot Report #433: “What is a Comet?” (June 11, 2007) at: http://www.semp.us/publications/biot_reader.php?BiotID=432&Letter=; accessed June 13, 2007.
10. NASA: “2006 Near-Earth Object Survey and Deflection Study: Final Report.” December 2006, p. 23. Available online at: http://www.b612foundation.org/papers/NASA-finalrpt.pdf; accessed June 13, 2007
11. Ibid, p. 24.
12. National Aeronautics and Space Administration: “Near-Earth Object Survey and Deflection Analysis of Alternatives. Report to Congress.” March 2007, p. 19. Available online at: www.nasa.gov/pdf/171331main_NEO_report_march07.pdf; accessed June 13, 2007.
13. Ibid, p. 20.
14. Ibid, pp. 22-23.
15. See SEMP Biot Report #432: “Worlds in Collision: Velikovsky on Earth’s Catastrophic History”. June 13, 2007. Available at:
    http://www.semp.us/publications/biot_reader.php?BiotID=432&Letter=; accessed June 13, 2007.
16. National Aeronautics and Space Administration: “Near-Earth Object Survey and Deflection Analysis of Alternatives. Report to Congress.” March 2007, p. 26. Available online at: www.nasa.gov/pdf/171331main_NEO_report_march07.pdf; accessed June 13, 2007.