potentially hazardous asteroid

A potentially hazardous asteroid is a near-Earth asteroid (NEA) whose orbit can bring it so close to Earth that there is a risk (typically very small) of a future collision. Past asteroid and comet impacts have had dramatic effects both on a local scale (as in the case Tunguska event) and globally (for example, the mass extinction at the Cretaceous-Tertiary boundary).

Scientists have adopted a very specific definition for the term "potentially hazardous asteroid," which is any asteroid with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less. In other words, an asteroid that can't get any closer to the Earth than 0.05 AU (roughly 7,480,000 km or 4,650,000 miles) or is smaller than about 150 m (500 ft) in diameter (i.e., H = 22.0 with assumed albedo of 13%) is not considered to be a PHA. There are currently more than a thousand known potentially hazardous asteroids.

Measuring the risk

Planetary scientists have developed a means of conveying the risks associated with asteroids and comets that might collide with the Earth. This risk-assessment scale, called the Torino scale, is similar to the Richter scale used for measuring earthquakes. The Torino scale runs from zero to 10. An object with a value of zero or one will have virtually no chance of causing damage on Earth; a 10 means a certain global climatic catastrophe.

Detection and monitoring of potentially hazardous asteroids

The growing realization that Earth will inevitably be hit hard again at some point, with potentially catastrophic results for human and other life, has led to the setting up of several programs to detect and monitor NEAs (or, more generally, NEOs – near-Earth objects – which also include short-period comets) and near-Earth threatening objects. Among these projects are the Spacewatch Project, LINEAR, the Spaceguard Foundation, and the Jet Propulsion Laboratory's NEO Program.

Recent close approaches

Among recent close encounters was one on June 14, 2002, when an asteroid the size of a football pitch, catalogued as 2002MN, passed Earth at a distance of around 120,000 km – less than a third the distance to the Moon – traveling at over 10 km/s (23,000 mph). This was bettered by 1994XM1, which came to within 105,000 km of Earth in December 1994.

The threat from Apophis

potentially hazardous asteroid approaching Earth

The current biggest threat of a collision with Earth is posed by the asteroid Aphophis. This 400-meter-wide space rock will come closer to Earth in 2029 than the orbits of many communications satellites – but it will not hit the planet. The concern centers on the small chance that its orbit could be altered enough during the flyby to put the rock on a collision path for its return in 2036. In 2006, the Planetary Society announced a $50,000 prize to be given to the designers of the mission that would allow the asteroid's orbit to be tracked with the most precision. The competition has support from the US and European space agencies. It was won in February 2008 by a team led by Atlanta-based SpaceWorks Engineering. The Foresight spacecraft, if it goes ahead, will launch in about 2012, rendezvous with Apophis, determine its center of mass, and take regular measurements of its position relative to the asteroid. These measurements will be used to reduce the uncertainties in Apophis' orbit.

TNT equivalent of asteroid impacts

Even a relatively small asteroid, with a diameter of 100 meters, could have a devastating effect on the Earth and its life because of its high incoming velocity. The kinetic energy (energy of motion) of an object is given by the formula

KE = ½mv²
where m is the mass of the object and v its velocity. If the velocity is high, then velocity squared is large and this makes the kinetic energy large if the mass is significant.

The table below shows the equivalent in megatons of TNT of the impact of asteroids of various masses and velocities, assuming a typival rock-like density for an asteroid of 2.5 kg/m³. For comparison, the yields of the Hiroshima and Nagasaki atomic bomb explosions were approximately 15,000 tons and 21,000 tons, respectively. In other words, the smallest impact considered in the table (that of a 100-meter-wide asteroid traveling at 15 km/s) would be roughly 100 times more powerful than the devastating Hiroshima-Nagasaki attacks.

TNT equivalent in megatons of asteroid impacts
diameter (meters)	mass (million tons)	velocity (km/s)
		15	20	25	30
100	1.3	35	62	97	140
200	10	280	500	780	1,100
300	35	950	1,700	2,600	3,800
400	84	2,200	4,000	6,200	9,000
600	280	7,600	13,000	21,000	30,000
800	670	18,000	32,000	50,000	72,000
1,000	1,300	35,000	62,000	97,000	140,000
2,000	10,000	280,000	500,000	780,000	1,100,000
3,000	35,000	950,000	1,700,000	2,600,000	3,800,000
4,000	84,000	2,200,000	4,000,000	6,200,000	9,000,000
5,000	160,000	4,400,000	7,800,000	12,000,000	18,000,000
6,000	280,000	7,600,000	13,000,000	21,000,000	30,000,000
7,000	450,000	12,000,000	21,000,000	33,000,000	48,000,000
8,000	670,000	18,000,000	32,000,000	50,000,000	72,000,000
9,000	950,000	26,000,000	45,000,000	71,000,000	100,000,000
10,000	1,100,000	35,000,000	62,000,000	97,000,000	140,000,000

Archived news

Asteroid makes close Earth pass (Jan 30, 2008)
Near-Earth asteroids buzz US government (May 24, 2005)

Related entries

• Torino scale
• cosmic collisions, biological effects

Related categories

• ASTEROIDS AND OTHER MINOR PLANETS: TYPES AND GROUPS
• MEGACATASTROPHES

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