ASTEROID BASICS
What is an asteroid?
An asteroid is a small, solid object in our solar system, orbiting the Sun. The asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the solar system's formation. The vast majority of the asteroids are found within the asteroid belt, with elliptical orbits between those of Mars and Jupiter.
What is the shape of an asteroid?
Asteroid geometries are extremely variable with some asteroids being singular solid bodies of rock while other asteroids are composed of many smaller asteroids orbiting in proximity as a rubble pile. It is estimated that 15% of asteroids are rubble piles. These piles are usually greater than 150 m in diameter while asteroids smaller than this in diameter are often cohesive rock formations. A diameter greater than 200 m is generally structurally weak, meaning it is loosely consolidated and porous. The larger size signifies a slower rotation rate as otherwise the asteroid will not remain consolidated. One study states that no asteroid greater than 200 m in diameter has a rotational period less than 2.3 hours.
What is an asteroid composed of?
Although the exact compositions of many asteroids are not known, a classification system for asteroids based on the albedo, or the fraction of electromagnetic energy a body reflects, is used to identify potential compositions. The albedo is strongly correlated with the surface composition of an asteroid but does not guarantee that the composition is uniform throughout. A table of some of the main asteroid types is included below. A complete list can be found here.
Type of Asteroid
Composition Albedo % S-type stony, silicon 15% C-type carbon 2-5% M-type metallic 10% What is an Apollo asteroid?
An Apollo asteroid is a type of Near Earth Asteroid (NEA) which crosses Earth's orbit with a period greater than 1 year. Near Earth Asteroids are those asteroids with orbits that bring them within 1.3 AU of the Sun (1 AU = Distance from Sun to Earth).
Have asteroids impacted Earth before?
Absolutely! The evidence can be found in the impact craters of Earth. It is believed that an asteroid was responsible for creating the conditions that led to the extinction of the dinosaurs. Ironically, although asteroids may be a threat to life, it is believed that life may not have been able to exist without them. In an infantile solar system, impacting asteroids brought some of the basic elemental building blocks of life to Earth.
How probable is an Earth impact?
Over a large enough time frame and without human interference, it is almost a statistical certainty that an asteroid will impact Earth. The US military has detected many upper atmosphere impacts of asteroids using satellites that are designed to detect nuclear explosions.
What would happen if an impact occurred?
The effect of an impact varies greatly with the size and composition of the asteroid as well as the location of impact. For asteroid 2004WR, we have calculated that the impact speed would be approximately 8 km/sec. A worst case calculation of the impact energy, assuming a solid, dense asteroid, is of the order of 1017 J transferred from the asteroid to Earth. This the equivalent of 10,000 Hiroshima bombs or 0.1 of the estimated energy released in the 2004 Indian Ocean Earthquake.
How are asteroids detected?
Today many asteroids are detected by Charged Couple Device (CCD) cameras that scan the night sky. Computers subsequently process the images for potential asteroids and notify the operator of likely asteroids. More information about the speed and direction of the asteroid can be gleaned using radio astronomy.
What is currently being done to prevent Earth impacts?
NASA and JPL’s Near Earth Asteroid Tracking (NEAT) program makes extensive use of CCD cameras to identify potential threats and track the orbits of asteroids. The main sites used by NEAT are the Haleakala station in Maui and Palomar Observatory in California. NEAT has been successful in detecting thousands of asteroids since its inception in 1995. Despite these successes, NEAT is limited in the amount of time and resources it can dedicate to scanning the vast Solar System.
Can I see an asteroid impact?
Although witnessing an impact event itself is unlikely without the aid of a telescope, there are instances when impacts on other planets can be predicted and witnessed. The comet Shoemaker Levy-9 impacted in 1994 and was witnessed by many with a telescope.
However you can view the effect of asteroid impacts by simply looking at the cratered surface of Moon!
