What are asteroids?   Fragments of primordial bodies dating back to the formation of the Solar System.

Why detect, characterize, and monitor asteroids?  The primary objective is planetary defense:  Many have hit our planet in the past and more will hit it in the future.   Sixty-five million years ago, a ten kilometer wide asteroid or comet is thought to have precipitated the extinction of the non-avian dinosaurs and three fourths of all animal and  plant life during the late Cretaceous period.  The impact created a 100 mile-wide crater on what is now called the Yucatan peninsula on the Gulf of Mexico.

Where are the asteroids?  They are primarily in the asteroid belt.  This is between the orbits of Mars and Jupiter (2.1 - 3.2 AUs from the sun).  They vary in size from1 km - 457 km (Ceres is the largest).  Others move out towards Saturn or in towards the Earth.  See map of locations below.

What are Near Earth Objects (NEOs)? Asteroids and comets whose orbits come between 0.983 and 1.3 AUs from the Sun.  The Earth is 1 AU from the Sun.  

What are Near Earth Asteroids (NEAs)? Near Earth Asteroids (NEAs) include 3 groups: Aten (6%), Apollo (62%), Amor (32%), and Inner Earth Objects (6 known).

What are Potentially Hazardous Asteroids (PHAs)?  Asteroids that come within 0.05 AU of the Earth and are greater than 150 meters.  These could take out a country.  One that is 45 meters could wipe out a city and by this definition is NOT a PHA.

How are asteroids classified?   By their albedo (reflectivity) .

What is albedo?  Intrinsic brightness.  A white body is a perfect reflector and has an albedo of 1.0.  A black body with perfect absorption has an albedo of 0.0.

Left:  S-type asteroid, 951 Gaspra 

Image credit:  NASA's Galileo spacecraft 1991

Right:  S-type asteroid, 243 Ida

Image credit:  NASA's Galileo probe 1993

Should we lasso an asteroid and sling it into an orbit around the moon?  Astrophysicists Carl Sagan (deceased) formerly of Cornell University and Steven J. Ostro from JPL in Pasadena, California raise concerns over a catastrophic collision with Earth resulting from the altering of the asteroids path in space or trajectory.   

Mining Asteroids:   There has been recent commercial interest in mining valuable resources from asteroids such as iron, water, nickel, and platinum.   Space travel costs need to be significantly reduced if this is to become a worthwhile investment.   

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Six orbital elements describing an space object's trajectory

Image credit: Orbit1.svg Source: Lasunncty

Left:  C-type asteroid, 253 Mathilde 

Image credit:  NASA NEAR Shoemaker spacecraft

Right:  C-type asteroid, 2005-YU55 Radar Image 11/7/2011 at 3.6 lunar distances from Earth

Image credit:  NASA/JPL-Caltech Goldstone

Left:  M-type asteroid, 16 Psyche

Image credit:  NASA

Right:  M-type asteroid, 216 Kleopatra

Image credit:  Arecibo Observatory

Left:  G-type asteroid, Ceres cutaway

Image credit:  NASA, ESA, and A. Feild (STScI)

Right:  G-type asteroid, Ceres

Image credit:  NASA Hubble Space Telescope 1/24/2004

Asteroid Locations: Main belt, Trojans, Hildas, Greeks

Image Credit:  en.wikipedia

Trajectory of Russian Chelyabinsk Meteor and 2012 DA14
Image Credit: NASA / MSFC / Meteroid Environment Office
Asteroid Locations: Apollo, Amor, and Atens
Near Earth Asteroids
Image Credit: ESA
Left: Orbit parameters
Right: Recent encounters
Asteroid Locations
Left: S-types
Right: G-type
Left: C-types
Right: M-types

What are the main classifications of asteroids?  C, M, S, G, and numerous rarer types.

  • C-types (carbonaceous type) are made of carbon and complex organic (carbon containing) compounds.  They are usually dark in color and located in the outer part of the asteroid belt.  They are the most common.  They may contain water ice.  They are very dark with albedos (reflectivity) at around 0.03 - 0.09. They account for more than 75% of known asteroids. An example is 10 Hygiea, the largest C-type and 4th largest (by volume and mass) asteroid in the Solar System.  The 253 Mathilde shown below is 50 km in diameter.  2005 YU55 is also shown below and is about 360 meters in diameter.                                            

  • M-types (formerly called metallic types) were once thought to be composed of pure nickel and iron.  However, some are non-metallic like Lutetia and some are both metallic and non-metallic.  To avoid composition issues, some define them by a reflected spectrum.  Thermal emissions such as those measured by the Infrared Astronomical Satellites (IRAS) are used to verify membership to this type.  Lutetia for instance has an albedo measurement of 0.22 and is thought to be hydrated (non-metallic).     M-types are located in the middle of the asteroid belt.  They are relatively bright with albedos at around 0.10 - 0.18.  About 8% of the asteroids are this type.  An example is the Psyche asteroid which is about 253 km in diameter and appears to be iron-nickel in composition.  One source estimates that it contains 170 million trillion tons of nickel-iron.  (Kit Eaton article )  216 Kleopatra is another metallic asteroid and is about 217 km.  16 Psyche and 216 Kleopatra are shown below.

  • S-types (silicaceous type) are made of silicate rocks with small amounts of iron.  They are lighter in color (slightly red) and located mainly in the inner part of the asteroid belt.  They have high albedos (reflectivity) at around 0.10 - 0.22.  They account for about 17% of known asteroids.  951Gaspra and 243 Ida are S-types and are shown below.

  • G-types are relatively uncommon types of carbonaceous asteroids.  They are similar to C-types but exhibit a strong UV activity below 0.5 microns (one millionth of a meter).  Ceres, the largest asteroid is often classified as a G-type but sometimes as a C-type.  In 2006 Ceres as designated a dwarf planet but is still designated the largest known asteroid.   Ceres is a rock-ice body and is about 950 km in diameter.  It was the first asteroid identified (Giuseppe Piazzi in 1801).    It may have an ocean of water-ice in its core.  NASA's $466 million spacecraft DAWN is expected to reach Ceres in February 2015.

How big are asteroids?   According to the  Non Luminary World Classification System (NoLWoCS), an asteroid is 50 meters to 50 km in diameter while a Planetoid is 50km  - 100 km.  However, the terms asteroids and planetoids are often used interchangeably.   Ceres is 457 km and is the largest asteroid (planetoid) in the asteroid belt. 

How to we calculate rotation?  Measuring the changes in their brightness enables the calculation of their rotation.  Some make one full rotation in 3 days, others in 30 days.  When available, Doppler radar can measure the rotation rate almost instantly.

How to we calculate orbits? Ground-based measurements include the angles to the object (azimuth and elevation) and the range and/or range-rate, Earth's rotation, Earth's movement around the Sun, and observer's position.  Multiple time-tagged measurements are needed.  Other methods don't require range data to calculate the orbits.  See Lambert's and Gauss' methods for more information.  The orbital elements can then be calculated.  They are eccentricity, semimajor axis, inclination, longitude of the ascending node, argument of periapsis, and mean anomaly at epoch.

How large an impact is needed to cause a continental fire?  A global fire? The Journal of Geophysical Research (8/20/2004) contained an article by Doctors Durda and Kring indicating that an impact which creates an 85 km crater can produce a continental-scale fire; 135 km for a global-scale fire.  Recall from above that it is estimated that a 10 km diameter asteroid created a 100 km crater precipitating the end of the dinosaur era.

What is the Asteroid Grand Challenge? A 2013 NASA initiative focused on detecting and characterizing asteroids and finding mitigation plans.

What is the 1998 Congressional Directive?  To identify 90% of all 1 km or larger Near-Earth Objects. (Clinton Administration)

Asteroids 101

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