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The footprint of asteroid E ros


M anhattan


> Im pacting the Earth. An asteroid im pacting the Earth som e 4 9 , 000 y ear ago scarred the Earth leaving this 1. 2-K m [ 0. 7 -m ile] crater. This view from the Space Shuttle show s the dram atic ex pression of the crater in the arid landscape of Arizona, USA. ( Im age courtesy of the Earth Sciences and Im age Analy sis Lab oratory , NASA J ohnson Space Center, STS04 0_ STS04 0-614 -5 8. )


amount of material— putting all of the asteroids together would form a body about 1,5 00 km [ 930 miles] in diameter, Earth’s moon.1 7


roughly half the size of Not all asteroids are far away in the asteroid


belt. Some, called near-Earth asteroids (NEAs), have orbits that bring them close to Earth. Astronomers believe NEAs to be fragments ejected from the main asteroid belt by asteroid- asteroid collisions or by gravitational perturba- tions from Jupiter. Some NEAs could also be the nuclei of dead, short-period comets. Since many asteroids have historically struck Earth and its moon, understanding their composition and origin may be a key to our past as well as our future. Scientists believe that the chemical building blocks of life and much of Earth’s water may have arrived on asteroids or comets that bombarded the planet in the early stages of its development (above left). One widely accepted theory suggests that an asteroid measuring at least 10 km [ 6 miles] across, impacted the Earth some 65 million years ago, causing mass extinctions among many life forms, including the dinosaurs.


Astronomers suspect that the approximately


800 NEAs found to date represent only a small percentage of their total population. The largest presently known is 1036 Ganymede, with an approximate diameter of 41 km [ 25 .5 miles] . NEAs with diameters greater than 1 km


[ 0.6 miles] are known as potentially hazardous asteroids,


suggesting that should they strike Earth, they could threaten life as we know it.


Of the more than 700 known potentially hazardous asteroids, one of the largest is Toutatis, an asteroid that is nearly 1.6 km


[ 1 mile] long and orbits around the Sun within one-half degree of Earth’s orbital plane. In December 1992, Toutatis passed within 0.024 astronomical units (AU), or 9.4 lunar distances from Earth.1 8


Then, on September 29,


2004, Toutatis’s orbital path brought it within 0.01 AU of Earth— the closest approach of any large asteroid in the 20th century.


Although astronomers have known about asteroids for nearly 200 years, until recently, their basic properties, their relationship to meteorites found on Earth and their origins remained a mystery. NASA and the scientific community, driven by both the desire to understand asteroids and the threat to Earth presented by NEAs more than 1 km in diameter, set in motion the plans for the NEAR project.


A Mission of Many Firsts


In 1990, NASA introduced a new program of planetary missions called the Discovery program. By 1991, the first mission was chosen— a rendezvous with near-Earth asteroid 433 Eros. The Johns Hopkins University Applied Physics Laboratory (JHUAPL) was chosen to manage the project, and in 1995 , the NEAR spacecraft was shipped to the Kennedy Space Center in Florida.1 9


> A large asteroid. The outline of Eros ( red) is superim posed on the island of Manhattan, New City , show ing the relative size of the asteroid.


Y ork


Discovered in 1898, the NEA Eros is one of the largest and best-observed asteroids.2 0


With


dimensions 33 by 13 by 13 km [ 21 by 8 by 8 miles] , Eros is about the size of Manhattan, New York, USA (above). It accounts for nearly half of the volume of all near-Earth asteroids.


15 . The term b inary star refers to a doub le-star sy stem , or a union of tw o stars into one sy stem b ased on the law s of attraction. Any tw o closely spaced stars m ight appear from Earth to b e a doub le-star pair w hen, in fact, they are a foreground and b ack ground star pair w idely separated in space. These sy stem s are ty pically referred to as optical b inaries.


16. NASA– Eros or Bust: http: / / science. nasa. gov/ headlines/ y 2000/ ast08feb _ 1. htm ( accessed April 14 , 2006) .


17 . NASA, reference 16.


18. NASA/ J et Propulsion Lab oratory – Asteroid 4 17 9 Toutatis: http: / / echo. j pl. nasa. gov/ asteroids/ 4 17 9 _ Toutatis/ toutatis. htm l ( accessed April 14 , 2006) .


An astronom ical unit ( AU) is eq uivalent to the distance from the Earth to the Sun, or approx im ately 14 9 , 000, 000 k m [ 9 2, 5 00, 000 m iles] .


19 . The NEAR spacecraft w as renam ed NEAR– Shoem ak er to honor planetary geologist Eugene Shoem ak er ( 19 28– 19 9 7 ) .


20. Farq uhar RW: “ NEAR Shoem ak er at Eros: Mission Director’ s Introduction, ” J ohns Hopk ins APL Technical Digest 23 , no. 1 ( 2002) : 3 – 5 .


Spring 2006


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