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Asteroids are rocky and metallic
objects that orbit the Sun but are too
small to be considered planets. They
are known as minor planets.
Asteroids range in size from Ceres,
which has a diameter of about 1000
km, down to the size of pebbles.
Sixteen asteroids have a diameter of
240 km or greater.
They have been found inside Earth's
orbit to beyond Saturn's orbit. Most,
however, are contained within a main
belt that exists between the orbits of
Mars and Jupiter. Some have orbits
that cross Earth's path and some have
even hit the Earth in times past.
Near Earth Asteroids (NEAs)
Asteroids with orbits that bring them within 1.3 AU (121 million miles/195
million kilometers) of the Sun are known as Earth-approaching or near-Earth
asteroids (NEAs). It is believed that most NEAs are fragments jarred from the
main belt by a combination of asteroid collisions and the gravitational
influence of Jupiter. Some NEAs may be the nuclei of dead, short-period
comets. The NEA population appears to be representative of most or all
asteroid types found in the main belt.
NEAs are grouped into three categories, named for famous members of each:
1221 Amor, 1862 Apollo, and 2062 Aten.
Amors: Asteroids which cross Mars' orbit but do not quite reach the orbit of
Earth. Eros -- target of the NEAR mission -- is a typical Amor.
Apollos: Asteroids which cross Earth's orbit with a period greater than 1 year.
Geographos represents the Apollos.
Atens: Asteroids which cross Earth's orbit with a period less than 1 year. RaShalom is a typical Aten.
In 2004, it was reported that the asteroid 2004 MN4 (later named 99942 Apophis) had as much as a 1
in 37 chance of impacting Earth on 13 April 2029. The asteroid is about 1,300 ft (400 m) across and
its impact would cause local or regional devastation. Further study of the orbit of Apophis found that it
will miss Earth in 2029 but could strike in 2036. Still more research concluded that Apophis poses
little danger of impacting Earth in the 21st century, but the object should be regularly monitored.
Asteroids are material left over
from the formation of the solar
One early hypothesis
suggested that they were the
remains of a planet that was
destroyed in a massive collision
long ago. Given the variety of
asteroids, a single parent body
is highly unlikely.
More likely, asteroids are
material that never coalesced
into a planet. In fact, if the
estimated total mass of all
asteroids was gathered into a
single object, the object would
be less than 1,500 kilometers
(932 miles) across -- less than
half the diameter of our Moon.
The first 10 asteroids profiled against the Earth's Moon. From
left to right, 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 5 Astraea, 6
Hebe, 7 Iris, 8 Flora, 9 Metis, and 10 Hygiea.
Asteroid Classification
Asteroids are classified into a number of types according to their spectra (and hence
their chemical composition) and albedo:
C-type (and rarer B-, F-, and G-types)
• more than 75% of known asteroids
• extremely dark (albedo 0.03)
• similar to carbonaceous chondrite meteorites?
• 15-20% of known asteroids
• relatively bright (albedo .10-.22)
• metallic nickel-iron mixed with iron- and magnesium-silicates
• similar to stony-iron meteorites and ordinary chondrites?
• most of the rest
• bright (albedo .10-.18)
• nickel-iron
• similar to iron meteorites?
D- and P-type
•Outer edge of main belt, Trojans, and Jupiter’s small moons
•Very dark
•Ultra-primitive organic compounds
Some Notable Asteroids
1 Ceres - The largest and first discovered asteroid, by G.
Piazzi on January 1, 1801. Ceres comprises over onethird the 2.3 x 1021 kg estimated total mass of all the
2 Pallas - The 2nd largest asteroid and second asteroid
discovered, by H. Olbers in 1802.
3 Juno - The 3rd asteroid discovered, by K. Harding in
4 Vesta - The 3rd largest asteroid, Vesta appears to have
a basaltic crust overlying an olivine mantle, indicating
differentiation has occurred. Imaged by the Hubble Space
Telescope in 1995.
Much of our understanding about
asteroids comes from examining
meteorites and observing spectral
data from Earth-based telescopes.
In 1991 asteroid 951 Gaspra was
visited by the Galileo spacecraft and
became the first asteroid to have hiresolution images taken of it. In 1993
Galileo made a close encounter with
asteroid 243 Ida. Both Gaspra and
Ida are classified as S-type asteroids
composed of metal-rich silicates.
In 1997 the spacecraft NEAR made
a high-speed close encounter with
asteroid 253 Mathilde. This
encounter gave scientists the first
close-up look of a carbon rich C-type
asteroid. NEAR continued on to
encouter, orbit, and touch down on
asteroid Eros.
Reflectance Spectra
Reflection & Emission
Main Belt
Primitive versus Differentiated Asteroids
Based on heating by decay of aluminum-26,
a short-lived radionuclide.
951 Gaspra
orbit: 330,000,000 km from the Sun (average) size: 19x12x11 km
Gaspra is an S-type asteroid, believed
to be composed of a mixture of rocky
and metallic minerals.
Gaspra was encountered Oct 29, 1991 by the Galileo
spacecraft on its way to Jupiter
Ida was encountered Aug.
28, 1993, by the Galileo
spacecraft on its way to
243 Ida and Dactyl
Dactyl (right of Ida) is about
1.6 x 1.2 km, surprisingly
round for such a small body. It
orbits Ida at approximately 90
orbit: 428,000,000 km from the Sun (average) size: 58x23 km
Ida was originally thought to be an S-type asteroid, like Gaspra,
composed of nickel-iron and some silicates. But a density of 2.9 is too
low for that. Instead, Ida could well have a composition like that of
ordinary chondrite meteorites, which are primitive and largely unaltered.
253 Mathilde
orbit: 394,000,000 km from the Sun (average) size: 59 x 47 km
As dark as soot, carbonaceous asteroid 253
Mathilde is rich in carbon compounds and other
dark substances common in the outer region of
the Main Asteroid Belt. Mathilde is like a
loose rubble pile with the density of water.
The spacecraft NEAR made a flyby of Mathilde on 27 June 1997
433 Eros
orbit: 172,800,000 km from the Sun (average) size: 33x13x13 km
Eros is an S-type asteroid and was the main target of the NEAR Mission.
NEAR Spacecraft
Asteroids Mathilde and Ida, shown at their correct relative brightness.
MUSES-C (Hayabusa)
Asteroid Sample Return Mission
MUSES-C was launched by the M-V launch vehicle into a transfer orbit
toward asteroid Itokawa in May 2003, and it arrived at the asteroid in
September 2005. Landing took place on November 20, 2005.
After staying in close proximity to the asteroid for about 5 months and
performing scientific observations and sample collections, it departed from
the asteroid and is returning to earth in the summer of 2010.
MUSES-C Spacecraft
Sampling Technique
MINERVA (MIcro/Nano Experimental Robot Vehicle for Asteroid) is a small
robot lander whose weight is less than 600g. Although it is a tiny lander,
MINERVA can investigate the surface of ITOKAWA using three small color
CCD cameras.
Unbowed by Robot
Loss, Japan's
Asteroid Probe
Readies For
By Leonard David
Senior Space Writer
posted: 15 November 2005
02:41 pm ET
Despite a glitch in deploying a mini-robot onto asteroid Itokawa, Japanese space
officials plan to proceed in a milestone-making touchdown on the space rock to obtain
samples of the object for return to Earth.
The problems encountered Nov. 12 with the release by Japan’s Hayabusa space probe
of its camera-toting robot highlight the difficulty of this kind of mission. The robot probe,
called the MIcro/Nano Experimental Robot Vehicle for Asteroid (MINERVA), was lost
upon release from the mother ship Hayabusa.
MINERVA was successfully released, but the device appeared to start drifting away
from the asteroid's surface, according to a release from the Japan Aerospace
Exploration Agency (JAXA). The robot was expected to land and hop around on the
asteroid's surface collecting data with three small color cameras.
Launch 2007
Arrival at Vesta 2011
Arrival at Ceres 2015