ASTEROIDS AND COMETS
• Asteroids and comets constitute a vast number of minor objects
which revolve around the Sun, but differ from each other in their
primary locations and compositions.
• Asteroids are similar to the terrestrial planets in composition,
and are made up mostly of rock and metal.
• Asteroids inhabit the inner solar system, and are largely
concentrated in the “asteroid belt” between the orbits of Mars
and Jupiter.
• Collisions between asteroids are thought to be responsible for
most of the meteoroids which impact Earth at present.
• Comets are mainly concentrated in the region beyond the orbit
of Neptune, but can travel over highly eccentric orbits which
bring them into the inner solar system, where solar heating
causes their volatile materials to be released.
• Comets are similar to Pluto, Triton, and the outer planet
satellites in composition; they are made up mostly of water ice
and other frozen volatiles, mixed with rock and metal particles.
ASTEROIDS
• Asteroids range in size from below the limit of measurement by
ground-based telescopes, up to the largest and first-discovered,
named Ceres.
• Ceres has a diameter of about 940 km; the next two largest are
Pallas (580 km) and Vesta (540 km).
• Several asteroids have been observed at close range, by the
Galileo spacecraft on its way to Jupiter, and by the Near Earth
Asteroid Rendezvous (NEAR) mission.
• NEAR also accomplished the first detailed study of an asteroid,
Eros, from close-up orbits around Eros which were decreased in
altitude until the spacecraft actually landed on the asteroid’s
surface.
• Ground-based studies of asteroids indicate a variety of
compositions for these objects, to some extent dependent on
their distances from the Sun.
• Some asteroids have orbits that come closer to the Sun than
Earth’s orbit, hence pose a potential hazard of impact.
• It is believed that the impact of an asteroid with Earth 65 million
years ago was a major cause of the extinction of the dinosaurs.
The Asteroid Belt - January 1, 2001
Eccentric Asteroid Orbits
Spacecraft Observations of Asteroids
• The Galileo mission, primarily intended to fly by and go into orbit
around Jupiter, flew past, and obtained close-up imagery of, the
asteroids Gaspra and Ida, on its way to Jupiter.
• The observations of these asteroids were the first to be obtained
at close range by any space mission, and revealed (among
other things) a tiny satellite of the asteroid Ida.
• The Near-Earth Asteroid Rendezvous (NEAR) mission was
specifically intended for close-up studies of the asteroid Eros, in
much greater detail than possible with ground-based
astronomical telescopes.
• The NEAR mission went into orbit around Eros, and obtained
not only visible-light imagery, but also measurements of its
surface composition using infrared and gamma-ray
spectroscopy.
• At the end of its nominal mission, NEAR was landed on the
surface of Eros (an event the spacecraft was not designed or
originally intended for), and in the process, obtained detailed
imagery from very close to the surface.
The Asteroid Ida and Satellite, as seen by the Galileo Spacecraft
The Asteroid Mathilde, as seen by the Near Earth Asteroid
Rendezvous (NEAR) Spacecraft
Two Views of the Asteroid Eros, from NEAR
Closeup Views of Asteroid Eros Obtained by NEAR
Near Final Image of the Asteroid Eros from NEAR, Altitude 250 Meters
NEAR Near-Infrared Spectrum of Eros
NEAR Gamma-Ray Spectrum of Eros
COMETS
• The visual appearance of comets is quite different from other solar
system objects. They appear not as solid objects (like the inner
planets, satellites, and asteroids) nor as sharply defined gaseous
objects (like the giant outer planets and the Sun), but as diffuse
objects, non-uniform in brightness, and typically of elongated shape
(head and tail).
• The light emitted by comets is a combination of gaseous emission
and scattering of sunlight by dust particles.
• The nuclei of comets are similar to Pluto, Triton, and the outer planet
satellites in composition; they are made up mostly of water ice and
other frozen volatiles, mixed with rock, metal, and carbonaceous
particles.
• Comets are thought to have originated (and currently are mainly
concentrated) in the region beyond the orbit of Neptune known as
the Kuiper belt, or possibly in an even larger region of space (known
as the Oort Cloud), but can travel over highly eccentric orbits which
bring them into the inner solar system, where solar heating of their
nuclei causes their volatile materials to be released.
Comet Kohoutek, January 1974
Comet Hale-Bopp, March 1997
Structure and Typical Orbit of a Comet
Note, both the dust tail and the ionized gas tail normally point away from the Sun, because of the solar
radiation pressure (dust tail) and ionized solar wind (ion tail).
GROUND-BASED SPECTRA OF COMETS
GROUND-BASED SPECTRUM OF COMET BRADFIELD
Spectrum is Negative Image
ULTRAVIOLET SPECTRUM OF COMET WEST
NASA Goddard Space Flight Center
ULTRAVIOLET SPECTRA OF COMET WEST
Johns Hopkins University Sounding Rocket
Far Ultraviolet
Middle Ultraviolet
Comet Halley as Seen from Earth, 1986
Orbit of Comet Halley
Comet Halley Far Ultraviolet Camera and
Spectrograph Sounding Rocket Payload, 1986
29.5 s.
Far Ultraviolet Spectra of Comet Halley, 1986
Comet Shoemaker-Levy 9, Broken Up by Close
Encounter with Jupiter
Effects of Collision of Shoemaker-Levy 9 Fragments with
Jupiter, July 1994
Comet Borrelly and Nucleus Close-up, Observed by
Deep Space I Spacecraft
Nucleus of Comet Wild-2, Observed by Stardust
THE STARDUST SAMPLE-RETURN MISSION TO
COMET WILD-2
• The Stardust mission, launched February 7, 1999, and returned to
Earth January 15, 2006, is the first ever mission to bring back
preserved samples of cometary dust back to Earth.
• These samples will be studied in detail in Earth-based laboratories, to
determine their detailed composition and other properties.
• These will be the oldest samples ever of the materials that existed in
our solar system, since the comets are thought to have been created
at the same time as the major planets, about 4.6 billion years ago.
Aerogel Sample Collection Tray
Stardust Return Capsule – Pre-Launch Drop Test
Comet Tempel 1 Views from Deep Impact Probe
3 July 2005
The Deep Impact mission was the first to study the composition of a
comet, by observation of its internal as well as surface materials.
“Deep Impact” on Tempel 1
As Viewed From Fly-By Spacecraft
Post-Impact View of Comet Tempel 1, from Deep Impact
Flyby Spacecraft, Following Impact of Deep Impact Probe
Infrared Spectra of Comet Tempel-1, Post-Impact
View Prior to Impact
Other molecules tentatively detected include HCN, H2CO, and CH3OH.
METEOROIDS, METEORS, AND METEORITES
• Meteoroids are the smallest objects known in the solar
system, ranging in size from the smallest observable
asteroids to smaller than sand grains.
• Meteors are defined as the optically visible result of the
impact of meteoroids with Earth’s atmosphere, where
extremely high temperatures result from aerodynamic heating
of the meteoroid and surrounding air.
• Meteorites are the actual solid remnants of those meteoroids
which survive entry to Earth’s atmosphere and reach Earth’s
surface.
• Very large meteorites can impact Earth’s surface with
sufficient energy to produce craters or other significant
damage.
• The very largest craters, on Earth and other planets, are
believed due to impacts of asteroids or comets; these are
distinguished from meteoroids only in size.
METEOROIDS AND ASTEROIDS: THE CONNECTION
• Astronomers have long believed that most meteoroids are
simply the smallest members of the group of objects we call
asteroids.
• It has been proposed that meteoroids are also produced by
collisions between asteroids, which both produce these smaller
objects, and put them into more eccentric orbits which bring
them into the inner part of the solar system (including Earthcrossing orbits).
• Ground-based telescopic remote-sensing measurements of
asteroids have shown that asteroids have varying compositions,
generally similar to those of meteorites recovered on Earth, but
many questions remain unanswered.
• Some meteoroids are also believed to be fragments of comets,
particularly, those participating in periodic “meteor showers” that
are connected to the orbits of extinct comets.
TYPES OF METEOROIDS
• Meteorites (and their parent meteoroids) can be classified, in
accord with their compositions, into three major categories:
o Iron meteorites (iron-nickel compositions, similar to Earth’s core)
o Stony (chondritic) meteorites (similar in composition to Earth’s
mantle, Moon rocks, and the observed surfaces of main-belt
asteroids)
o Carbonaceous chondrites (some containing organic compounds)
• The first two types are thought to be representative of the main
asteroid belt objects; the iron meteoroids (in particular) are
thought to be the result of collisions between larger asteroids
having iron cores.
• The carbonaceous meteoroids are thought to be representative
of objects originating farther out in the solar system than the
stony and iron meteoroids, and some may result from the
breakup of comet nuclei.
• Carbonaceous meteorites are much rarer than stony or iron
meteorites; this may be largely due to the lesser ability of their
parent meteoroids to survive entry to Earth’s atmosphere.
Ultraviolet Spectrum of Meteor Entry to Earth’s Atmosphere
Observed from Earth Orbit
Examples of Meteorites
Chondritic Meteorite
Iron Meteorite
TYPES OF METEOROIDS
• In addition to the primary sources of meteoroids, there are also
some that result from collisions of asteroids or very large
meteoroids with our Moon or smaller planets (such as Mars).
• A number of meteorites collected on Earth, primarily in
Antarctica (where they are much easier to find than elsewhere,
due to the permanent ice cover) are known or strongly
suspected to be from the Moon or from Mars.
• The most famous of this latter type, the Allan Hills meteorite, is
confirmed (by detailed laboratory analyses) to have come from
Mars; more controversial is the hypothesis that some of the
features observed are fossils of microbial organisms (similar to
the unicellular life forms that came into existence on Earth more
than 3 billion years ago).
Meteorite From Mars, Found in Antarctica
Allan Hills Meteorite
Electron Microscopic View of
ALH Meteorite Sample
HAZARDS TO EARTH DUE TO METEORITES
AND ASTEROIDS
• Collisions of meteorites of sizes large enough to produce major
damage on Earth’s surface are very rare, and the probability of
such an encounter decreases rapidly with the size of the impactor.
• However, the historical record of impacts on Earth, that have not
been totally eroded by atmospheric and aquatic weathering,
verifies that major events HAVE occurred, in the relatively recent
(10%) of Earth’s history, that could result in major, or even total,
extinction of life on Earth.
• The most recent large crater in the continental United States, is
the Meteor Crater in Arizona, more than 1 kilometer in diameter,
which was created by a major impact about 50,000 years ago.
• A much larger event, which occurred about 65 million years ago,
was an asteroid impact at the northern edge of the (current period)
Yucatan Peninsula, that is held responsible for the extinction of
the dinosaurs.
• The asteroid involved in this impact is estimated to have been at
least 5 kilometers in diameter, and created a crater more than 100
kilometers in diameter.
Meteor Crater Formation Process
Meteor Crater, Arizona
Asteroid Crater, Manicouagan, Canada