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Asteroids, Comets, and
Meteoroids
Asteroids are small,
rocky objects. The name
“asteroid” actually means
‘star-like bodies’.
The largest known
asteroid is Ceres.
Ceres is 940 km
across. There were
4000 known asteroids
by the mid-1990’s.
The total mass of all these
asteroids is less than 1/10
the mass of our Moon.
Asteroids generally have
very eccentric orbits
between the orbits
of Mars and Jupiter.
Asteroids were first
discovered when
astronomers were
looking for a planet
that fit the “TitiusBode” law.
Giuseppe Piazzi first
detected Ceres in 1801 at
an orbit 2.8 A.U.’s from the
Sun. This distance was
perfect for the expected
location of a planet by the
T-B law.
Soon thereafter, three more
asteroids were discovered:
Orbit
• Pallas
• Juno
• Vesta
Diameter
2.8 A.U.580 km
2.7 A.U.320 km
3.4 A.U.540 km
Most of the asteroids
are confined to the
asteroid belt,
between 2.1 and 3.3
A.U. from the Sun.
The asteroids may have
been a planet at one time,
or they may have never
formed a planet due to
the gravitational pulls
of Jupiter (probably
the latter).
Most asteroids always
stay between the orbits of
Mars and Jupiter. Some
stray inside Earth’s orbit,
the Apollo Asteroids.
The Amor Asteroids
cross Mars’ orbit.
In 1968, the asteroid
Icarus missed Earth by
6 million km. In 1989,
asteroid 1989FC passed
Earth at a distance of
800,000 km (2 times the
distance to the Moon).
In 1991, 1991BA
missed by less than
half the distance to
the Moon at
170,000 km.
The possibility of
a collision with an
asteroid is a real
concern; however,
there is not much
we could do about
such an occurrence.
Statistically, over a
million year period
there should be 3
Apollo asteroid
collisions.
Most Apollo
asteroids are
about 1 km in
size, but one
is 10 km.
Collision with even a
1 km asteroid would
be catastrophic.
It would devastate
an area 100 km
in diameter.
The impact would be
equal to one million
1-megaton nuclear bombs
(this is 100 times ALL the
nuclear weapons now on
Earth). It would also
produce a fatal blast wave.
Spaceguard is a systematic
search to try and predict
potential collisions with Apollo
asteroids. The entire staff of
Spaceguard is less than the
number of employees at the
local McDonalds.
Asteroid 3753 follows the
yellow path while also orbiting
the Sun. Each orbit the yellow
path shifts, taking 385 years
to complete one cycle.
Asteroid 3753’s orbit is
inclined to Earth’s, so
they can never collide.
There are families
of asteroids called
“Hirayama Families”.
Each family contains 5
to 10 members that all
have similar orbits.
They are probably the
remains of a single
asteroid that broke up
after a collision with
another asteroid. The
remnants then spread
out along the orbit.
There are also
binary asteroids:
two asteroids that
orbit each other in
addition to orbiting
the Sun.
Trojan Asteroids are
asteroids that orbit in
Jupiter’s orbit. They
are locked at LaGrange
points with Jupiter (60°)
as a result of the
synchronism of gravity
with the Sun and Jupiter.
The main asteroid belt has
gaps called “Kirkwood
Gaps”. These are
produced by gravitational
resonances with planets
(much as Saturn’s moons
produce gaps in the rings).
There are two basic
types of asteroids:
•C-type asteroids and
•S-type asteroids.
The C-type asteroids
contain lots of carbon
(“C” stands for
carbonaceous).
They are the least
reflective asteroids with
albedos around 0.05.
The S-type (“S” means
silicon) are made up
more of rocky materials.
Containing less carbon,
they are more reflective
with albedos of 0.15 to
0.25.
The asteroids at the edge
of the asteroid belt that is
closest to the Sun are
primarily S-type.
The asteroids transition
gradually to being more
C-type as you move out
further from the Sun.
The size of asteroids is
estimated by the amount
of light reflected and the
amount of heat radiated.
Rare stellar occultations
aid in determining size.
Larger asteroids are
roughly spherical,
while the smaller
ones can be highly
irregular in shape.
The Jupiter probe
Galileo passed through
the asteroid belt twice.
It passed close to
and took pictures of
Gaspra in 1991.
Gaspra is irregularly
shaped, has a
maximum diameter of
20 km, is pitted with
craters up to 2 km
across, and has a
regolith (dust) layer.
On Feb 14, 2000 the NEAR
spacecraft became the first
artificial satellite of an
asteroid when it moved into
orbit around the asteroid
Eros, a 20 mile long asteroid
with a 3 mile crater.
The NEAR spacecraft
arrived at Eros (the
goddess of love) on
February 14, 2000,
so here is your
valentine!!
Eros is a large, S-type near-earth
asteroid that is too small to have
enough gravity to form it into a
sphere. By comparison, Eros has
less than a thousandth Earth's
gravity, so a 100 pound object on
Earth would weigh about 1 ounce
on Eros. A baseball thrown at 22
miles per hour would completely
escape into space!
Other spacecraft
have been sent to
research asteroids
as seen in the next
slide.