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11. Solar System Extras
Halley Shows Comets are Periodic
In 1705, British mathematician, Edmund Halley, using historical
reports from 1456, 1531, 1607, and 1682, predicted the return of a large comet in 1758. This
established a new class of objects in the Solar System.
Piazzi Discovers Ceres
Based on a mathematical pattern, known as the Titius-Bode Law,
astronomers began a search for a planet between Mars and Jupiter. Unrelated to this, in 1801, Italian
Giuseppe Piazzi, while cataloging stars, discovered a moving object—Ceres, classified as a planet. After
the discovery of Pallas (1802), Juno (1804), and Vesta (1807), all these bodies were “demoted” from
being called planets to being asteroids, or Minor Planets.
Tombaugh Discovers Pluto After years of work, comparing pairs of photographs at the Lowell
Observatory in Flagstaff, Arizona, Clyde Tombaugh discovered a ninth planet, Pluto, in 1930.
Oort Proposes a “Cloud” of Comets
In 1950, Jan Oort of the Netherlands postulated a distant group
of comets, now referred to as the Oort Cloud.
Kuiper Proposes a Belt of Minor Planets
In 1951, Dutchman Gerard Kuiper, working in the US,
proposed a large group of solar system objects located out past Neptune. This Kuiper Belt includes
Eris, Pluto, Haumea, Makemake, etc.
Pluto is Re-Classified
In 2006, after the discovery of Eris (a Kuiper Belt object larger than Pluto),
the International Astronomic Union, formalized the definition of “planet,” and created a new classification,
“dwarf planet,” which includes Pluto.
Meteors
A meteor is a quick flash of light produced when a piece of space rock falls through our
atmosphere. The light is not produced from the rock “burning up.” The meteor moves so fast that it
compresses and super-heats the air, making the air glow.
Comet
A comet is a “dirty snowball” of mostly frozen water and gasses, traveling in an elliptical orbit
around the Sun. When a comet approaches the inner Solar System, its frozen gasses are heated, and
released, forming a tail. A comet does not produce its own light. It shines by reflecting light from the
Sun. A comet can be seen for days or weeks, as it slowly moves in its orbit.
Asteroids
Asteroids are largish, mostly rocky objects in orbit around the Sun. Only the very largest
can be seen (barely) without a telescope, looking like a distant planet. We recognize them by their
motion relative to the stars.
Asteroids: Location & Formation
Most asteroids are located in the asteroid belt, orbiting between
Mars and Jupiter, in the same plane as the planets. Other locations include: Jupiter’s orbit (Trojan
asteroids) and Earth-crossing orbits (Apollo asteroids). The asteroids have not joined together into a
small planet, because they are continually perturbed by Jupiter’s gravity.
Asteroid Composition
Asteroid material is mostly rock and/or metals, left unchanged since the
formation of the Solar System. The largest asteroids (1 Ceres, 2 Pallas, 3 Juno) are roughly spherical,
but most asteroids are irregularly shaped. Some asteroids are apparently loose jumbles of material.
Comet: Major Parts
Nucleus—central, solid part; 1-10 km across; composed of frozen water, carbon
dioxide, carbon monoxide, and dust. Coma—cloud of expanding gas surrounding the nucleus; ten times
size of the Earth (more or less). Tail—long stream of gas and dust, pushed away by the solar wind; up
to 1 AU long.
Comets: Tale of Two Tails
Comets often produce two tails: Gas (Ion) Tail—gases emitted from the
comet are pushed away by the solar wind; ultraviolet light from the Sun can cause the material to
fluoresce blue. Dust Tail—small dust particles are influenced by both gravity and the solar wind, so the
dust tail doesn’t point directly away from the Sun
Halley’s Comet
First comet discovered to be periodic (returning). Will next return in 2061
Shoemaker-Levy 9
1994, Broke into pieces and fragments struck Jupiter
Comet Orbits and The Oort Cloud
Comets have very elliptical orbits, moving slowly when far from
the Sun, and quickly when near the Sun. Comet orbits are not all oriented in the same plane as the
planets—they can come from any direction. Long period comets spend most of their time in orbits far
from the Sun—a region called the Oort cloud, as far out as 100,000 AU. Many comets are believed to
have originated in the inner Solar System, but were then gravitationally flung outward by the planets.
Disappearing Comets Produce Meteor Showers
Each time a short-period comet passes near the
Sun, it loses mass—up to 10%. Over time, this produces a band of small particles (a meteoroid swarm)
along the comet’s orbit. If the Earth passes through this swarm of particles, we see many meteors—a
meteor shower.
Meteorites
If a large meteor survives the plunge through the atmosphere and strikes the ground it’s
called a meteorite. Prior to this, while still above our atmosphere (orbiting the Sun), the rock is called a
meteoroid. Collecting meteorites allows us to study material that was originally part of: the early solar
system, other planets, asteroids, and comets.
Recognizing Meteorites
Some meteorites are primarily iron-nickel. But most meteorites have a rocky
composition, while still containing some elemental iron. They are attracted to magnets. Chemical
analysis reveals the age and composition of meteorites, including nickel which is diagnostic. Fresh
meteorites have a black fusion crust.
Famous Meteorite Impacts
50,000 years ago, the 300,000 ton (50 m across) Canyon Diablo
meteorite created the Barringer Crater (Meteor Crater) in Arizona—almost a mile wide.
Famous Meteorite Impacts
In 1947, in Eastern Siberia, a bolide was observed—the Sikhote Alin
meteorite. A painter captured the sight. 28 tons. In 1954, a 4-kg meteorite smashed through the roof
of a house in Alabama, destroyed a radio cabinet, and struck Ann Hodges—the only recorded human
struck by a meteorite. The Willamette Meteorite (15 tons, discovered in Oregon), at view at the New
York Museum of Natural History. The world’s largest meteorite, at Hoba, Namibia.
What happens to meteor material that doesn’t survive the atmosphere?
Small meteors are
destroyed by their descent through the atmosphere. The material from these meteors rains down over
the entire Earth. Molten material can coalesce into cosmic spherules.
Are large impacts a danger?
We have overwhelming evidence that some past mass extinctions have
been caused by celestial impacts. Iridium between rock layers is irrefutable.
Do we have a program ready to avert a catastrophic impact, when we identify a collision-bound
object?
No. But this might be the single most important thing that we can do to preserve human
society.
Pluto: Basic Characteristics
Pluto is only visible with a telescope—it looks like a faint star. Size: one
fifth the size of Earth. Composition: ice and frozen methane; Temperature: 50 K. Axial Tilt: 118°.
Rotational Period: - 6.4 days. Moons: five (so far). Largest moon, Charon, has orbital period 6.4 days.
Pluto and Charon are tidally locked in 1:1 spin-orbit resonance. Considered a binary pair.
Pluto: Orbital Data
Distance from Sun: 39 AU. Orbital period: 250 years 3:2 orbital resonance with
Neptune. Perhaps saved from getting pushed away, or captured, by its resonance with Neptune.
Kuiper Belt
At distances ranging from 30 AU to 50 AU, over a thousand Kuiper Belt objects have
been identified.
Why isn’t Pluto classified as a planet anymore?
For 60 years, Pluto was the only known object in
the far Solar System. Now, we have observed 200 objects with the same orbital radius as Pluto. With
the discovery of Eris, which is larger than Pluto, a formal definition of “planet” was developed that reclassified Pluto as a dwarf planet. The IAU definition: A “planet” is a celestial body that: (a) is in orbit
around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around
its orbit.
Spacecraft Exploration
NASA’s ICE (International Comet Explorer) passed through the tail of Comet
Giacobini-Zinner in 1985. European Space Agency’s Giotto Mission. Launched in 1985, it made close-up
observations of Halley’s Comet in 1986 and Comet P/Grigg-Skjellerup in 1992.
Launched in 1989, the Galileo Spacecraft, on its way to Jupiter, captured close-up images of asteroids
Gaspra and Ida. Launched in 1996, the Near Earth Asteroid Rendezvous - Shoemaker (NEAR
Shoemaker), passed near asteroid 253 Mathilde and established orbit around 433 Eros. In 2001,
NEAR Shoemaker “landed” on 433 Eros. Launched in 2003, the Japanese Hayabusa mission goal is to
return a sample of Comet Itokawa. The craft made collection contact with the asteroid, and will return
with possible samples in 2011. The miniature Minerva hopping lander failed to make contact with the
surface. Deep Impact July 4, 2005, Projectile smacked into comet Tempel 1 to study interior. NASA’s
Stardust mission, Collected material from the tail of Comet Wild-2 and returned to Earth in 2006.
Launched 2007, NASA’s Dawn mission is scheduled to explore Vesta between 2011 and 2012, and
Ceres in 2015. One is on its way to Pluto- New Horizons lifted off in January 2006, Fastest probe ever
(Traveling 10 miles per second), Will reach Pluto in 2015, Fly by and continue toward other Kuiper Belt
objects.
Questions to be Answered:
•
How do meteors, comets, asteroids, and Kuiper belt objects look in the night sky? Do they
produce light?
•
What are the differences between meteors, asteroids, comets, and Kuiper Belt (Trans-Neptunian)
Objects?
•
What is the composition and orbits of comets, and how do comets produce meteor showers?
•
What is the composition, origin, and value of meteorites?
•
What are asteroids, where are they found, and what is their composition?
•
What is the origin of the asteroids and why haven’t they formed into a planet?
•
Do asteroids pose a threat, and what can we do about it?
•
What are the major characteristics of Pluto and the other Kuiper Belt Objects?
•
Why is Pluto no longer classified as a planet?