The Search for the “Missing” Planet
• Bode’s Law relates the sizes of planetary orbits
• Astronomers noticed a
“missing” planet if this rule of thumb was correct.
To find the mean distances of the planets, beginning with the following simple sequence of numbers:
0 3 6 12 24 48 96 192 384
Add 4 to each number:
4 7 10 16 28 52 100 196 388
Then divide by 10:
0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 The resulting sequence is very close to the
distribution of mean distances of the planets from the Sun:
Body Actual distance (A.U.) Bode's Law Mercury 0.39 0.4 Venus 0.72 0.7 Earth 1.00 1.0 Mars 1.52 1.6 2.8 Jupiter 5.20 5.2 Saturn 9.54 10.0 Uranus 19.19 19.6
The Discovery of Ceres
• Discovered on 1 January 1801 by Giuseppe Piazzi – Semi-major axis = 2.7654 AU – Orbital period = 4.60 yr – Mass = 0.00015 Earths = 0.0128 Moons• Now classified as a minor planet, or asteroid.
• Also classified as a dwarf planet.
NASA's Hubble Space Telescope color image of Ceres, the largest object in the asteroid belt.
Diameter comparis on of the dwarf planet– asteroid Ceres with the Moon and Earth.
How do we find asteroids?
•
Asteroids appear to
move relative to
background stars
Path of 1 Ceres in late 2012
As an asteroid moves along its orbit it will produce an elongated trail in an image taken with a telescope following the background of "fixed" stars, which are not moving in this way.
The Asteroid Belt
Sizes and shapes of the largest asteroids, compared to the moon
Small, irregular objects, mostly in the apparent gap between the orbits of Mars and Jupiter.
Thousands of asteroids with accurately determined orbits known today.
Asteroids pose virtually no hazard to space navigation. The average
The Asteroid
Belt
This is where we find the majority of asteroids As seen from
above
The Asteroid Belt
•
Jupiter’s gravity likely
prevented the formation
of a planet between Mars
and Jupiter.
•
Most of the planetesimal
were “kicked” out of this
region, leaving the
objects that now make
up the asteroid belt.
•
Jupiter’s gravity also
shapes the asteroid belt
Kirkwood Gaps
• The distribution ofasteroids in the main belt is NOT uniform.
• Gaps form at specific regions where there are whole number ratios (1/3, 2/5, 3/7, etc.) of Jupiter’s orbital period.
• Mean motion resonances
Asteroids
Picture of the asteroid Gaspra from the Galileo spacecraft
Studying Asteroids
•
15 asteroids have by
visited by spacecraft
•
Radar can be used to
determine the
shapes of asteroids
that are too small to
resolve with
telescopes
•
Optical telesopes can
be used to infer an
asteroid’s shape and
rotation period
Motion of near-Earth object 2001 FE90 showing brightness variation over two rotations on
6/26/2009.
Computer model of asteroid (216) Kleopatra, based on radar analysis.
Non-Belt Asteroids
Not all asteroids orbit within the asteroid belt!
Asteroids with elliptical orbits, reaching into the inner solar system.
Some potentially colliding with Mars or Earth.
Trojans: Sharing stable orbits along the orbit of Jupiter: Trapped in the Lagrangian points of Jupiter.
Near Earth Objects
(NEOs)
•
Objects that cross
Mars’ orbit or orbit
entirely within the orbit
of Mars are called
near-Earth objects
.
•
These objects
occasionally pass very
close to Earth, and can
potentially pose a
threat.
Orbits of near Earth asteroids wider than .88 miles are in yellow; comets in blue.
NEOs
•
As of February 21, 2013,
9738
Near-Earth objects have
been discovered. Some
863
of these NEOs are asteroids
with a diameter of approximately 1 kilometer or larger.
Also,
1379
of these NEOs have been classified as Potentially
Hazardous Asteroids (PHAs).
Evidence of Impacts on Earth
Meteor Crater, Arizona
Pingualuit Crater, Quebec Nördlinger Ries, Germany
The K-T event
•
Sixty-five million years
ago about 70% of all
species then living on
Earth disappeared
(including dinosaurs)
within a very short
period.
•
K-T boundary clay
contains iridium, an
element common in
iron-rich minerals like
asteroids and meteorites
Impacts in Modern Times
Tunguska River in Siberia, Russia in 1908
2150 square kilometers affected Airburst of large meteoroid or comet (100 meters in size) 5–10 kilometers above the surface of the Earth.
largest impact event on or near Earth in modern times.
Some Nomenclature
• Meteoroid = small body in space
• Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky
• Meteorite = meteor that survives the plunge
through the atmosphere to strike the ground...
• Sizes from microscopic dust to a few centimeters. • Estimated 300 tons of meteoritic material falls on Earth each day.
• Typically impact onto the atmosphere with 10 – 30 km/s (≈ 30 times faster than a rifle bullet).
Analysis of Meteorites
3 broad categories:
• Iron meteorites • Stony meteorites • Stony-Iron meteorites (4%) (95%) (1%)What Does a Meteorite Look Like?
Selection bias:
Iron meteorites are easy to recognize as meteorites (heavy, dense lumps of iron-nickel steel) – thus, more likely to be found and collected.
Every year between 30,000 and 80,000
meteorites larger than 20g in mass fall from space to Earth.
Goose Lake meteorite (iron)
Willamette Meteorite
The Origins of Meteorites
Probably formed in the solar nebula, ~ 4.6 billion years ago.
Almost certainly not from comets (in contrast to meteors in meteor
showers!).
Probably fragments of stony-iron planetesimals
Some melted by heat produced by 26Al decay (half-life ~ 715,000 yr).
26Al possibly provided by a nearby supernova, just a few 100,000
years before formation of the solar system (triggering formation of our sun?)
The Origins of Meteorites (2)
Planetesimals cool and differentiate (if they are large enough)
Collisions eject material from different depths with different compositions and temperatures.
Meteorites can not have been broken up from planetesimals very long ago
so remains of planetesimals should
still exist.
Carbonaceous Chondrites
• A rare type of stony meteorite which contains large amounts of the magnesium-rich minerals and a variety of organic
compounds, including amino acids.
• No evidence of melting (many were never heated above 50° C)
• Primitive and undifferentiated meteorites
• Most of them contain water or minerals that have been altered in the presence of water, and some of them contain larger amounts of carbon as well as organic compounds.
Comets
Comet McNaught C/2009 R1 was visible on June 6, 2010. CREDIT: Michael Jäger
Comet Holmes, 2007 Hale-Bopp, 1997 Comet Halley, 1986
Comets
•
Small objects made of frozen
gases, rock and dust,
"dirty
snowballs“ –
the
nucleus
•
When close enough to the Sun,
displays a visible
coma
(a thin,
fuzzy, temporary atmosphere) and
sometimes also a
tail
.
•
Coma is surrounded by a giant
hydrogen envelope
.
•
Breakup of comets lead to meteor
showers on Earth.
•
Orbital periods from 20 years to
100,000 years or more
– Short period
– Intermediate period
– Long period
Halley's Comet becomes visible to the unaided eye about every 76 years as it nears the sun. Image credit: Lick Observatory
The Comet Nucleus
Composite image of the nucleus of Comet Halley produced from 68 original photographs taken by the Halley Multicolour Camera on board the Giotto spacecraft on March 13 and 14, 1986.
Comet Hartley, as imaged by NASA's EPOXI spacecraft. (Credit: NASA)
The Hydrogen Envelope
• Surrounding every moderately
active comet is a sparse but extensive envelope of neutral hydrogen atoms. The hydrogen is liberated when ultraviolet radiation from the Sun splits the water vapor molecules released from the nucleus of the comet into the constituent components, oxygen and
hydrogen.
• Up to 10 million km is size
• Not visible to the human eye
• Visible in UV light
The hydrogen cloud surrounding comet Hale Bopp in 1997 far exceeds the comet’s visible tail (inset). Although not visible from the Earth, the hydrogen envelope is enormous,
completely dwarfing the Sun which is shown as the yellow dot in the lower right corner.
Credit: SOHO/SWAN (ESA & NASA) & J.T.T. Mdkinen et al.
Two Types of Tails
Ion tail: Ionized gaspushed away from the comet by the solar wind.
Pointing straight away from the sun.
Dust tail: Dust set free from vaporizing ice in the comet; carried away from the comet by the sun’s
radiation pressure.
Lagging behind the comet
along its trajectory The yellow, because its microscopic dust particles dust tail of a comet appears whitish-reflect sunlight.
The ion tail glows blue by emitting light when elections re-combine with electrically charged ions to make uncharged molecules.
Evolution of Comets
•
Comets lose 0.5% to 1% of
its ice each perihelion
passage
•
Over time comets lose all
their ice leaving nothing
but rubble and dust called a
meteoritic swarm
•
These particles orbit in the
comet’s orbit but now
spread out along it.
•
Gravitational interactions
with planets can tidally
disrupt comets
(Shoemaker-Levy 9)
A NASA Hubble Space Telescope (HST) image of comet Shoemaker-Levy 9, taken on May 17, 1994, with the Wide Field Planetary Camera 2 (WFPC2) in wide field mode.
Short-Period Comets
• Orbital periods of less than 20 years, also called Jupiter-family comets
• Orbit more-or-less in the ecliptic plane in the same direction as the planets.
• Example: Comet Tempel 1 and Hartley 2
• These comets could not have formed here… Where do they come from? Kuiper Belt?
Intermediate-Period
Comets
•
Orbital periods of
between 20 and 200
years and inclinations
extending from zero to
more than 90 degrees
•
As of 2012, only 64
Halley-type comets
have been observed
•
Probably originate in an
outer icy asteroid belt,
the Kuiper Belt, beyond
Neptune.
The orbit of Halley's Comet is pretty typical. It has an eccentricity of .967, meaning an extremely elongated ellipse with the Sun very close to one end.
The Origin of Long-Period Comets
Long period comets are believed to originate in the Oort cloud: Spherical cloud of several trillion icy bodies, ~
10,000 – 100,000 AU from the sun.
Oort Cloud
Gravitational influence of occasional passing stars may perturb some orbits and draw them towards the inner solar system.
Interactions with planets may perturb orbits further, capturing comets in short-period orbits. Highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years
Meteoroid Orbits
• Meteoroids contributing to a meteor shower are
debris particles, orbiting in the path of a comet. • Spread out all along the orbit of the comet. • Comet may still exist or have been destroyed.
Meteor Showers
• Meteor showers are mostly caused by the trails of dust and debris left in the wake of a comet. When Earth passes through this
material we see a meteor shower.
• Meteors are observed to radiate from one point in the night sky called the
radiant.
• Zenith hourly rate varies between several meteors per hour to over 100 per hour depending on the shower.