Solar System Debris
Keys to Our Origin

Solar System Debris
Keys to Our Origin

May 22, 2011, jeb

OUTLINE

Asteroids
Comets
Beyond Neptune
Meteoroids
Other Sources on Debris

Asteroids

Over a hundred-thousand rocky objects concentrated in the Asteroid belt
between Mars and Jupiter

Quite eccentric orbits (not circular)
largest: Ceres
- 940 km diameter
- 1/10,000 M_Earth
Total mass of all asteroids is about 10% M_Moon
- therefore, they are a small fraction of the Solar System
Very few in retrograde orbits and only a couple of the larger asteroids
Why is there an Asteroid belt?
- Theories
  1. fragments of a planet that broke up long ago?
  2. rocks that never managed to accumulate
- Researchers favor 2.
  - Jupiter's gravity continuously disturbs these chunks of matter, preventing them from accumulating

Types of Asteroids

C-type
- carbonaceous(largely carbon, dark)
- 75% of the asteroids
- outer orbits
- very old (4.6 billion years)
S-type
- silicate(largely silicon, light)
- 15% of the asteroids
- inner orbits
M-type
- nickel-iron
- about 10% of the asteroids

Earth and Mars-Crossing Asteroids

Apollo asteroids
- Earth-crossing with semi-major axis greater than 1 AU
Aten asteroids
- Earth-crossing with semi-major axis less than 1 AU
Amor asteroids
- only cross the orbit of Mars
Several thousand Earth-crossing asteroids have been discovered
- Icarus, perihelion of 0.2 AU, frequently passes close to Earth
- Icarus passed within 6 million km in 1968!
Most of the Earth crossing asteroids will eventually collide with the Earth
- about 3 every million years
- In 2002 a 100-m diameter asteroid passed within 120,000 km (less than 1/3 distance to Moon)
- None of the known asteroids are expected to impact in next century
- April, 2029 - 350-m asteroid Apophis will pass just 30,000 km from Earth
- List of Forthcoming Close Approaches To The Earth

Orbital Resonances of Asteroids

Trojan asteroids
- orbit with Jupiter
  - 60^o ahead or behind at the Lagrange points
Kirkwood gaps
- caused by resonance with Jupiter

Close-up Views of Asteroids

Galileo (on journey to Jupiter)
- visit to Gaspra
  - S-type
  - maximum diameter of 20 km
  - about 200 million years old
  - fragment of larger object
- visit to Ida
  - S-type
  - maximum diameter of 60 km
  - about 1 billion years old
  - fragment of larger object
  - tiny moon: Dactyl (1.5 km across)
NEAR-Shoemaker (Near Earth Asteroid Rendezvous) - official web site
- spacecraft misson to rendezvous with asteroid Eros
- launched February 17, 1996
  - planned Eros rendezvous in early 1999
- passed by Mathilde on June 27, 1997
  - C-type
  - low density of 1400 kg/m³
    - porous interior
- missed planned rendezvous with EROS in January, 1999, when spacecraft engine mis-fired
- but obtained initial up close data
  - Eros is S-type
  - Eros is a bit smaller then thought(33 km x 13 km x 13 km)
    - Twice the size of Manhattan
  - density is 2700 kg/m³
  - appears homogeneous, not porous as Matilde
- NEAR-Shoemaker entered orbit around Eros February 14, 2000
  - orbited Eros for a year and did detailed study
  - being a minor body, Eros has little gravity
    - 200 pound person would weigh 2 ounces
    - escape velocity is 0.01 km/sec (22 miles/hour)
  - crater densities indicated Eros is old, like Ida and Matilde, but unlike Gaspra
- 5 km flyby on October 20, 2000
  - mosaic image obtained during flyby
  - detailed image
- landed on Eros, February 12, 2001
  - survived low-velocity impact and maintained radio contact for 16 days

Collisions with Earth

Plot of the Inner Solar System
Plot of the Innermost Solar System
about 3 per million years
- Solar System Collisions web site
lists
- Forthcoming Close Approaches to Earth
- Closest Approaches to the Earth by Minor Planets
65,000,000 years ago - ???
- (see below)

Comets

Icy matter in highly elliptical orbit
- like "dirty snowballs"
left-over material, unchanged since the formation of the Solar System
as they near the Sun
- heat up and emit radiation
- develop tail
IAU Minor Planet Center

Comet Structure

Nucleus - few km in diameter
- Density : 100 kg/m³
- Evaporation Rate: 10 tons/ second near the Sun
- Lifetime: about 5000 orbits or about 400,000 years for Halley's Comet (in other words - short on cosmological scale)
Coma- evaporated gas and dust forms halo around nucleus as comet approaches the Sun
Hydrogen envelope
Tail
- grows as comet approaches Sun
- two types of tails
  - ion tail (straight out, blown by solar wind)
  - dust tail (broad, diffuse, curved, less deflected by solar wind)

Two sets of orbits

short-period comets:
- from Kuiper Belt (KBOs - Kuiper Belt Objects)
  - orbits of 30 - 50 AU in diameter
  - flat (near ecliptic plane) set of orbits
  - barely observable by ground based telescopes
  - over a thousand Kuiper Belt Objects have been discovered
  - Most of these objects move in roughly circular orbits
  - Occasional close encounter of two, or gravitational influence of outer planet, "kicks" object into highly eccentric orbit into the inner solar system
long-period comets:
- from Oort Cloud
  - orbits of up to 100,000 AU in diameter
  - surrounds Sun in all directions

Halley's Comet

Probably the most famous comet
In 1705, Edmund Halley predicted that this comet would reappear in 1758
- Halley died in 1742, and his comet appeared right on schedule in 1758
period of 76 years
- retrograde orbit
recorded every passage since 240 BC
- 1910 images
last visit in 1986
- met by armada of spacecraft in 1986
  - USSR, Japan, Western European
  - Vega 2 and Giotto resolved Halley's nucleus
  - advanced understanding of Halley's nucleus

Cometary Impact on Jupiter in 1994

Comet Shoemaker-Levy crashed into the surface of Jupiter in July, 1994
Comet was discovered in March, 1993
- composed of several pieces
- "string of pearls"
How did Comet get this way?
- July, 1992 passed within Roche Limit of Jupiter -> broke apart
Series of impacts
- Huge fireballs (hundreds of km)
  - (like a billion nuclear explosions)
- Jupiter disturbed over thousands of kilometers
interesting comparison tables

Comets of 1996/1997

Hyakutake
- in January of 1996, an amateur comet astronomer in Japan discovered this comet with a pair of binoculars
- the orbit was predicted to pass 0.1 AU from Earth
- a spectacularly bright, naked eye image was predicted, and observers were not disappointed
- Hyakutake turned out to be the brightest comet in 20 years
Hale-Bopp
- discovered in July, 1995
  - simultaneously by Alan Hale in New Mexico and Thomas Bopp in Arizona
- Comet Hale-Bopp was visible to naked eye in the 1997
- Perihelion on April 1, 1997
  - period is about 4000 years
- One of the brightest comets of the 20th century
  - passed 1.3 AU from Earth (compared to 0.1 for Hyakutake), but still was brighter

Stardust Mission

Mission launched in February, 1999 to collect cometary material and return it to Earth
Passed through tail of Comet Wild-2 in January, 2004
- Onboard detector made of foam-like gel (aerogel), which is 99.8% air, collected cometary dust particles
Returned to Earth in 2006 with dust tracks collected in the aerogel that reveals detailed physical, chemical, and even biological properties of a body probably unchanged since the formation of the solar system

Deep Impact

400-kg projective crashed into comet Tempel 1 on July 4, 2005 spacecraft observed explosive collision from 500 km distance
spectroscopic analysis confirmed presence of water ice and many organic molecules, in this remnant of the early universe
- confirming "snowball" picture of cometary structure

Beyond Neptune

Discovery of Pluto in 1930

Percival Lowell estimated position of ninth planet from deviations in orbits of Uranus and Neptune
Discovered 14 years after Lowell's death very close to Lowell's predicted location by American Clyde Tombaugh
- Lowell's calculations now appear to have been incorrect and predicted location was accidentally accurate
- Pluto is named for the mythical Greek god of the underworld
  - note 1st two letters of Pluto stand for Percival Lowell

Basic Properties of Pluto

Orbital period
Semi-major axis
Mass
Radius
Density
Rotation Period
Known Moons

Pluto
248 years
39.5 AU
0.002 x Earth's mass
0.2 x Earth's radius
2100 kg/m³
-6.4 days
3

Hubble Space Telescope Images of Pluto

Pluto has not been visited by spacecraft
Most revealing images were taken with the Hubble Space Telescope
- Pluto surface map

Pluto's Orbital Properties

Semi-major axis of 39.5 AU
very faint object in Earth's sky
- highly elongated and tilted orbit
  - eccentricity of 0.25 (largest in Solar System)
    - approaches within about 30 AU of Sun (perihelion)
    - at farthest point, about 50 AU from Sun (aphelion)
  - inclination of 17^o to ecliptic (largest in Solar System)

Passes inside Neptune's orbit at perihelion
- Inside Neptune from 1979 to 1999
  - making Neptune the most distant planet from the Sun during this period
Locked in 3:2 resonance with Neptune
- Does not come close enough to Neptune to collide

Pluto's moon, Charon

Discovered in 1978
- named for the mythical boatman who ferried the dead across the river Styx into Hades
HST images much more distinct than ground-based observations
Inclined orbit (118^o)
- orbital period of 6.4 days
- Furthermore, Charon rotates once for each revolution
  - just as the Earth's Moon does
by chance, a series of several eclipses (1985-1991) occurred
- provided detailed info on size and orbits of Pluto and Charon
- about one-half the radius of Pluto
  - most closely matched planet and moon in the Solar System
    - "double planet"
- about one-sixth the mass of Pluto

Pluto's other properties

Radius = 1135 km (more like a moon than any other planet)
Density (2100 kg/m³) midway between Jovian and Terrestrial
- Density similar to Triton (Neptune's moon)
- Probably mostly water-ice composition
- Surface temperature of about 50K (solid methane)

Pluto's origin

Escaped moon?
Icy asteroid
- and Charon?
  - more icy asteroids beyond Pluto?
Do Triton and Pluto have a similar origin?
- They are similar
  - density, mass, radius
  - Pluto: 2100 kg/m³, 0.002 M_E, 1135 km
  - Triton: 2100 kg/m³, 0.0035 M_E, 1350 km
- Did both once move in independent orbits around the Sun?
- Charon?

Exploration of Pluto

Pluto has not been explored up close by spacecraft
NASA launched a mission to Pluto-Charon and beyond in 2006.
- New Horizons Pluto-Kuiper Belt mission
- should arrive at Pluto-Charon in 2015
  - 9 year flight-time
  - trajectory

Properties of Trans-Neptunian Objects

More than a thousand faint, beyond Neptune objects are known
most in the Kuiper belt
- Pholus, discovered in 1992, about 185 km across
- Quaowar, discovered in 2002, about 1200 km across
- Sedna, discovered in 2003, about 1500 km across
- Haumea, discovered in 2003, about 1400 km across
- Makemake, discovered in 2005, about 1500 km across
- Eris, discovered in 2005, about 2400 km across, larger than Pluto
estimated total number of Kuiper belt objects larger than 100 km is more than 100,000
comparison of the sizes of some of largest trans-Neptunian objects
orbits of some of the prominent occupants of the outer solar system

King of the Kuiper Belt

definition of a planet
1. it orbits the Sun
2. it is massive enough that its own gravity has caused its shape to be approximately spherical
3. it has "cleared the neighborhood" around its orbit of other bodies
Pluto satisfies 1 and 2, but does not satisfy 3
- bodys that satisfy 1 and 2, but not 3, are now called dwarf planets
- Pluto, Eris, Makemake, Haumea, and Ceres are all dwarf planets
- icy dwarf planets beyond Neptune are known collectively as plutoids

Meteoroids

Terminology

Meteoroid - chunk of debris before entering Earth's atmosphere
Meteor - bright streak in the sky caused by piece of debris
- "shooting star"
Meteorite - a piece of a meteoroid that survives and lands on Earth
Meteoroid swarm - fragments from comet that follow same orbit
Meteor shower - event where frequent meteors can be seen

Cometary Fragments

smaller meteoroids are rocky remains of broken up comets
certain meteor showers are regular and (fairly) predictable events
- named for "radiant", or constellation from which they appear to come.

Stray Asteroids

larger meteoroids (more than a few centimeters) are generally from the asteroid belt
- these objects are responsible for most cratering in the inner Solar System
massive meteoroids (more than a ton and a meter across)
- can make it to the surface of the Earth
Earth is scarred with nearly 100 craters larger than 0.1 km in diameter
- Barringer Crater (1 km)
- Quebec's Manicouagan Reservoir
  - 70 km diameter
- Tunguska event of 1908 (in Siberia)
  - exploded in Earth's atmosphere before reaching surface
Major collisions of large meteoroids with the Earth occur every few hundred-thousand years
- Demise of the dinosaurs 65,000,000 years ago?

Meteorite Properties

Fairly dense: up to 5000 kg/m³ (like asteroids)
Most have rocky composition
Some are composed mostly of iron and nickel
Some show geologic history
- indicating strong heating sometime in their past, perhaps when they were liberated from a larger body
Most are very old (4.4 - 4.6 billion yrs)

Lunar and Martian Meteorites on Earth

Many meteorites have been established as having come to Earth from Mars,
- typically billion year old
and many are from the Moon

Conclusion

Someday,
One of these large objects will strike the Earth and life will be significantly affected

Remember the Dinosaurs!

Other Sources on Debris

Asteroids

Solar System Debris Keys to Our Origin

May 22, 2011, jeb

OUTLINE

Types of Asteroids

Earth and Mars-Crossing Asteroids

Orbital Resonances of Asteroids

Close-up Views of Asteroids

Collisions with Earth

Comet Structure

Two sets of orbits

Cometary Impact on Jupiter in 1994

Comets of 1996/1997

Deep Impact

Basic Properties of Pluto

Orbital period Semi-major axis Mass Radius Density Rotation PeriodKnown Moons

Pluto 248 years 39.5 AU 0.002 x Earth's mass 0.2 x Earth's radius 2100 kg/m3 -6.4 days3

Terminology

Cometary Fragments

Meteorite Properties

Lunar and Martian Meteorites on Earth

Conclusion

Solar System Debris
Keys to Our Origin

Orbital period
Semi-major axis
Mass
Radius
Density
Rotation Period
Known Moons

Pluto
248 years
39.5 AU
0.002 x Earth's mass
0.2 x Earth's radius
2100 kg/m³
-6.4 days
3