1. South pole of Moon as seen by the Lunar Reconnaissance Orbiter

2. Final Exam: Wednesday, May 4, 12:30 - 2:30 p.m., Swain West 119
Final Exam Review Session
Tuesday, May 3, 7:15pm-9:15pm
Swain West 007  

3. Solar system: 1 large object several medium-sized objects
Sun
several medium-sized objects
Planets
Moons
lots of debris
Asteroids
Comets
Meteoroids
Dust

4. Asteroids, Comets, Meteorites & Dwarf Planets

5. Nomenclature asteroid – a rocky leftover planetesimal
comet – an icy leftover planetesimal orbiting the Sun
meteor – a flash of light in the sky caused by a particle entering the atmosphere, regardless of its origin (“falling star”)
meteorite – any piece of rock that fell to the ground from space, regardless of its origin
meteoroid – the name of an object that becomes a meteor, while it is still in space

6. Asteroids: Small rocky or metallic objects believed to be left over from the beginning of the solar system
More than a million may exist; 10,000 have well determined orbits

7. Most asteroids orbit the Sun between Mars and Jupiter
Some share orbits with planets and cross into the inner solar system (Apollo asteroids, Earth Crossing or Near Earth Objects)
Trojan asteroids share orbit of Jupiter, but are located 60 degrees ahead and behind Jupiter in its orbit
Mass of all known asteroids < 1/10 mass of Moon.
Range in size from Ceres (diameter of ~1000 km), down to objects a few centimeters or less across.

9. Distribution of asteroid orbit semi-major axes shows prominent gaps caused by resonances with Jupiter's orbital motion. These are known as the Kirkwood Gaps (after Daniel Kirkwood of Indiana U).
Asteroid in resonance with Jupiter receives a strong gravitational tug from planet each time they are close together. If asteroid's period is in a well-defined ratio with that of Jupiter’s, the effects reinforce each other.
Kirkwood Gaps

10. Formation
Believed to represent material left over from the formation of the solar system
It was once believed that asteroids are the remains of a planet that was destroyed in a massive collision. It is more likely that they represent material that never formed a planet.
Highest concentration of asteroids is in the asteroid belt, the region lying between orbits of Mars and Jupiter.
Gravitational perturbations by Jupiter kept these planetesimals from coalescing into larger bodies.

11. Ida – Dactyl a binary asteroid
Ida is more heavily cratered, ~1 billion years old
satellite, Dactyl, 1.5 km diameter
Orbital period = 24 hours;
orbital distance = 100 km
Ida’s density ~ 2.5 g/cm3

12. 25143 Itokawa
The Japanese spacecraft Hayabusa landed on this asteroid.

13. Eros

14. Gaspra 7 hour rotation period 16 x 11 x 10 km, irregular shape
sparse crater count implies 200 million years old

15. Asteroids Mathilde (59 x 47 km)
NASA NEAR Mission
Jun 1997
Range: 2,400 km

16. Vesta: An Unusual Asteroid
HST resolves features as small as 50 miles across, allowing astronomers to map Vesta's geologically diverse terrain.
The surface is a complex record of Vesta's four billion-year history.
Features include ancient lava flows, and a gigantic impact basin that is so deep, it exposes the asteroid's subsurface, or mantle.

17. Mars Global Surveyor images of Phobos
Most or all of the small moons in the solar system may be captured asteroids
Mars Global Surveyor images of Phobos

18. Earth-Crossing Asteroids
Certain groups of asteroids have elliptical orbits that cross the orbit of Earth and other inner planets.
About 300 asteroids are known to cross Earth's orbit.
However, about 1500 unknown NEO’s are estimated to exist.
These may pose a serious risk to the Earth (as the dinosaurs found out)
None of the known asteroids will strike the Earth.
We have discovered only a fraction of the total number of Earth-crossing asteroids, so there are many for which we do not know the orbit.

19. Icy Planetesimals: Comets
small bodies made out of dust and ices ("dirty snowballs").
name derived from the Greek aster kometes, "long-haired star”
Since the observations of Tycho Brahe, comets are known to be members of the Solar System well beyond Earth's atmosphere.
Most on long elliptical orbits (perhaps parabolic in some cases)
When they come near the Sun they are heated and emit gases and dust that are swept by the Solar Wind into the characteristic tail that always points away from the Sun.

20. Parts of a Comet
Comets have several distinct parts when near the Sun and active:
nucleus
relatively solid and stable, mostly ice and gas with a small amount of dust and other solids
coma
dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus
hydrogen cloud
huge (millions of km in diameter) but very sparse envelope of neutral hydrogen
dust tail
up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases;
most prominent part of a comet to the unaided eye
ion tail
as much as several hundred million km long
composed of plasma; laced with rays and streamers caused by interactions with the solar wind.

21. Structure of a Comet
Ion Tail
Dust Tail
Orbital motion
Coma
To Sun

22. Comet Nucleus
Borrelly
Halley
Hartley2
Temple 1
Wild 2

23. Comet Hale Bopp 1997

24. Hale Bopp Animation

25. Comet Tails
Tails of bright comets can be 150 million kilometers (1 AU) in length, making them the "largest" objects in the Solar System.
Many comets have two tails:
gas tail (or ion tail) composed of ions blown out of the comet away from the Sun by the solar wind, and
dust tail composed of dust particles liberated from the nucleus as the ices are vaporized.

27. Comet Orbits
Comets interact gravitationally with the Sun and other objects.
Their motion is also influenced to some degree by gases jetting out of them, so their orbits are not completely determined by gravity.
Most comets’ orbits appear to be elliptical, or in some cases parabolic.
The most common comets are called short-period comets that have only mildly elliptical orbits that carry them out to a region lying from Jupiter to beyond the orbit of Neptune. These are normally seen only with telescopes.
Comets visible to the naked eye are rare and are thought to come from a great spherical cloud of cometary material surrounding the Solar System called the Oort Cloud.

28. Long term comets most are in the Oort cloud (up to 50,000 AU from Sun)
normally orbit far from the Sun, very few enter planetary region of solar system
highly elongated orbits
not confined to ecliptic, all orbital inclinations
prograde and retrograde orbits
roughly uniform distribution

29. Short term comets periods < 200 year
most originate in region beyond Neptune called Kuiper belt
approximately circular, prograde orbits AU
normally orbit outside jovian planets, occasionally kicked into inner solar system

30. Comet Halley
English astronomer Edmund Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time.
He found that the bright comets of 1531, 1607, and 1682 had the same orbits, and concluded that these were different appearances of the same comet.
He used his calculations to predict the return of this comet in 1758.
If one traces back in the historical records for recordings of bright comets and their positions in the sky, it can be concluded that Comet Halley has been observed periodically as far back as 240 B.C.
Halley in 1910
Halley in 1986

31. Comet Shoemaker-Levy 9
In July of 1994, fragments of Comet Shoemaker-Levy 9 impacted the planet Jupiter. The points of impact could be observed by the Galileo spacecraft.