A planet is a celestial body that is: In orbit around the Sun Has sufficient mass for its self-gravity to overcome its rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape Not massive enough to produce fusion reactions at the core Has cleared the neighborhood around its orbit
Which of the above criteria does Pluto fail to meet?
All planetary orbits are nearly circular and lie nearly in the same plane
ALL planets orbit the Sun counterclockwise when viewed looking at Earth’s north pole
Most planets rotate in the same direction as they orbit Small axis tilts The Sun rotates in the same direction
Large moons tend to orbit in planets’ equatorial planes Same direction as planet rotates
Terrestrial planets have few moons Look “out of place” Jovian planets have many moons Larger Jovian moons follow parent planet’s rotation
Uranus rotates practically on its side Its axis of rotation is nearly in the plane of its orbit
Venus rotates clockwise about its own axis (viewed from high above Earth’s north pole)
Terrestrial planets besides Earth either have no moons or small moons Earth’s Moon is very large Almost a quarter of the mass of Mercury
Terrestrial Planets: Mercury, Venus, Earth, Mars Small and dense Rocky exteriors, metal interiors Solid surface Few moons No rings Jovian Planets: Jupiter, Saturn, Uranus, Neptune Large! Lower average density (“gas giants”) No solid surface Rings Many moons H, He, and H compounds
Conjunction: Planet lines up with the Sun as seen from Earth, looking toward the Sun!
Opposition: Planet lines up with the Sun as seen from Earth, looking away from the Sun!
Greatest Eastern Elongation: An inner planet is as far east of the Sun as it can get as viewed from Earth
Perihelion: point along orbit where closest to the Sun Aphelion: the point along orbit where farthest from the Sun
In 2003, Mars had a better-than-usual opposition (in terms of being close) Consider elliptical orbits It was claimed that Mars would be the size of the full moon Would Mars be at conjunction or opposition for it to be as large as possible in the sky? Was only 25% closer than usual closest- approach
We have sent spacecraft to: All terrestrial and jovian planets Moons Asteroids Comets
Launched in 1972 First encounter with Jupiter Lost contact with it in 2003 (7 billion miles from Earth) Could reach Aldebaran in 2 million years
Launched in April 1973 Passed by Jupiter in December 1974 Passed Saturn in September 1979
Launched in 1977 Visited Jupiter and Saturn Opted to study Titan instead of Pluto; flung out of Solar System Voyager 1 is 0.002 light years (11 billion miles) from the Sun Will pass by a star in 40,000 years 10 miles per second
Launched in 1979 Voyager 2 visited all four Jovian planets Used a chain of gravitational slingshots Planets were lined up just right for the “Grand Tour”
Notable Orbiters Galileo Orbited and probed Jupiter and its moons Cassini Orbits and probes Saturn and its moons
Mars Rovers (Sojourner, Spirit, and Opportunity) Analyze chemistry and geology of Mars Searching for past and present signs of water