PHYSICS 1040 - ELEMENTARY ASTRONOMY - HOMEWORK #8
1. Uranus data: Average distance from Sun = 19.2 A.U.
Orbital period = 84.1 years
Rotational period = 17.2 hours
Diameter = 4.01 x Earth=s diameter
Average density = 1.3 x water=s density
Average temperature = -218oC (at cloudtops)
Neptune data: Average distance from Sun = 30.1 A.U.
Orbital period = 164.9 years
Rotational period = 16.1 hours
Diameter = 3.88 x Earth=s diameter
Average density = 1.6 x water=s density
Average temperature = -218oC (at cloudtops)
2. The following summarizes the discoveries of Uranus and Neptune: Uranus was found by accident while William Herschel was scanning the sky with this telescope in 1781. Uranus did not perfectly follow its orbit according to Newton's Laws of Motion and Gravity, and scientists were worried that Newton's Laws might be wrong. However, the suggestion was made that a more distant planet could be disturbing Uranus' motion. Calculations predicting the position of this more-distant planet were made, and Neptune was found exactly where it was predicted. This turned out to be a great triumph of Newton's Laws!
3. Both Uranus and Neptune have atmospheres composed primarily of hydrogen and helium, with about 2 percent methane. Methane absorbs red light, which gives these two planets their distinctive greenish-blue color. Both have a system of thin, dark rings. Uranus= axis of rotation lies nearly 8o below the plane of its orbit, so it rotates in a retrograde direction on its side! However, both Uranus= and Neptune= s magnetic fields are oriented in a direction similar to the magnetic fields of Earth, Jupiter, and Saturn, although the magnetic fields are offset from the centers of Uranus and Neptune.
4. Uranus= moon Miranda has a patchwork surface that has been altered drastically since its formation, either by a huge impact that nearly split the moon apart or by tidal heating. Neptune=s moon Triton orbits Neptune in a retrograde direction, and its orbit is inclined by 23
o from the plane of Neptune=s equator. These observations indicate that Triton probably formed elsewhere and was later captured by Neptune=s gravity. Neptune=s other moon, Nereid, orbits Neptune in a prograde direction, but it has the most eccentric orbit of any satellite in the solar system.
5. Pluto has a very eccentric orbit which sometimes carries it inside of Neptune=s orbit! Pluto=s orbit is also inclined by 17 degrees to the ecliptic plane of the other planets= orbits. Pluto=s axis of rotation lies about 32o below the plane of its orbit, so (like Uranus) it rotates in a retrograde direction its side! Pluto=s moon Charon is only about 20,000 km away from Pluto. Pluto and Charon rotate synchronously with each other, so each always keeps the same side toward the other.
6. More than 100,000 asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter in the ecliptic plane. Most of the asteroids have diameters of less than a kilometer, but a few of them are very large. The largest, Ceres, was the first to be discovered; it has a diameter of 934 km. Like the rings of Saturn, the asteroid belt was formed from small objects called planetesimals that never came together to form a larger object. Just as Saturn's gravitational tidal force kept its ring particles from forming a moon, so the gravitational tidal force of the planet Jupiter prevented the planetesimals in the asteroid belt from forming a larger object. Also, just as an orbital resonance with Saturn's moon Mimas maintains the Cassini division between the A and B rings, orbital resonances with the planet Jupiter maintains the Kirkwood gaps in the asteroid belt.
7. Geological evidence indicates that, 65 million years ago, an asteroid about 10 km in diameter traveling about 40,000 km/hr slammed into Mexico= s Yucatan Peninsula and left the crater, 180 km in diameter, that we see today. This collision could have produced enough dust and smoke in Earth= s atmosphere to block out sunlight for several years. At this time, two-thirds of all the species on Earth died out, including the dinosaurs. This allowed small, furry mammals to rise and eventually dominate life on Earth.
8. A meteoroid is a small chunk of rock in space, less than fifty meters across, produced by the collisions of asteroids. When a meteoroid plunges through our atmosphere, it heats up and produces a streak of light called a meteor (a Ashooting star@ ). If a piece of that rock reaches the ground intact, it is called a meteorite. About 300 tons of extraterrestrial material, mostly dust, fall on Earth every day!
9. The nucleus of a comet is a dirty snowball, a loose mixture of ice and dust a few kilometers across. In the diagram of a comet orbiting the Sun at right, identify the comet=s
a) nucleus,
b) coma,
c) hydrogen envelope,
d) ion tail, and
e) dust tail.
The ion tail always points directly away from the Sun, so you can/can not (circle one) tell which direction the comet is moving by looking at its tail.
10. The orbits of comets around the Sun are different than the orbits of the planets in two ways:
a. Although both planets and comets move in elliptical orbits around the Sun, the orbits of the planets are nearly circular, while the orbits of the comets are highly eccentric or elongated.
b. The planets all orbit the Sun in the same orbital (the ecliptic plane), while the comets orbit the Sun in many different orbital planes.
Comets come from two regions of the solar system. Tens of thousands of comet nuclei in the Kuiper Belt orbit the Sun in the ecliptic plane between 40 and 500 AU from the Sun). Billions of comets are found in the Oort Cloud, a spherical distribution of comet nuclei that extends out to 50,000 AU from the Sun.