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Lesson 5: The Solar System
Summary of Discovery of the Solar System
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Pre-History
 Earth
 The sun and the moon
 The five naked-eye planets: Mercury, Venus, Mars, Jupiter, and Saturn
 Comets and meteors
th
17 Century
 The four Galilean moons of Jupiter: Io, Europa, Ganymede, and Callisto (1609)
 Saturn’s rings (1659)
th
18 Century
 Uranus (1758)
th
19 Century
 Ceres (1801)
 Ceres is the first discovered and largest member of the asteroid belt.
 Asteroid belt
 We continue to discover new asteroids, both in and out of the asteroid belt,
to the present day.
 Planetary moons
 We continue to discover new moons around Jupiter, Saturn, Uranus, and
Neptune to the present day.
 Neptune (1846)
20th Century
 Pluto (1938)
 Pluto is the first discovered member of the Kuiper belt. It was
misclassified as a planet but continues to be called one for historical
reasons.
 Planetary rings
 In addition to around Saturn, rings have been discovered around Jupiter,
Uranus, and Neptune.
 Kuiper belt
 The discovery of the Kuiper belt has only just begun. We are discovering
Kuiper belt objects at an increasing rate.
st
21 Century
 Sedna (2003)
 The first and possibly largest member of the Oort cloud.
 “Xena” (2005)
 Larger than Pluto, this Kuiper belt object is forcing astronomers to better
define what “planet” means.
Summary of Interplanetary Debris
Object
Asteroid
Meteroid
Comet
Dust
Composition
Mostly rocky
Mostly rocky
Mostly icy
Graphites and silicates
Size
>100 m
<100 m
≈1 – 10 km
< ≈1 m
Summary of Exploration of the Solar System
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Missions in bold face are still active or were recently active.
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We have successfully flown satellites by:
 The moon
 Mercury
 Mariner 10 (1974)
 Venus
 Mariner 2 (1962)
 Mariner 5 (1967)
 Mars
 Mariner 4 (1965)
 Mariner 6 (1969)
 Mariner 7 (1969)
 Jupiter
 Pioneer 10 (1973)
 Pioneer 11 (1974)
 Voyager 1 (1979)
 Voyager 2 (1979)
 Saturn
 Pioneer 11 (1979)
 Voyager 1 (1980)
 Voyager 2 (1981)
 Uranus
 Voyager 2 (1986)
 Neptune
 Voyager 2 (1989)
 Comets
 Many satellites have now flown through the tails of many comets,
beginning with Halley’s comet in 1985. Recently, Stardust flew through
the tail of comet Wild 2 in 2003 and collected a sample for return to Earth
in 2006.
 Asteroids
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Many satellites have now flown by many asteroids, beginning with
Galileo’s flybys of the asteroids Gaspra (1991) and Ida (1993) while on its
way to Jupiter. Ida was discovered to have a moon, which was named
Dactyl.
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We have successfully put satellites into orbit around:
 Mercury
 Messenger (2011)
 Messenger will fly by Mercury twice in 2008, once in 2009, and
will enter orbit in 2011.
 The moon
 Venus
 Soviet Venera 9 (1975)
 Soviet Venera 10 (1975)
 Pioneer Venus Orbiter (1978)
 Soviet Venera 15 (1983)
 Soviet Venera 16 (1983)
 Magellan (1990)
 Mars
 Soviet Mars 2 (1971)
 Soviet Mars 3 (1971)
 Mariner 9 (1971)
 Soviet Mars 5 (1973)
 Viking 1 Orbiter (1976)
 Viking 2 Orbiter (1976)
 Mars Global Surveyor (1997)
 2001 Mars Odyssey (2001)
 European Mars Express (2003)
 Jupiter
 Galileo Orbiter (1995)
 Saturn
 Cassini (2004)
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We have successfully launched satellites/probes into the atmospheres of:
 Venus
 Soviet Venera 4 (1967)
 Soviet Venera 5 (1969)
 Soviet Venera 6 (1969)
 Pioneer Venus Large Probe (1978)
 Pioneer Venus North Probe (1978)
 Pioneer Venus Night Probe (1978)
 Jupiter
 Galileo Probe (1995)
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We have successfully landed satellites/probes on:
 The moon
 Venus
 Soviet Venera 7 (1970)
 Soviet Venera 8 (1972)
 Soviet Venera 9 (1975)
 Soviet Venera 10 (1975)
 Pioneer Venus Day Probe (1978)
 Soviet Venera 11 (1978)
 Soviet Venera 12 (1978)
 Soviet Venera 13 (1981)
 Soviet Venera 14 (1981)
 Soviet Vega 1 Probe (1985)
 Soviet Vega 2 Probe (1985)
 All of these melted within about an hour of landing.
 Mars
 Viking 1 Lander (1976)
 Viking 2 Lander (1976),
 Mars Pathfinder and its rover Sojourner (1997)
 Mars Exploration Rover Spirit (2004)
 Mars Exploration Rover Opportunity (2004)
 Saturn’s moon Titan
 Huygens (2005)
 Huygens is a probe that was launched by Cassini.
 Near-Earth asteroid 433 Eros
 This depends on your definition of successful. We purposefully crashlanded the NEAR Shoemaker satellite on 433 Eros at the end of its mission
in 2001. It did not survive, but it was never designed to. However, it
continued to collect and transmit data to Earth until the moment it hit.
 Comet Temple 1
 Again, this depends on your definition of successful. On July 4, 2005, a
probe launched by the Deep Impact satellite purposefully crashed into
comet Temple 1, ejecting comet material into space so it could more easily
be studied.
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We have successfully landed humans on:
 The moon
 Apollo 11 (1969)
 Neil A. Armstrong
 Edwin E. Aldrin
 Apollo 12 (1969)
 Charles Conrad, Jr.
 Alan L. Bean
 Apollo 14 (1971)
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 Alan B. Shepard, Jr.
 Edgar D. Mitchell
Apollo 15 (1971)
 David R. Scott
 James B. Irwin
Apollo 16 (1972)
 John W. Young
 Charles M. Duke, Jr.
Apollo 17 (1972)
 Eugene A. Cernan
 Harrison H. Schmitt
Math Notes
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Planetary Diameter
 D = diameter of planet
  = angular diameter of planet
 d = distance to planet
 D = ( / 1 rad) x d
 Note: This equation, sometimes called the “skinny triangle” equation, is only
valid if  is much less than 1 radian.
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Planetary Mass
 P = orbital period of satellite (natural or artificial)
 a = orbital semimajor axis of satellite
 M = mass of planet
 Newton’s form of Kepler’s Third Law tells us that:
 P2 is proportional to a3 / M
 Solving for M yields:
 M is proportional to a3 / P2
 Consider the case of Earth orbiting the sun. Then:
 Msun is proportional to aearth3 / Pearth2 = (1 AU)3 / (1 yr)2
 Dividing this equation into the previous equation yields:
 M / Msun = (a / 1 AU)3 / (P / 1 yr)2
 Hence, as long as M is measured in solar masses, a is measured in AU, and P is
measured in years, Newton’s form of Kepler’s Third Law should be very easy to
use.
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Average Density
  = average density of planet
 M = mass of planet
 V = volume of planet
 R = radius of planet (= half of diameter of planet)
 =M/V
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V = 4R3/ 3
Hence,  = 3M / 4R3
Example values:
 Water: 1000 kg/m3
 Rock: 2000 kg/m3 – 3000 kg/m3
 Iron: 8000 kg/m3
The Titius-Bode “Law”
 n = -∞, 2, 3, 4, 5, 6, 7, 8, 9, and 10
 an = predicted semimajor axis in AU
 a = actual semimajor axis
 an = 0.4 + 0.3 x 2n-2 AU
n
-∞
2
3
4
5
6
7
8
9
10
Formula
0.4 + 0.3 x 2-∞
0.4 + 0.3 x 20
0.4 + 0.3 x 21
0.4 + 0.3 x 22
0.4 + 0.3 x 23
0.4 + 0.3 x 24
0.4 + 0.3 x 25
0.4 + 0.3 x 26
0.4 + 0.3 x 27
0.4 + 0.3 x 28
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an [AU]
0.4
0.7
1.0
1.6
2.8
5.2
10.0
19.6
38.8
77.2
a [AU]
0.39
0.72
1
1.52
2.8
5.2
9.5
19.2
30.1
39.5
67.9
Object
Mercury
Venus
Earth
Mars
Ceres (1801)
Jupiter
Saturn
Uranus (1781)
Neptune (1846)
Pluto (1938)
“Xena” (2005)
Proposed in 1766 by Johann Titius and later popularized by Johann Bode, this
empirical equation accurately accounted for the semimajor axes of Earth and the
five naked eye planets, accurately predicted the semimajor axes of Uranus and the
asteroid belt, but proved increasingly inaccurate for Neptune and Pluto. The
physical meaning of the Titius-Bode “law” remains unclear.
Homework #5
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Extra Credit Homework
Write a two-page essay on any of the missions, or related series of missions, listed above in the
section Summary of Exploration of the Solar System. I will grade this optional homework on a
pass/fail basis. If you pass, I will drop your second lowest homework score when I average your
homework grades at the end of the semester (I already drop your lowest homework grade). The
Extra Credit Homework is due the same time as the final exam.
To pass, follow these simple rules:
 The essay should be single spaced with 1.25-inch margins.
 Use 12-point Times New Roman font.
 The header should consist of a title, your name, and a single space before the beginning
of the essay. Do not put spaces between paragraphs.
 The essay should run at least one line onto page three.
 Stay on topic and don’t make things up 