A Student's Guide to Traveling the Solar System

The solar system is made up of: the Sun, four inner planets, four outer planets, dwarf planets, moons, satellite objects, asteroids, comets, dust, and gas. The most widely accepted model of the formation of the solar system is referred to as the nebular model. This model theorizes that a cloud of dust and gas contracted and formed the Sun, the planets, and the other objects in the solar system. The dust and gas cloud, like the nebular clouds that can be observed today, rotated and flattened along its axis, at which time the center contracted into dust and hot gases that would eventually form the Sun. The planets were formed in a process called accretion, in which the gases that were not contracted into the protosun solidified into planetesimals. These planetesimals collided with each other and as they grew, their gravitational force attracted more material and formed planets. Understanding what happened 4.5 billion years ago to form the solar system is difficult, since there is no way to re-create those events and test a hypothesis. Scientists who study the formation of the solar system use computer models that use rules of physics and chemistry to test their hypotheses of how the solar system was formed. Some scientists also study comets, asteroids, and meteoroids to find more information about conditions in the solar system. Others are searching for solar systems formed around other stars.

The Sun is a star comprised of superheated helium and hydrogen gases. The Sun is approximately 4.5 billion years old and has a diameter of 1.39 X 10^6 km (109 times the diameter of Earth). The temperature of the Sun’s interior is 15,000,000 Kelvin (26,999,540° Fahrenheit) and 5,800 Kelvin (9,980° Fahrenheit) on the surface. The Sun is classified as a G2V, a “yellow star”. The Sun is actually white, but appears yellow on Earth because of the scattering of blue light in the atmosphere. The Sun’s energy is generated through nuclear fusion of hydrogen into helium. The Sun is 14.6 million kilometers from the Earth, and it takes 8 minutes and 19 seconds for light from the Sun to reach the Earth.

Many space missions and observation stations have been launched to study the Sun and its effects on the Earth and the solar system. The Hinode (Solar-B), led by the Japanese Aerospace Exploration Agency in collaboration with NASA, the European Space Agency, and the Particle Physics and Astronomy Research Council, is a three-year mission to study the Sun, its corona, its magnetic sphere, and solar explosions. NASA’s STEREO mission (Solar TErrestrial RElations Observatory) will study the Sun and the Earth. One probe will lead the Earth along its orbit and the second will follow behind to study the movement of matter between the Sun and the Earth. SDO, or the Solar Dynamics Observatory, launched on February 11, 2010, will study the Sun’s effect on space weather. SOHO, the SOlar and Heliospheric Observatory, was launched in 1995 by NASA and the European Space Agency, orbits the Sun to study the Sun’s internal structure, its outer atmosphere, and solar wind.

• The Sun: An overview of the sun, including a diagram of the sun.
• About the Sun: Images of the sun and information on sunspots and solar physics.

The Moon is a natural satellite of the Earth. The Moon has a mean radius of 1,737.10 kilometers and its diameter is one quarter of the Earth’s. Scientists think that the Moon was formed 4.5 billion years ago when a large object hit the Earth, causing large chunks of material to fly into orbit, one of which became the Moon. The Moon’s surface is made of silica, alumina, lime, iron oxide, magnesia, titanium dioxide, and sodium oxide. The Moon rotation is synchronous to the Earth’s, meaning that the same side of the Moon always faces the Earth. The Earth’s ocean tides and the slight lengthening of the day with the seasons is due to the Moon’s gravitational influence.

The Soviet Union reached the Moon with an unmanned spacecraft as part of the Luna Program in 1959. The Luna 3 mission returned successfully to Earth with the first pictures of the Moon’s dark side in 1959 and the Luna 9 mission in 1966 returned with close-up photographs of the Moon’s surface. Later Luna missions returned to Earth with samples of lunar soil. The United States sent humans to the Moon in the Apollo program. Apollo 8 orbited the Moon in 1968, and Apollo 11 became the first manned lunar landing in 1969. Five other Apollo lunar landings followed, ending in 1972. Unmanned spacecraft have visited the Moon since Apollo 17 in 1972, but there have been no more manned Moon landings. Recent lunar orbiters sent by the United States, India, China, Japan, and the European Space Agency have discovered lunar water ice. The Outer Space Treaty makes the Moon available for all nations to explore for scientific and peaceful purposes.

• All About the Moon: Photos, lunar resources, missions, and facts about the moon.
• Lunar Atlas: Photos of the moon by a lunar orbiter.

Mercury is the planet closest to the Sun; its distance from the Sun varies from 46 million kilometers to 69.8 million kilometers. Mercury is slightly larger than the Earth’s Moon, with a diameter of 4,878 kilometers. Mercury’s rotation period is 58.65 days and its period of revolution around the Sun is 0.24 years, or 87.969 Earth days. Temperatures on the surface range from -300°F to 800°F. Mercury’s surface is marked by craters and plains and does not have a substantial atmosphere. The atmosphere is made of hydrogen, helium, sodium, oxygen, potassium, and calcium. Mercury does not have any satellites. Mercury has an iron core that accounts for 42 percent of its volume and generates a magnetic field that is 1 percent as strong as the Earth’s magnetic field. Mercury’s composition is 70 percent metals and 30 percent silicate.

The unmanned NASA spacecraft Mariner 10 reached Mercury in 1974 and mapped nearly half of its surface. The probe ran out of fuel before its final approach and is orbiting the Sun. The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft mapped another 30 percent of Mercury’s surface in 2008, and will reach orbit in 2011 to map the remainder. MESSENGER will also study Mercury’s magnetic field, its geology, its core, its poles, and its atmosphere.

• Exploring Mercury: Facts and views of Mercury.
• Mercury Questions and Answers: “Ask an Astronomer” answers to questions about Mercury.

Venus is the second-closest planet to the Sun. The distance between Venus and the Sun ranges from 108.9 million kilometers to 107.5 million kilometers, with a mean distance of 108.2 million kilometers. Venus orbits the Sun in 225 Earth days. Venus rotates around its axis in a retrograde direction (in the opposite direction from the direction in which it orbits the Sun) and one rotation takes 243 Earth days. Venus is similar in size to the Earth, with a diameter of 12,100 kilometers, only 644 kilometers smaller than the Earth’s diameter. Thick clouds of sulfuric acid surround Venus, and its atmosphere is made of carbon dioxide, with some nitrogen, water vapor, argon, carbon monoxide, neon, and sulfur dioxide. The atmosphere is very heavy; its atmospheric pressure 90 times that of Earth’s atmosphere. The surface is hot and dry, averaging 870°F. Venus has mountains, canyons, valleys, and volcanoes, and 65 percent of the surface is flat plains. Scientists think that Venus’s current surface is only 1 billion years old because there are few impact craters, such as those on the Moon and Mercury.

NASA’s Mariner 2 spacecraft passed near Venus in 1962 and measured its temperature. The Soviet Union’s unmanned Venera 2 spacecraft passed near Venus in 1966 and Venera 3 crashed into the surface later that year. Venera 4 dropped a capsule onto the surface in 1967 and NASA’s Mariner 5 passed near Venus the same year. The first spacecraft to land on Venus was Venera 7 in 1970. Mariner 10 took the first close-up image of the planet in 1974. Venera 9 landed on Venus in 1975 and transmitted a photograph of the surface. Venera 10 landed the same year, photographed the surface, measured atmospheric pressure, and measured the composition of the surface rocks. NASA’s Pioneer Venus 1 began its orbit of Venus in late 1978 and transmitted radar images of the planet, mapped its surface, and measured its temperature. Pioneer Venus 2 measured the atmospheric density and composition the same month. Venera 11 and 12 entered the lower atmosphere and relayed data on its composition later that year. Other probes have since reached Venus to transmit photographs and analyze soil samples.

• Venus Facts: Facts and images about the planet Venus.
• Geography of Venus: Photos and facts about the geography of the planet Venus.

Earth is the fifth-largest planet in the solar system and the third from the Sun. The Earth’s diameter is 13,000 kilometers and its orbit keeps it 150 million kilometers from the Sun. The Earth is 147.1 million kilometers from the Sun in January and 152.1 million kilometers from the Sun in July. The Earth rotates on its axis in 24 hours, or a solar day, and orbits the Sun in 365 days, 6 hours, 9 minutes, and 9.54 seconds. The Earth has one natural satellite, the Moon. The Earth is comprised of a crust, with a rocky mantle beneath the crust, and a core in the center of the planet. The outer core is liquid and the inner section is solid. Seventy-one percent of the Earth’s crust is covered by water. The atmosphere is made up of nitrogen, oxygen, argon, water vapor, and carbon dioxide.

• Planet Earth: An e-book on the geography of the planet Earth.
• The Earth from Space: Photos of the Earth from space. Also includes information on the Earth’s surface and atmosphere.

Mars, the fourth planet from the Sun, is 4.5 billion years old. The iron-rich minerals of Mars’ surface make it appear red or rust-colored when viewed from Earth. Mars has a radius of 3,390 kilometers and rotates on its axis once every 24 hours, 39 minutes, and 35 seconds. Mars’ orbit takes it from a distance of 206,620,000 kilometers to 249,230,000 kilometers from the Sun. Mars orbits the Sun in 687 Earth days. Mars’ surface, or curst, is made of basalt and andesite, its mantle is peridotite, and its solid core is iron, nickel, and sulfur. The temperature on the surface of Mars ranges from -195°F to 70°F, with an average temperature of -80°F. The surface of Mars has mountains, plains, canyons, volcanoes, and many impact craters. The atmosphere of Mars is carbon dioxide, nitrogen, argon, carbon monoxide, water vapor, and oxygen. Mars has two moons, Phobos and Deimos.

NASA’s Mariner 4, 6, and 7 spacecraft flew near Mars and took pictures of its surface in 1694 and 1969. Mariner orbited Mars in 1971 and mapped 80 percent of its surface. NASA’s Viking orbiter and landers reached Mars in 1976. The two landers visited the surface to take photos and soil samples. NASA’s Pathfinder probe and Mars Global Surveyor orbiter were launched in 1996 and landed on the surface. The Pathfinder carried Sojourner, a rover, which analyzed the soil and rocks on the surface. NASA launched the Mars Odyssey probe in 2001 to analyze the chemical composition of the surface and rocks below the surface, to look for evidence of ice, and to study radiation. The Odyssey probe discovered water ice beneath the surface of the southern pole. The European Space Agency launched the Mars Express mission in 2003, which released the lander Beagle 2 and transmitted images of the planet. NASA’s rovers, Spirit and Opportunity, took photos of Martian land features and analyzed soil samples. The rovers found evidence that Mars once had liquid water on its surface.

• Mars Facts: Information on the planet Mars and the Mars Exploration Rovers.
• Chronology: Chronology of astronomy and space exploration related to Mars. Also includes a page on Mars in popular culture.

Jupiter is the solar system’s largest planet, with a diameter of 142,984 kilometers, and the fifth planet from the Sun. Jupiter’s orbit keeps the planet an average of 778,570,000 kilometers from the Sun. Jupiter orbits the Sun in 4,333 Earth days and rotates around its axis in 9 hours and 56 minutes. Jupiter is classified as a gas giant: it is a ball of gas, mostly hydrogen and helium, and liquid with a surface of dense clouds. There is no solid surface or crust. Jupiter’s atmosphere also contains methane, ammonia, phosphine, water, ethane, carbon monoxide, acetylene, and germanium. The gases in the atmosphere form rings and spots that are visible from Earth. Jupiter is heavier than other planets in the solar system: its mass is 318 times the mass of the Earth. Jupiter has a low density, about one quarter of Earth’s density. The temperature on Jupiter ranges from -230°F on the top of its clouds to 70°F in the lower atmosphere. The core temperature of Jupiter is 43,000°F. Jupiter has 16 larger satellites and many smaller ones. The four largest of Jupiter’s moons are named Io, Europa, Canymede, and Callisto.

NASA has launched six probes to Jupiter. Pioneer 10 flew near Jupiter in 1973 and analyzed the atmosphere. Pioneer-Saturn transmitted photographs of the Polar Regions and the Great Red Spot in 1974. Voyager 1 and 2 transmitted information on the atmosphere and mapped Jupiter’s moons. Ulysses passed Jupiter in 1992 and transmitted information on solar wind. Galileo released a probe into the atmosphere of Jupiter in 1995 to measure the chemical composition of the atmosphere.

• Jupiter: Jupiter facts and images.
• Exploring Jupiter: Information on Jupiter and its moons.

Saturn is the second-largest planet in the solar system, with a diameter of 120,540 kilometers. Saturn’s distance from the Sun ranges from 1,514,500,000 kilometers to 1,352,550,000 kilometers. Saturn orbits the Sun in 10,759 Earth days. Saturn rotates on its axis in 10 hours and 39 minutes. Saturn is a gas giant, with no solid surface. Saturn may have a solid core of iron and rock surrounded by a layer of methane, water, and ammonia and another of liquid hydrogen. A viscous region of hydrogen and helium surround this outer core. Saturn’s surface and atmosphere are made of gaseous hydrogen and helium. The surface of Saturn is covered with cloud layers that form belts or bands of different colors. Saturn has seven flat rings that orbit the planet. The temperature on the surface is -285°F. The temperature below the clouds is hotter. Saturn has 25 natural satellites over 10 kilometers in diameter and additional smaller satellites.

NASA’s Pioneer-Saturn probe reached Saturn in 1979 and transmitted close-up images of the planet and data about its outer rings. The probe also discovered the planet’s magnetic field and radiation in the Saturn’s magnetosphere. Voyager 1 and Voyager 2 flew near Saturn in 1980 and 1981 and discovered the seventh ring of Saturn. The Voyager probes also found that the larger rings are composed of smaller ringlets and explored some of Saturn’s moons. NASA’s Cassini probe went into orbit around Saturn in 2004 and launched a smaller probe, Huygens, to land on Titan, one of Saturn’s moons.

• Saturn: Facts about Saturn and the history of astronomy.
• Saturn Overview: Information about Saturn’s atmosphere, radiation, interior, and ring system.

Uranus is the seventh planet from the Sun. Its orbit keeps it an average distance of 2,872,460,000 kilometers from the Sun. Uranus orbits the Sun in 30,685 Earth days. Light from the Sun takes 2 hours and 40 minutes to travel to Uranus. Uranus is a gas giant, a ball of gas and liquid with a diameter of 51,118 kilometers. Uranus does not have a solid surface. The surface of Uranus is covered with blue-green clouds of frozen methane crystals, below which are thicker clouds of ammonia crystals and liquid water. There may be an ocean of liquid water and ammonia below the clouds with a rocky core beneath. The core of rock and ice makes Uranus an “ice giant”. The ocean and core rotate on the planet’s axis in 17 hours and 14 minutes, but the atmosphere rotates faster. The atmosphere of Uranus contains hydrogen, helium, methane, and ethane and its temperature is -355°F. The temperature of the oceans is 4200°F and the core may reach 126, 00°F. Uranus has 27 known satellites and a planetary ring system, with 13 known rings. The largest of the satellites are Miranda, Ariel, Umbriel, Oberon, and Titania. NASA’s Voyager 2 spacecraft flew near Uranus in 1986 and discovered ten of Uranus’s moons and the ring system. There have not been any other space probes or missions to Uranus.

• Uranus Facts: Statistics, exploration, atmosphere, and moons of Uranus.
• Uranus: Information on the features of Uranus, its orbit, and atmosphere.

Neptune is the eighth planet from the Sun, although Pluto’s orbit takes it inside Neptune’s for a 20-year period every 248 years. Neptune’s orbit keeps it an average of 4,495,060,000 kilometers from the Sun. Neptune orbits the Sun in 165 Earth years. Neptune rotates on its axis every 16 hours and 7 minutes. Neptune has a diameter of 49,528 kilometers. Neptune is comprised of hydrogen, water, helium, and silicates. The surface of Neptune is covered with thick blue clouds of frozen methane and lower cloud layers of hydrogen sulfide. The interior of the planet is made of dense gases and a liquid layer that surrounds the planet’s core of ice and rock. Neptune is categorized as an “ice giant” because of its core. Neptune has 11 moons and 4 rings. The largest of the moons is Triton. Triton orbits Neptune in the opposite direction from that of Neptune.

NASA’s Voyager 2 spacecraft passed near Neptune in 1989 and transmitted images of the planet and two of its moons, Triton and Nereid. The spacecraft also transmitted information on Neptune’s atmosphere and magnetic field. Voyager 2 discovered six new moons and three rings.

• Neptune: Facts about Neptune, its orbit, and its atmosphere.
• Seasons on Neptune: Images of Neptune and Uranus, and information on Neptune’s seasons.

Pluto is a dwarf planet that orbits the Sun beyond Neptune. The area of Pluto’s orbit also includes the Kuiper belt, a region of icy objects called Kuiper belt objects. Pluto was considered a planet from its discovery in 1930 until the discovery of the Kuiper belt, and was reclassified a dwarf planet in 2006. Pluto’s orbit keeps it an average of 5,869,660,000 kilometers from the Sun. For a twenty-year period every 248 Earth years, Pluto’s orbit moves it inside Neptune’s orbit. Pluto rotates on its axis in six Earth days. Pluto has a diameter of 2,300 kilometers. The temperature on Pluto’s surface is -375°F. Pluto’s surface is made of frozen methane gas, with a thin atmosphere of methane. Pluto has three moons, Charon, Nix, and Hydra. The only mission to Pluto, NASA’s New Horizons mission, was launched in 2006 and will approach Pluto in 2015.

• Is Pluto a Planet: Information on Pluto and whether it fits the definition of a “planet”.
• Pluto: An overview of Pluto and its place in the solar system.

Exploration of the solar system continues, with many more things to discover. There is much more information on the planets and the Sun than contained in this article. The resources below will provide more information on them, as well as on other features of the solar system, such as comets, asteroids, and meteors.

• The Solar System: An introduction to our solar system. Includes modules on each planet and an overview of astronomy, both ancient and modern.
• Amazing Space: Astronomy basics and teaching tools about the solar system.
• Voyager: Information on the Voyager spacecraft, its mission, and facts about Jupiter, Saturn, Uranus, and Neptune.
• Asteroids: Information on asteroids, meteors comets, and other features of our solar system.
• Planets and Stars: Information on the planets in our solar system, stars, asteroids, and comets.
• Meteor Showers: Information about meteor showers and how to observe them.
• Meteorites: Characteristics of meteorites and how to identify them.
• Meteors and Meteorites: The definition of meteors and meteor showers.  
• All about Comets: The history of comets, their origin, and characteristics.
• Space Stations: An overview of the history of space stations.
• Space Flight: Information on space flights, space shuttles, and space stations.