Our solar system consists of our star, the Sun, and everything bound to it by gravity — the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, dwarf planets such as Pluto, dozens of moons and millions of asteroids, comets and meteoroids. Our solar system orbits the center of the Milky Way Galaxy at about 515,000 mph (828,000 kph). We’re in one of the galaxy’s four spiral arms.

Mercury is the smallest planet in our solar system and nearest to the Sun at a distance of about 36 million miles (58 million kilometers) or 0.39 AU. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system – that title belongs to Venus. Mercury is only slightly larger than Earth's Moon and makes a complete orbit around the Sun (a year in Mercury time) in just 88 Earth days. Daytime temperatures can reach 430 degrees Celsius (800 degrees Fahrenheit) and drop to -180 degrees Celsius (-290 degrees Fahrenheit) at night. It is unlikely life as we know it could survive on this planet.

Venus, a second planet from the Sun and our closest planetary neighbor is slightly smaller than Earth (similar in structure and size) and the only planet in our solar system that rotates clockwise. Venus is also the hottest planet in our solar system with a surface temperature of over 450 degrees celsius. One day on Venus lasts 243 Earth days because Venus spins backwards, with its sun rising in the west and setting in the east.

The third planet from the Sun is Earth, our home planet, and the fifth largest planet in our solar system. Earth is the only place we know of so far that’s inhabited by living things and the only planet in our solar system with liquid water on the surface. Just slightly larger than nearby Venus, Earth is the biggest of the four planets closest to the Sun, all of which are made of rock and metal. Earth's distance from the Sun is about 93 million miles (150 million km). A day on Earth is 24 hours. Earth makes a complete orbit around the sun (a year in Earth time) in about 365 days. Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen and 1 percent other ingredients which makes the perfect balance to breathe and live. Our atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface.

Mars is a small rocky planet (about half the size of Earth) that makes a complete orbit around the Sun (a year in Martian time) in 687 Earth days. Mars is the fourth planet from the Sun at an average distance of about 228 million km (142 million miles) or 1.52 AU. One day on Mars takes a little over 24 hours. Mars is a planet with solid surface that has been altered by volcanoes, impacts, winds, crustal movement and chemical reactions. Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), argon (Ar), nitrogen (N2), and a small amount of oxygen and water vapor. Mars is also one of the most explored bodies in our solar system.

Jupiter is the largest planet in our solar system and more than twice as massive as all the other planets combined. More than 1,300 Earths would fit into Jupiter's vast sphere. Fifth in line from the Sun, Jupiter has more than 75 moons and cannot support life as we know it. But some of Jupiter's moons have oceans beneath their crusts that might support life.

Saturn is the sixth planet from the Sun and the second largest planet in our solar system. Saturn is unique among the planets surrounded by a spectacular ring system that stretch out into space. Saturn orbits at a distance of about 886 million miles (1.4 billion kilometers) from the Sun and cannot support life as we know it, but some of Saturn's 53 known moons have conditions that might support life. Saturn's atmosphere is made up mostly of hydrogen (H2) and helium (He). Saturn takes about 29 Earth years to orbit the Sun and has short day (about 10.7 hours).

The seventh planet from the Sun at a distance of about 1.8 billion miles (2.9 billion kilometers) is Uranus, a planet about four times wider than Earth. Uranus is an ice giant with atmosphere made mostly of molecular hydrogen and atomic helium, with a small amount of methane. Uranus spins lying on its side and takes about 17 hours to rotate once, which means a day on Uranus is 17 hours. Uranus takes about 84 Earth years to orbit the Sun (a Uranian year). Uranus was the first planet discovered by telescope in 1781 by astronomer William Herschel, although he originally thought it was either a comet or a star. It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode.

The eighth and most distant planet in our solar system is Neptune with a distance the from the Sun of about 2.8 billion miles (4.5 billion kilometers). Neptune takes about 16 hours to rotate once (a Neptunian day), and about 165 Earth years to orbit the sun (a Neptunian year). Neptune is a large planet, nearly four times the size of Earth with atmosphere made up mostly of molecular hydrogen, atomic helium and methane which gives Neptune the same blue color as Uranus. Neptune has six known rings and 13 moons.

The first Earthling to orbit Earth was dog Laika aboard the Soviet Union's Sputnik 2 in 1957. She did not survive the trip. A few years later, the next two Soviet space dogs Belka and Strelka became the first living creatures to return from space alive - paving the way for future human explorers.

NASA became operational on October 1, 1958 -- one year after the Soviets launched Sputnik 1, the world's first artificial satellite.

On January 31, 1958, Explorer 1 became the first artificial satellite launched into space by the United States. Onboard was a cosmic ray detector designed to measure the radiation environment in Earth orbit.

Pioneer 10, the first NASA mission to the outer planets, garnered a series of firsts perhaps unmatched by any other robotic spacecraft in the space era: the first vehicle placed on a trajectory to escape the solar system into interstellar space, the first spacecraft to fly beyond Mars, the first to fly through the asteroid belt, the first to fly past Jupiter, the first to use all-nuclear electrical power, and the first human-made object to fly beyond the orbit of the outermost known planet in our solar system.

Apollo 7, the first piloted Apollo mission, took place October 11-12, 1968, with astronauts Wally Schirra, Donn Eisele, and Walter Cunningham.

Galileo, launched in 1989, was the first U.S. spacecraft to orbit Jupiter. Galileo entered Jupiter's orbit in 1995.

Ganymede, one of the moons of Jupiter, is the largest moon in our Solar System.

The first U.S. woman to walk in space was Katherine Sullivan. During STS-41G, she and Commander Dave Leestma successfully conducted a 3-1/2 hour Extravehicular Activity (EVA) on October 11, 1984 to demonstrate the feasibility of actual satellite refueling.

The first African-American woman in space was Dr. Mae Jemison. She was selected for the astronaut program in June 1987 and served as the science mission specialist on STS-47 Spacelab-J (September 12-20, 1992).