As Earth's only natural satellite, the moon holds a unique fascination that has already inspired dozens of missions and scientific studies. Still, the moon contains many mysteries we have yet to uncover. What is the moon's composition? How did the moon evolve? Does it have a solid core? What's the true nature of its odd gravity field? These are just some of the questions NASA's GRAIL mission, which launched in September 2011, will help to answer. The mission's twin spacecraft will fly together around the moon to measure its gravity field and better understand its origin and evolution.

These fun facts and activities are designed to give you an exciting and hands-on guide to the moon, plus an introduction to NASA's GRAIL mission, which will uncover some of the moon's most hidden secrets. Start reading and see what you discover. Or click the links below to go directly to fun facts and activities you want to explore most!

› Cool Things About the Moon
› Cool Things About the GRAIL Mission
› Cool Things About the GRAIL Spacecraft

Cool Things About the Moon

SIZE
With a surface area of about 14.6 million miles (more than four times the size of China!) our moon is the fifth largest satellite in the solar system.

LOCATION
The moon is our only known natural satellite, which means it's the only natural object that orbits Earth. At its closest, it is 223,700 miles away from Earth. So if you were riding in a car going 70 miles per hour, it would take you 135 days to get there. Imagine how long it would take you when the moon is at its farthest from Earth: 251,700 miles!

APPEARANCE
Because of the speed at which the moon rotates, we only ever see one side of it from Earth. The side that we cannot see is sometimes called the "far side" or "dark side" even though it's not actually dark. If you look up at the moon from Earth, you may see light and dark spots which look a lot like a face -- or a "man on the moon." These features are actually craters (white spots) and plains called maria (dark spots) formed by ancient volcanic eruptions.

TEMPERATURE
Temperatures on the moon are much more extreme than on Earth. During the day, it can get as hot as 273 degrees Fahrenheit at the moon's equator and dip to minus 244 degrees Fahrenheit at night.

SURFACE & COMPOSITION
Just like Earth, the moon has layers: a core at the center, a mantle just beyond that and an outer layer, the crust. Scientists know very little about the moon's interior, a mystery that NASA's GRAIL mission will study when it arrives at the moon in early 2012. The surface, which has been studied in much more detail by astronauts and spacecraft alike, is full of unique features like giant craters - one of the largest known craters in the solar system, in fact, with a diameter of 1,553 miles - and plains called maria caused by ancient volcanic eruptions.

GRAVITY
The moon's gravitational pull - the force that attracts, or pulls on, other objects - creates ocean tides on Earth. Each day on Earth, every ocean in the world experiences two high tides and two low tides. These are caused by the changing gravitational forces as the sun, moon and Earth rotate in space.

PHASES
The moon orbits Earth at an average speed of 2,288 miles per hour! As it orbits Earth, it goes through a series of phases that we can see from Earth: new moon, crescent moon, gibbous moon and so on. These phases are caused by the positions of the Earth, moon and sun as they travel on their orbits. It takes the moon 29.5 days to go through the complete cycle of its phases - new moon to new moon. This is called a lunar month. If you were standing on the moon, it would take the same amount of time - 29.5 days - to see the sun rise and then set.

LEARN MORE - EDUCATIONAL ACTIVITIES
› Lunar Math
› Distance to the Moon
› Diameter of the Moon
› Moon Phases
› Lunar Surface
› How to Make a Crater
› Reaping Rocks
› Video: NASA Brain Bites - Why Do We Only See One Side of the Moon?
› Lithograph: Earth's Moon

Cool Things About the GRAIL Mission

NAME
The mission's name, GRAIL, is an acronym that stands for Gravity Recovery And Interior Laboratory.

SCIENCE GOALS
Gravity -- the force that keeps us firmly planted on Earth -- isn't the same everywhere in the world. For example, if you stand on top of any mountain on Earth, your weight will decrease just slightly because gravity decreases the higher you go. Gravity also changes with depth, the density of rocks around you and your location on Earth. So studying gravity can help scientists understand all of those properties -topography, geology, altitude, depth and so on - on other worlds without actually visiting them. The GRAIL spacecraft will do just that: It will study the moon's gravity, so scientists can better understand what the moon is made of and how it might have evolved.

LAUNCH & MISSION PHASES
The two GRAIL spacecraft were launched on Sept. 10, 2011 from Cape Canaveral Air Force Station in Florida. They'll travel more than 2.5 million miles to reach the moon, arriving one day apart on Dec. 31, 2011 and Jan. 1, 2012. Soon after they arrive, the spacecraft will begin the science phase of their mission, gathering information on the moon's gravity field, which will help scientists learn about the composition and evolution of the moon. This is also when middle school classrooms can start using GRAIL's special MoonKAM camera to take photos of the moon.

MOON MISSIONS
The United States has sent dozens of successful missions to the moon. The first U.S. spacecraft to ever reach another body in the solar system beyond Earth was NASA's Ranger 4 spacecraft, which made a crash landing on the moon in 1962. Starting with the first human moon landing in 1969, the United States has explored the moon not only with unmanned spacecraft - spacecraft without humans on board -- but also through the work of astronauts for more than 40 years!

LEARN MORE - EDUCATIONAL ACTIVITIES
› Field Trip to the Moon
› Launch it!
› Build a Moon Habitat
› Moon Mining Activity
› Moon Anomalies
› Video: Astronauts Armstrong and Aldrin Arrive on the Moon

Cool Things About the GRAIL Spacecraft

ORBIT
The GRAIL mission will actually use two spacecraft to study the moon. They launched together on the same rocket, but they'll arrive at the moon one day apart (Dec. 31, 2011 and Jan. 1, 2012). Once they arrive, the two GRAIL spacecraft will go through a tricky maneuver so that they can follow each other - 40 to 140 miles apart -- on the same path around the moon. It will be the first time this kind of maneuver has been used around another body beyond Earth.

SIZE & WEIGHT
The twin GRAIL spacecraft are each about the size of a standard washing machine, but they weighed a lot more than a washing machine when they launched: 677 pounds each!

COMMUNICATION
We only ever see one side of the moon. When a spacecraft travels to the side that we can't see, we aren't able to talk to it because the signals we send from Earth are blocked by the moon. In order to study the entire moon, even the side we can't see, the GRAIL mission will fly two spacecraft around the moon. They'll constantly measure the distance between one another to determine the moon's gravity and talk to one another so they can continue to collect new data even when the Earth is not in view.

CAMERA
The two GRAIL spacecraft each carry a special camera that can take snapshots of the moon as they pass over its surface. What makes this camera system, called MoonKAM, especially unique is that it was built for middle school students, who will be able to sign up to control the cameras and tell the

LEARN MORE - EDUCATIONAL ACTIVITIES
› Roving on the Moon
› GRAIL MoonKAM