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Messages from Earth
  

Space Topics

Earth


Earth, the third planet from the Sun, is our home. It is the only world in the universe known to host life. Living things have been here for at least 3.5 of the 4.5 billion years of Earth's history and are found everywhere on the planet, from the air to the oceans, from mountaintops to ocean basins, from polar glaciers to desert sands, and from the surface to many kilometers below it, deep within rocks. This "living skin" is called the biosphere, and it has profoundly influenced the chemistry, geology, and climate of our planet over most of its history.

Liquid water, which we take for granted, is what sets Earth apart from all the other planets. Earth's oceans cover 70 percent of its surface, leaving less than one third of it as dry land. (For that reason, Earth, peculiarly, is less well mapped from space than the Moon, Venus, or Mars, because 70 percent of Earth's surface is hidden from view, underwater.) The water acts as an important buffer to the Earth's temperature and climate.

The geologic history of Earth has been shaped by the motions of tectonic plates. Colliding plates build mountains, and diverging plates open ocean basins. Earth has also been shaped by asteroid impacts that dig craters and, occasionally, exterminate some (or even most) of the life on the planet. Historically, geologic processes internal to Earth have occasionally caused great loss of human life; but even the greatest of earthquakes and volcanic eruptions cannot match the potential disaster caused by a major asteroid impact. Fortunately, humans have the power to mitigate these disasters and prevent loss of life -- if they choose to do so.

Earth's history has also been shaped by the presence of its enormous Moon. The Moon is fully one-quarter the diameter of Earth; the only planet with a bigger moon, relative to its own diameter, is Pluto. The large size of the Moon causes tides in Earth's oceans and, less noticeably, in its rocks.

There are more than three thousand spacecraft currently orbiting Earth. Spacecraft in geostationary orbits provide near-real-time views of the entire globe. And many planetary exploration spacecraft have shot photos of Earth and the Moon as they left our neighborhood, or flew by. These spacecraft views have given humans new respect for Earth as a planet, a "pale blue dot" floating in the blackness of space.

Earth Numbers
Size: 5th largest planet - 12,756 kilometers (7,654 miles)
Orbit: 149,600,000 kilometers (89,760,000 miles)
Axial tilt: 23.45 degrees
Number of moons: 1