General

• One of five planets known to ancient astronomers; in mythology Mercury was the fleet-footed messenger of the gods, a fitting name for a planet that moves quickly across the sky
  
  
• Closest planet to the Sun; second smallest planet in the solar system (larger only than Pluto)
  
  
• Visited by only one spacecraft: NASA's Mariner 10 (1974-75), which examined less than half the surface in detail

Physical Characteristics

• Diameter is 4,878 kilometers (3,031 miles), about one-third the size of Earth and slightly larger than our Moon
  
  
• Densest planet in the solar system (when corrected for compression), with density 5.3 times greater than water
  
  
• Largest known crater on Mercury's pockmarked surface is the Caloris Basin (1,300 kilometers or 800 miles in diameter), likely created by an ancient asteroid impact
  
  
• Surface is a combination of craters, smooth plains and long, winding cliffs
  
  
• Possible water ice on the permanently shadowed floors of craters in the polar regions
  
  
• Enormous iron core takes up 60 percent of the planet's total mass - twice as much as Earth's

Environment

• Experiences the solar system's largest swing in surface temperatures, from highs above 450 degrees Celsius (840 degrees Fahrenheit) to lows below -212 C (-350 F)
  
  
• Only inner planet besides Earth with a global magnetic field, though Mercury's field is about 100 times weaker than Earth's (at the surface)
  
  
• Extremely thin atmosphere contains hydrogen, helium, oxygen, sodium, potassium and calcium

Orbit

• Average distance from the Sun is 58 million kilometers (36 million miles), about two-thirds closer to the Sun than the Earth
  
  
• Highly elliptical (elongated) orbit, ranging from 46 million kilometers (29 million miles) to 70 million kilometers (43 million miles) from the Sun
  
  
• Orbits the Sun once every 88 Earth days; moving at an average speed of 48 kilometers (30 miles) per second, it's the "fastest" planet in the solar system
  
  
• Rotates on its axis once every 59 Earth days, but because of its slow rotation and fast speed around the Sun, one solar day (from noon to noon at the same place) lasts 176 Earth days, or two Mercury years
  
  
• Distance from Earth (during MESSENGER's orbit) ranges from about 87 million to 212 million kilometers, about 54 million to 132 million miles