Mercury

Credit: NASA/USGS/JHUAPL

Mercury is the closest planet to the Sun in Earth’s solar system, but has no moons and no substantial atmosphere. Mercury is named after the Roman swift-footed messenger God.

Diameter: 4,879 km
Mass: 3.30 x 10^23 kg (5.5% Earth)
Moons: None
Orbit Distance: 57,909,227 km (0.39 AU)
Orbit Period: 88 days
Surface Temperature: -173 to 427°C
First Record: 14th century BC
Recorded By: Assyrian Astronomers

As the planet nearest the sun, the surface of Mercury can reach a scorching 840 degrees F (450 degrees C). However, since this world doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 degrees F (minus 170 degrees C), a more than 1,100 degrees F (600 degree C) temperature swing that is the greatest in the solar system.

Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. For instance, about 4 billion years ago, a roughly 60-mile-wide (100-kilometer-wide) asteroid struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 kilometers) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas.

NASA's $446 million Messenger probe (which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was launched in 2004 and has been in orbit around Mercury since 2011.

Significant Dates:

• 1631: Thomas Harriott and Galileo Galilei observe Mercury with the newly invented telescope.
• 1631: Pierre Gassendi uses a telescope to watch from Earth as Mercury crosses the face of the sun.
• 1965: Incorrectly believing for centuries that the same side of Mercury always faces the sun, astronomers find that the planet rotates three times for every two orbits.
• 1974-1975: Mariner 10 photographs roughly half of Mercury's surface in three flybys.
• 1991: Scientists using Earth-based radar find signs of ice locked in permanently shadowed areas of craters in Mercury's polar regions.
• 2008-2009: MESSENGER observes Mercury during three flybys.
• 2011: MESSENGER begins its orbital mission at Mercury, yielding a treasure trove of images, compositional data and scientific discoveries.

Credit: Karl Tate/Space.com

Reference: Karl Tate/spacedotcom/Nasa Solar System