The Planet Mercury

The history of physics from ancient times to the modern day, focusing on space and time. We physically explored all of the known planets in the Solar System in the 20th century. There have only been two missions to Mercury, in 1973 and 2011. Another mission is due next year. Mercury is small and made of rock.

Last updated on 5th August 2017 by Dr Helen Klus

1. Characteristics of Mercury

Mercury is the closest planet to the Sun. It takes about 88 days to orbit the Sun, and only half a day goes by on Mercury during this time. Mercury is the smallest and least massive planet in the Solar System, and its orbit is the most eccentric[1]. This was something that English natural philosopher Isaac Newton's theory of gravitation could not explain but German-Swiss-American physicist Albert Einstein's theory of general relativity could.

Photograph of Mercury.

Mercury, image taken by the MESSENGER orbiter. Image credit: NASA/Public domain.

Mercury is a rocky planet, similar to Earth's Moon. It has at least as many craters and is covered with dust. It has no satellites of its own, and, like the Moon, it is not massive enough to sustain an atmosphere. Unlike the Moon, however, it has a massive iron core, which generates a magnetic field about 1% as strong as the Earth's[2a].

Radar observations conducted by the Goldstone antenna in California and the Very Large Array in New Mexico, in the early 1990s, suggested that frozen water exists at the bottom of deep craters on Mercury's poles[3].

Despite the fact that Mercury is bright when viewed from Earth, it's difficult to observe as it's usually hidden in the light of the Sun. It is best viewed early in the morning or at twilight.

The Ancient Greeks gave Mercury two names: Apollo, which was visible at sunrise, and Hermes, which was visible at sunset. Hermes was later named Mercury by the Romans, and was considered the messenger of the gods[4].

Planets in the Solar system, sizes are approximately to scale. Jupiter is the largest planet, followed by Saturn.

The planets, sizes approximately to scale. Image credit: Dave Jarvis/CC-SA.

The four inner planets, Mercury, Venus, Earth, and Mars, and the Moon. Sizes are approximately to scale, and the composition is shown.

The four inner planets to scale. Image credit: NASA/Public domain.

2. Missions to Mercury

Only two spacecraft have ever been to Mercury. These are NASA's Mariner 10 and MESSENGER spacecraft. Mariner 10 launched in 1973, and mapped almost half of its surface by 1975[2b]. MESSENGER launched in 2004, and orbited Mercury between 2011 and 2015. It mapped the whole surface[5] and found further evidence of frozen water[6].

The next mission to Mercury, BepiColombo, is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It is due to launch in 2018, and should arrive in 2024.

Coloured photograph of Mercury, the colours are enhanced.

Enhanced colour map of Mercury, composed using data taken by MESSENGER. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Public domain.

3. References

  1. NASA, 'Planetary Fact Sheet', last accessed 01-06-17.

  2. (a, b) NASA, 'Mercury: In Depth', last accessed 01-06-17.

  3. NASA, 'Ice on Mercury', last accessed 01-06-17.

  4. ESO, 'Mercury and Mythology', last accessed 01-06-17.

  5. NASA, 'MESSENGER Has Imaged 100 Percent of Mercury', last accessed 01-06-17.

  6. NASA, 'MESSENGER Finds New Evidence for Water Ice at Mercury's Poles', last accessed 01-06-17.

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The Star Garden is a science news and science education website run by Dr Helen Klus.

How we came to know the cosmos covers the history of physics focusing on space and time, light and matter, and the mind. It explains the simple discoveries we made in prehistoric times, and how we built on them, little by little, until the conclusions of modern theories seem inevitable. This is shown in a timeline of the universe.

The Star Garden covers the basics for KS3, KS4, and KS5 science revision including SATs, GCSE science, and A-level physics.

Space & Time

Pre 20th Century theories

1. History of Constellations

2. History of Latitude

3. History of Longitude

4. Models of the Universe

5. Force and Energy

6. Newton's theory of Gravity

7. Age of the Universe

20th Century discoveries

1. Special Relativity

2. General Relativity

3. Big Bang theory

4. History of Galaxies

5. Life Cycles of Stars

6. Red Giants and White Dwarfs

7. Neutron Stars and Black Holes

Missions to planets

1. The planet Mercury

2. The planet Venus

3. The planet Earth

3.1 The Earth's Moon

4. The planet Mars

4.1 The Asteroid Belt

5. The planet Jupiter

6. The planet Saturn

7. The planet Uranus

8. The planet Neptune

Beyond the planets

1. Kuiper Belt and Oort Cloud

2. Pioneer and Voyager

3. Discoveries of Exoplanets