Mercury as seen by the Messenger probe January 14 2008 model of the heavens in the 16th century explained why Mercury and Venus always appear in close proximity to the Sun expressed a deathbed regret that he had never set eyes on the planet Mercury himself AN INTRODUCTION TO MERCURY Until the last part of the 20th century Mercury was one of the least-understood planets and even now the shortage of information about it leaves many basic questions unsettled unsettled Indeed the length of its day was not determined until the 1960s and Mercury’s nearness to the Sun gave scientists 10
Mercury scientists bound to Earth many observational hurdles which were overcome only by the Messenger MErcury Surface Space ENvironment GEochemistry and Ranging probe Messenger was launched in 2004 flew past the planet twice in 2008 and once in 2009 and settled into orbit in 2011 It mapped the entire surface of Mercury before crashing into the planet in 2015 Mercury’s proximity to the Sun has also been exploited to confirm predictions made by relativity theory about the way gravity affects space and time At first glance the surface of the planet looks similar to the cratered terrain of the Moon an impression reinforced reinforced by the roughly comparable size of the two bodies Mercury is far denser however having a metallic core that takes up about 61 percent of its volume compared with 4 percent for the Moon and 16 percent for Earth Moreover its surface shows significant differences from lunar terrain including a lack of the massive dark-coloured lava flows known as maria and the presence of buckles and scarps that suggest Mercury is shrinking Mercury is an extreme planet in several respects Because of its nearness to the Sun its average orbital distance is 58 million km 36 million miles it has the shortest year a revolution period of 88 days and receives the most intense solar radiation of all the planets With a radius of about 2,440 km 1,516 miles Mercury is the smallest major planet smaller even than Jupiter’s largest 11
The Inner PlaneTs moon Ganymede or Saturn’s largest moon Titan In addition Mercury is unusually dense Although its mean density is roughly that of Earth’s it has less mass and so is less compressed by its own gravity when corrected for self-compression Mercury’s density is the highest of any planet Nearly two-thirds of Mercury’s mass is contained in its largely iron core which extends from the planet’s centre to a radius of about 2,100 km 1,300 miles or about 85 percent of the way to its surface The planet’s rocky outer shell its surface crust and underlying mantle is only some 300 km 200 miles thick THE CHALLENGES WITH OBSERVING THE PLANET As seen from Earth’s surface Mercury hides in dusk and twilight never getting more than about 28 in angular distance distance from the Sun It takes about 116 days for successive elongations i e for Mercury to return to the same point relative to the Sun in the morning or evening sky This is called Mercury’s synodic period Its nearness to the horizon horizon also means that Mercury is always seen through more of Earth’s turbulent atmosphere which blurs the view Even above the atmosphere orbiting observatories such as the Hubble Space Telescope are restricted by the high sensitivity sensitivity of their instruments from pointing as close to the 12
Mercury Sun as would be required for observing Mercury Because Mercury’s orbit lies within Earth’s it occasionally passes directly between Earth and the Sun This event in which the planet can be observed telescopically or by spacecraft instruments as a small black dot crossing the bright solar disk is called a transit and it occurs about a dozen times in a century Mercury also presents difficulties to study by space probe Because the planet is located deep in the Sun’s gravity gravity field a great deal of energy is needed to shape the trajec tory of a spacecraft to get it from Earth’s orbit to Mercury’s in such a way that it can go into orbit around the planet or land on it The first spacecraft to visit Mercury Mariner 10 was in orbit around the Sun when it made three brief flybys of the planet in 1974 75 In developing subsequent missions to Mercury such as the U S Messenger spacecraft launched in 2004 spaceflight engineers calculated complex routes making use of gravity assists from repeated flybys of Venus and Mercury over the course of several years In the Messenger mission design after conducting observations observations from moderate distances during planetary flybys in 2008 and 2009 the spacecraft entered into an elongated orbit around Mercury for close-up investigations in 2011 In addition the extreme heat not only from the Sun but also reradiated from Mercury itself challenged spacecraft designers to keep instruments cool enough to operate 13
