Genre: Information/Explanation

NASA / TRACE / SMEX A transit is when one astronomical body passes in front of another . When Johannes Kepler was preparing tables of celestial events and planetary positions for the years 1629 to 1636 , he paid special attention to any possible transits of Mercury and Venus in front of the Sun . With the invention of the telescope , Kepler believed it would be possible to definitively observe a planetary transit . He calculated a Mercury transit for November 7 , 1631 . Kepler was not too sure about his calculations , so he urged astronomers to also observe both the day before and the day after . Kepler died on November 15 , 1630 . The next November astronomers eagerly awaited the transit . There was poor weather in Europe , so only a handful of astronomers saw the transit . About 9 AM on the 7th , just a few hours off from Kepler's prediction , a small spot started to move across the Sun . All who saw it thought it was a sunspot at first , because the then-accepted size of Mercury was much larger than reality . However , ideas about the size of Mercury ( and the other planets ) predated the telescope . The scale of things in the solar system changed that day . From Kepler's time on , the orbit of Mercury was determined more precisely . With Newton's law of gravitation , the orbits of the planets were explained . After the discovery of Uranus in 1781 , discrepancies in its orbit led to the prediction and discovery of Neptune in 1846 . French astronomer Urbain-Jean-Joseph Le Verrier began work on the Uranus problem in 1845 , and on September 23 , 1846 , he asked Johann Gottfried Galle of Berlin to look for the planet . Galle discovered Neptune that evening . With Uranus solved , Le Verrier turned his attention to the other big discrepancy in the solar system , the advancement of the perihelion of Mercury ( where Mercury is closest to the Sun ) . This point moved , and adding in the effects of all the other planets explained most but not all of this movement . Le Verrier knew the solution : there was another planet inside Mercury's orbit . On March 26 , 1859 , Edmonde Lescarbault , a French physician and avid amateur astronomer , saw a spot cross the Sun and took detailed notes . Lescarbault later read about Le Verrier's theory about Vulcan and contacted him . Le Verrier was convinced that Lescarbault had observed a new planet . After Le Verrier gave his stamp of approval to Lescarbault's observations , Vulcan became a heated subject in astronomy . Some claimed to have observed it ; others reported that they could see no such planet . Vulcan lost some of its luster as an explanation for Mercury's strange procession , but there was not really a better explanation available . The answer turned out to be something even more radical than a new planet . Since 1905 German physicist Albert Einstein had struggled to incorporate gravitation into his theory of relativity . In 1915 he succeeded . Gravity was not a force stretching across space as Newton had thought but mass causing a curvature in space-time , the very fabric of the universe . That November Einstein gave four lectures to the Prussian Academy of Sciences about his new theory of general relativity . In the third lecture , on the 18th , Einstein explained the perihelion of Mercury " without the special hypotheses that had to assume . " From first principles , Einstein calculated the advancement of Mercury's perihelion . ( He went on to figure out the perihelion advancement of Venus , Earth , and Mars but noted that their values seemed to be so small that only Mercury's could be observed . He graciously concluded his paper , " I will however gladly allow professional astronomers a final say . " ) Since Mercury is so close to the Sun , it is hard to see any surface features . On those occasions ( called elongations ) when Mercury was at its farthest from the Sun , the same vague surface features were always seen . Astronomers who attempted to map Mercury thus agreed that the planet probably had a rotation period as long as its orbital period . Its day was as long as its year : 88 days . Beginning on April 6 , 1965 , radio astronomers Gordon Pettengill and Rolf Dyce used the large 305-meter ( 1 , 000-foot ) radio telescope at Arecibo in Puerto Rico to bounce radio signals off the planet . They found that Mercury had a rotation period that was two-thirds of its year , or 58.7 days . The elongations of Mercury had been occurring every 350 days . This is close to six times its rotation period , so Mercury was always in the same position at elongation . NASA / JPL Mariner 10 was the first spacecraft to visit Mercury . It was launched in November 1973 and flew by Venus in February 1974 . It flew by Mercury twice that year , on March 29 and September 21 . During its final flyby on March 16 , 1975 , Mariner 10 came within 327 km ( 203 miles ) of Mercury's surface . Mariner 10 took the first close-up images of Mercury , but because it arrived when the same hemisphere was facing the Sun , it was able to map only about half the planet . However , Mariner 10 showed that Mercury is an airless cratered world , like the Moon . It also discovered the immense multiringed basin of Caloris , a remnant of a huge collision early in the solar system's history . Courtesy of John Harmon , Arecibo Observatory Scientists from the California Institute of Technology and the Jet Propulsion Laboratory on this date and later on August 23 made a radar map of Mercury , specifically the side that Mariner 10 did not photograph . They used the giant 70-meter ( 230-foot ) dish at Goldstone Deep Space Communications Complex as the transmitter and the 26 antennas of the Very Large Array as the receiver . Much to their surprise , they saw a strong reflection from Mercury's north pole . This reflection was similar to that seen from the polar ice caps of Mars and the ice-covered moons of Jupiter . Later observations by radar and the Messenger spacecraft ( see next item ) showed that despite Mercury's closeness to the Sun , ice – likely brought in cometary collisions – could survive at the bottom of permanently shadowed craters . If people ever visited Mercury , this ice would be a vital resource . NASA / JHU / APL / Carnegie Institution of Washington After Mariner 10's last flyby , no spacecraft visited Mercury until Messenger , which became the first spacecraft to orbit the planet . Messenger ( MErcury Surface , Space ENvironment , GEochemistry , and Ranging ) was launched in August 2004 and flew by Mercury three times before it settled into orbit . Messenger completely mapped Mercury's surface . It confirmed the water ice that had been seen by Arecibo . It also found evidence that there had been past volcanic activity and that the planet's core was much larger than previously believed , extending out 85 percent of the way to Mercury's surface . Messenger ran out of fuel and crashed onto the planet's surface in April 2015 .