![]() ![]() Space agencies often use gravitational boosts from other planets to gain speed without a high fuel cost. Improvements in propulsion aren't the most important determining factor in terms of how long the voyage will take. Traveling to a planet isn't like driving across the country. But only a few spacecraft have traveled to it. Mercury isn't that far away, as far as planets go. When they are at their closest, they are only 48 million miles (77.3 million km) apart. When both planets are on opposite sides of the sun, they can achieve their maximum distance of 137 million miles (222 million kilometers). The constant motions of planets around the sun means that the distance between Earth and Mercury is in constant flux. "These instruments help us make our observations, despite any earthly obstacles," Gurman said. NASA's Solar Dynamics Observatory and the Japanese Aerospace Exploration Agency-led Hinode solar mission also captured Mercury crossing the sun. Gurman is project scientist for NASA's Solar and Heliospheric Observatory (SOHO) project scientist, one of the instruments. If you were in a place that had bad weather, for example, you missed your chance and had to wait for the next one," Joseph Gurman said in a statement. (Venus' transits are significantly more rare, occurring only once every 243 years.)ĭuring the 2016 transit, three spacecraft observed the display. Astronomers used transits to help calculate the distance to Earth from the sun. Mercury only crosses the sun once every seven years. But the inclination of Mercury's orbit is such that a transit does not occur every time the two planets pass. Like Venus, Mercury also undergoes transits, passing between the Earth and the sun about 13 times each century. Italian astronomer Galileo Galilei could detect the phases of Venus with his telescope, but not the phases of Mercury. However, the phases of Mercury are more difficult to detect than the phases of Venus, which also orbits between the Earth and the sun. (Image credit: NASA's Goddard Space Flight Center/SDO/Genna Duberstein) Mercury and the sunīecause Mercury lies between Earth and the sun, it undergoes phases much like the moon. This image is a composite view of Mercury during the 7.5 hour event as seen by NASA's Solar Dynamics Observatory during the event. Mercury crossed the face of the sun on during a rare transit that delighted skywatchers around the world. It wasn't until Nicolaus Copernicus established that planets traveled around the sun rather than around the Earth that the matter was cleared up. Astronomers charting the motion of Mercury across the sky would see it moving backward over the course of several nights in relation to other stars in the sky. This gives Earth time to catch up to it about once every three and a half months. When Mercury is at its farthest from the sun, it is traveling the slowest. This bizarre orbit confused early astronomers as they tried to piece together how the solar system worked. (Pluto's orbit is even more eccentric, but Mercury holds the "most elliptical planetary orbit" since the far-flung body was downgraded to a dwarf planet Pluto's downgrade also made Mercury the smallest of the eight remaining planets.) When it is closest to the sun, it is only 29 million miles (47 million km), but at its farthest, the distance to Mercury is 43 million miles (70 million km). Mercury boasts an orbit that is the most elliptical of all of the planets, stretched out from a perfect circle. ![]()
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