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VISIBLE TO THE NAKED EYE AROUND SUNRISE AND SUNSET,
MERCURY WAS WELL KNOWN IN ANCIENT TIMES. IT IS THE
FASTEST-MOVING OF ALL THE PLANETS AND WAS NAMED
AFTER THE WINGED MESSENGER OF ROMAN MYTHOLOGY.
Mercury is so small and distant—and circles so close to the Sun— that it is difficult to see from Earth. Consequently, very little was known about the planet until comparatively recently. Although Mercury was first observed through a telescope by Italian scientist Galileo Galilei in the early 17th century, it was not until the late 20th century that telescopes with sufficient power to resolve surface details were developed. The big breakthrough in understanding the Sun’s closest neighbor came when spacecraft began to beam back close-ups from the planet. The first was Mariner 10, which made flybys in 1974 and 1975. This was followed, after a gap of more than 30 years, by the MESSENGER spacecraft, which remains in orbit today.
Babylon tablets The earliest known record of the observation of Mercury is on the Mul.Apin tablets—catalogs of celestial bodies from ancient Babylon. The Babylonians call the planet Nabu, after their messenger god.
Apollo and Hermes At first, the ancient Greeks believe that Mercury is two planets: they call it Apollo when it appears in the morning sky, and Hermes when they see it after sunset. In the 4th century BCE, they realize it is actually a single planet and name it Hermes
Mercury’s diameter By unknown means and without a telescope, an Indian astronomer estimates Mercury’s diameter with 99 percent accuracy—an astonishing achievement or a lucky guess. The result is recorded in the book Surya Siddhanta.
Galileo’s observations Galileo makes the first observations of Mercury through a telescope. He guesses it is a planet, but his telescope is not powerful enough to reveal that Mercury has phases, just like Venus and the Moon, and that these phases depend on how much we see of Mercury’s sunlit half.
Gassendi observes transit French astronomer Pierre Gassendi sees Mercury pass in front of the Sun. This is the first time the transit of a planet has been observed through a telescope. It enables Gassendi to make the first reliable measurement of a planet’s diameter.
Phases Italian astronomer Giovanni Zupi observes through a powerful telescope that Mercury has phases similar to those of Earth’s Moon. This proves that Mercury orbits the Sun, revealing varying amounts of its surface as it catches the Sun at different angles.
Occultation by Venus Occultations—when one planet passes in front of another, as seen from Earth—are rare events. English astronomer John Bevis sees the occultation of Mercury by Venus on May 28—the only time in history that this has been witnessed.
Clouds on Mercury German astronomer Johann Schröter claims, wrongly, to have seen features such as clouds and mountains on Mercury. Using Schröter’s drawings, the astronomer Friedrich Bessel estimates (wrongly) that Mercury spins at the same speed as Earth and tilts strongly.
Mercury by radar Soviet scientists led by Vladimir Kotelnikov at Moscow’s Institute of Radio-engineering and Electronics become the first to bounce a radar signal off Mercury and receive its echo, enabling them to make the first radar observations of the planet.
Rotation speed American astronomers Gordon Pettengill and Rolf Dyce use the radio telescope dish at Arecibo, Puerto Rico, to measure Mercury’s spin rate. From radar pulses reflected by the planet’s surface, they calculate that Mercury’s rotation is not tidally locked, as Schiaparelli had thought, but takes just 59 days—about two-thirds of its orbital period of 88 days. Most of Mercury has now been mapped by the Arecibo dish.
Mariner 10 NASA’s Mariner 10 is the first spacecraft to visit Mercury and photograph it up close. In three separate flybys, starting March 29, 1975, Mariner 10 images almost half of the planet’s surface, revealing a landscape similar to that of the Moon.
Skinakas Basin Astronomers at the Skinakas Astrophysical Observatory in Crete believe that they have found a giant crater missed by Mariner 10. Subsequently, the MESSENGER spacecraft shows that the crater, dubbed the Skinakas Basin, is in fact an illusion.
MESSENGER flyby Launched on August 3, 2004, NASA’s MESSENGER makes the first of its three flybys of Mercury in January 2008. During the flybys, MESSENGER maps most of the planet’s surface in color and studies the atmosphere and magnetosphere.
MESSENGER in orbit On March 18, MESSENGER goes into long-term orbit around Mercury. The craft completes its mapping of Mercury, discovers water at the planet’s north pole, and continues to send valuable data about Mercury back to Earth.
MISSIONS TO MERCURY MERCURY IS THE LEAST EXPLORED OF THE ROCKY PLANETS, VISITED BY JUST TWO MISSIONS TO DATE: MARINER 10 IN THE MID-1970S AND THE MORE RECENT MESSENGER SPACECRAFT, WHICH STUDIED MERCURY FROM ORBIT. One reason for the lack of missions to Mercury is the sheer technical difficulty. Spacecraft have to travel extremely fast to get to Mercury, and when they reach the planet, they must suddenly slow down enough to get into orbit just as the Sun’s gravity is trying to accelerate them even more. In addition, the Sun’s pull is so strong near Mercury that orbits around the planet are unstable, and proximity to the Sun makes it hard for spacecraft to maintain a stable temperature. Nonetheless, Mariner and MESSENGER have reached the planet successfully and studied its features and properties. A third major mission, the joint European– Japanese BepiColombo, may reveal more about this intriguing planet
Mariner 10 Mariner 10’s first Mercury flyby took place on March 29, 1974. Because getting a craft into orbit around the planet was so difficult, Mariner 10 was designed to orbit the Sun instead, enabling it to fly past Mercury three times. These flybys revealed a highly cratered surface and, to the great surprise of astronomers, a magnetic field around the planet.
MESSENGER MESSENGER (Mercury surface, space environment, geochemistry, and ranging) left Earth in 2004 but took over six years to achieve orbit around Mercury—the first craft ever to do so. On March 29, 2011, it sent the first photo from Mercury orbit. Since then, MESSENGER’s cameras and other instruments have returned a flood of data about the planet. Its investigations have discovered water ice and organic compounds in shadowed
MESSENGER’s journey MESSENGER had to circle the Sun seven times to get into its orbit around Mercury. It passed Earth a year after launch and then Venus twice, using both planets’ gravity to slingshot itself onward. It then made three flybys of Mercury to slow down before entering orbit. Its orbit is very eccentric: its lowest point is just 124 miles (200 km) above the surface, while the highest is
Surface topography MESSENGER has imaged the entire surface of Mercury and returned over 200,000 pictures. It has also mapped the topography of the northern hemisphere by using a laser altimeter to measure elevation. In the view below, looking toward the north pole, the lowest regions are shown in purple and the highest in white.
This map of the northern polar region covers an area about 1,320 miles (2,130 km) wide. It was produced by the spacecraft’s Mercury Laser Altimeter (MLA). The MLA fires eight laser pulses at Mercury each second, and the time taken for reflected light to return is used to calculate elevation. MESSENGER’s data provides evidence that the planet’s diameter has shrunk by 8.7 miles (14 km) over the last 4 billion years, warping the surface into wrinkles and the curved cliffs known as rupes.
Mercury is so small and distant—and circles so close to the Sun— that it is difficult to see from Earth. Consequently, very little was known about the planet until comparatively recently. Although Mercury was first observed through a telescope by Italian scientist Galileo Galilei in the early 17th century, it was not until the late 20th century that telescopes with sufficient power to resolve surface details were developed. The big breakthrough in understanding the Sun’s closest neighbor came when spacecraft began to beam back close-ups from the planet. The first was Mariner 10, which made flybys in 1974 and 1975. This was followed, after a gap of more than 30 years, by the MESSENGER spacecraft, which remains in orbit today.
Babylon tablets The earliest known record of the observation of Mercury is on the Mul.Apin tablets—catalogs of celestial bodies from ancient Babylon. The Babylonians call the planet Nabu, after their messenger god.
Apollo and Hermes At first, the ancient Greeks believe that Mercury is two planets: they call it Apollo when it appears in the morning sky, and Hermes when they see it after sunset. In the 4th century BCE, they realize it is actually a single planet and name it Hermes
Mercury’s diameter By unknown means and without a telescope, an Indian astronomer estimates Mercury’s diameter with 99 percent accuracy—an astonishing achievement or a lucky guess. The result is recorded in the book Surya Siddhanta.
Galileo’s observations Galileo makes the first observations of Mercury through a telescope. He guesses it is a planet, but his telescope is not powerful enough to reveal that Mercury has phases, just like Venus and the Moon, and that these phases depend on how much we see of Mercury’s sunlit half.
Gassendi observes transit French astronomer Pierre Gassendi sees Mercury pass in front of the Sun. This is the first time the transit of a planet has been observed through a telescope. It enables Gassendi to make the first reliable measurement of a planet’s diameter.
Phases Italian astronomer Giovanni Zupi observes through a powerful telescope that Mercury has phases similar to those of Earth’s Moon. This proves that Mercury orbits the Sun, revealing varying amounts of its surface as it catches the Sun at different angles.
Occultation by Venus Occultations—when one planet passes in front of another, as seen from Earth—are rare events. English astronomer John Bevis sees the occultation of Mercury by Venus on May 28—the only time in history that this has been witnessed.
Clouds on Mercury German astronomer Johann Schröter claims, wrongly, to have seen features such as clouds and mountains on Mercury. Using Schröter’s drawings, the astronomer Friedrich Bessel estimates (wrongly) that Mercury spins at the same speed as Earth and tilts strongly.
Mercury by radar Soviet scientists led by Vladimir Kotelnikov at Moscow’s Institute of Radio-engineering and Electronics become the first to bounce a radar signal off Mercury and receive its echo, enabling them to make the first radar observations of the planet.
Rotation speed American astronomers Gordon Pettengill and Rolf Dyce use the radio telescope dish at Arecibo, Puerto Rico, to measure Mercury’s spin rate. From radar pulses reflected by the planet’s surface, they calculate that Mercury’s rotation is not tidally locked, as Schiaparelli had thought, but takes just 59 days—about two-thirds of its orbital period of 88 days. Most of Mercury has now been mapped by the Arecibo dish.
Mariner 10 NASA’s Mariner 10 is the first spacecraft to visit Mercury and photograph it up close. In three separate flybys, starting March 29, 1975, Mariner 10 images almost half of the planet’s surface, revealing a landscape similar to that of the Moon.
Skinakas Basin Astronomers at the Skinakas Astrophysical Observatory in Crete believe that they have found a giant crater missed by Mariner 10. Subsequently, the MESSENGER spacecraft shows that the crater, dubbed the Skinakas Basin, is in fact an illusion.
MESSENGER flyby Launched on August 3, 2004, NASA’s MESSENGER makes the first of its three flybys of Mercury in January 2008. During the flybys, MESSENGER maps most of the planet’s surface in color and studies the atmosphere and magnetosphere.
MESSENGER in orbit On March 18, MESSENGER goes into long-term orbit around Mercury. The craft completes its mapping of Mercury, discovers water at the planet’s north pole, and continues to send valuable data about Mercury back to Earth.
MISSIONS TO MERCURY MERCURY IS THE LEAST EXPLORED OF THE ROCKY PLANETS, VISITED BY JUST TWO MISSIONS TO DATE: MARINER 10 IN THE MID-1970S AND THE MORE RECENT MESSENGER SPACECRAFT, WHICH STUDIED MERCURY FROM ORBIT. One reason for the lack of missions to Mercury is the sheer technical difficulty. Spacecraft have to travel extremely fast to get to Mercury, and when they reach the planet, they must suddenly slow down enough to get into orbit just as the Sun’s gravity is trying to accelerate them even more. In addition, the Sun’s pull is so strong near Mercury that orbits around the planet are unstable, and proximity to the Sun makes it hard for spacecraft to maintain a stable temperature. Nonetheless, Mariner and MESSENGER have reached the planet successfully and studied its features and properties. A third major mission, the joint European– Japanese BepiColombo, may reveal more about this intriguing planet
Mariner 10 Mariner 10’s first Mercury flyby took place on March 29, 1974. Because getting a craft into orbit around the planet was so difficult, Mariner 10 was designed to orbit the Sun instead, enabling it to fly past Mercury three times. These flybys revealed a highly cratered surface and, to the great surprise of astronomers, a magnetic field around the planet.
MESSENGER MESSENGER (Mercury surface, space environment, geochemistry, and ranging) left Earth in 2004 but took over six years to achieve orbit around Mercury—the first craft ever to do so. On March 29, 2011, it sent the first photo from Mercury orbit. Since then, MESSENGER’s cameras and other instruments have returned a flood of data about the planet. Its investigations have discovered water ice and organic compounds in shadowed
MESSENGER’s journey MESSENGER had to circle the Sun seven times to get into its orbit around Mercury. It passed Earth a year after launch and then Venus twice, using both planets’ gravity to slingshot itself onward. It then made three flybys of Mercury to slow down before entering orbit. Its orbit is very eccentric: its lowest point is just 124 miles (200 km) above the surface, while the highest is
Surface topography MESSENGER has imaged the entire surface of Mercury and returned over 200,000 pictures. It has also mapped the topography of the northern hemisphere by using a laser altimeter to measure elevation. In the view below, looking toward the north pole, the lowest regions are shown in purple and the highest in white.
This map of the northern polar region covers an area about 1,320 miles (2,130 km) wide. It was produced by the spacecraft’s Mercury Laser Altimeter (MLA). The MLA fires eight laser pulses at Mercury each second, and the time taken for reflected light to return is used to calculate elevation. MESSENGER’s data provides evidence that the planet’s diameter has shrunk by 8.7 miles (14 km) over the last 4 billion years, warping the surface into wrinkles and the curved cliffs known as rupes.
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