Mercury is the smallest planet in the solar system and the nearest from the Sun. Planet's name is taken from the name of the messenger god of ancient Rome. He was given the name because its motion in the sky very quickly.
Planet Mercury (Source: Wikipedia)
From Earth, Mercury can only be observed visually at a maximum distance of 28.3 degrees from the Sun. That is, the planet is only visible in the eastern sky before sunrise or after sunset in the west. With such a small angle range, we only have a maximum of 1 hour 53 minutes to observe the planet, which is when Mercury reaches maximum elongation. So, we will never be able to see Mercury in the zenith (see figure below). Due to the alternating appearance that this planet was identified by the ancient Greek society as two different bodies. At that time, Mercury appears in the eastern sky were named Apollo and which appeared in the western sky was named Hermes.
If we are in the Mercury, we can see the sun moving retrograde in the sky. In one location, after rising in the east and before crossing the meridian, the Sun will move back slightly and then returned moving west to sunset. Similarly, after the sun went down, he will have retrograde motion once again (although not observable). As a result, one day there (once a day and all night) equals 176 Earth days (about 6 months). Please see for yourself by using a simulation program Stellarium sky.
Causes of retrograde motion of the Sun is related to the period of revolution and rotation. Revolution period of Mercury is 88 Earth days, while the rotation period is 58.7 Earth days. We can see that the ratio of the rotation and revolution is 2/3. That is, the planet of revolution was completed two times in the same time with 3 times the rotation.
Planet in when viewed from Earth (Source: Wikipedia)
The relationship between rotation and revolution periods (also called resonance) is unique in the solar system. Resonance is common in the solar system is 1:1. That is, the rotation period is equal to the period of revolution. For example, Pluto and Charon system, which each have the same rotation period of the revolution period of Charon to Pluto. As a result, Pluto and Charon mutual showed that the surface remains. Moon also has a 1:1 resonance due to the rotational period equal to the period around the Earth. We know the consequences, namely the surface of the moon as seen from Earth always remains.
Physical characteristics
Planet rock is only 4800 km in diameter. This size smaller than Ganymede and Titan, two of the largest satellite in the solar system. But Mercury is still more massive of the two. And its density 5.43 g / cm ^ 3, making it the second highest density of objects in the solar system after Earth. Thickness of the core of this planet is more dominant relative to its size, reaching 60% of its mass. The distance from the sun between 46 million km to 70 million km. Eccentricity orbit greatest among all the planets, which is 0.21.
Mercury's orbit inclination to the ecliptic is 7 degrees. The angle of rotation axis to the axis of revolution is close to zero, about 0.027 degrees. Jupiter is still smaller than that of 3.1 degree. With such a small angle, there are no four seasons in the northern and southern hemisphere of Mercury. Temperature at the surface varies between 80-700 K.
Flight missions to Mercury
Mercury is one of the objects are difficult to observe, so not much information can be obtained from it. In fact, the new planet's rotation period is well known in 1965 after the Mercury observed by radar.
Delivery vehicle to examine Mercury up close was not easy. Position close to the Sun, the lack of atmosphere, and differences in the rate of the orbit is a difficult thing. As a result, until now there are only a successful mission to observe Mercury, the Mariner 10.
Mariner 10 spacecraft was launched on 3 November 1973. Departure process that take advantage of the planet Venus (as a "slingshot" gravity) is the first done in the history of space flight. When it passed near Venus, this vehicle took a recording of ultraviolet photography planet. Although Venus has been observed in previous ground-based telescopes, still images of Venus was Mariner 10 provided the researchers invite admiration.
This vehicle has remarkable knowledge of Mercury's surface. In addition, this vehicle also detected Mercury's magnetic field. One thing that surprised the researchers because the planet has a slow rotation. Finally, in 1975, Mariner 10 was defunct after the fuel runs out and the contact stopped.
Mariner 10 (left) and Messenger (Source: Wikipedia)
New in 1998, the latest mission to Mercury began planned. Spacecraft on a mission, named Messenger, which was launched on August 3, 2004. The target of this mission is the orbit of Mercury on March 18, 2011. There are six questions to be resolved by the Messenger: 1. Why is Mercury's density is so high?; 2. How the history of the planet's geologic history?; 3. How does the nature of Mercury's magnetic field?; 4. How the internal composition of Mercury?; 5. What materials found in Mercury's poles?; And 6. How does the composition of Mercury's atmosphere?
In the future, a mission will be run again. His name BepiColombo. This mission will complement the data obtained Messenger. Planned for launch in 2013 and orbit Mercury in 2019, BepiColombo will collect data for 1 or 2 years. Scientists hope the mission would be to bring people get to know the characteristics of this small planet.
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