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Approximate sizes of the planets relative to each other. Outward from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Jupiter's diameter is about 11 times that of the Earth's and the Sun's diameter is about 10 times Jupiter's. The planets are not shown at the appropriate distance from the Sun.
Terrestrial planets have a compact, rocky surfaces, and Venus, Earth, and Mars each also has an atmosphere. Their size, radius, and density are all similar. Terrestrial planets have numerous similarities to dwarf planets (objects like Pluto), which also have a solid surface, but are primarily composed of icy materials.
The surface of Venus is comparatively flat. When 93% of the topography was mapped by Pioneer Venus Orbiter, scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 kilometres (8.1 mi), about the same as the vertical distance between the Earth's ocean floor and the higher summits of the Himalayas.
Three of the four inner planets (Venus, Earth, and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes. [91] Mercury (0.31–0.59 AU from the Sun) [D 6] is the smallest planet in the Solar System.
The global surface of Venus was first mapped by the Magellan orbiter during 1990–1991 with 50 km spatial and 100 m vertical resolution. During three orbit regimes, the surface images were transmitted back to the Earth. These three orbiting motions of the spacecraft are called mapping cycle 1, 2 and 3.
Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet , two or three planetary-mass satellites – Earth's Moon , Io , and sometimes Europa – may also be considered terrestrial planets.
Crocco realized that the flight trajectory may be disrupted by the gravitational fields of Mars and Venus, delaying or ultimately preventing return to Earth. He solved this problem by directing the flight path through the gravitational fields of Venus and Mars in such a way that their attraction would cancel each other out. [4]
The orbits of Venus and Earth are the closest between any two Solar System planets, approaching each other in synodic periods of 1.6 years. Venus is the easiest destination to reach from Earth because of the low delta-v needed, and is a useful gravity assist waypoint for interplanetary flights from Earth.