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Mercury has an orbital speed of 47.4 km/s (29.5 mi/s), whereas Earth's orbital speed is 29.8 km/s (18.5 mi/s). [112] Therefore, the spacecraft must make a larger change in velocity ( delta-v ) to get to Mercury and then enter orbit, [ 188 ] as compared to the delta-v required for, say, Mars planetary missions .
In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or, if one body is much more massive than the other bodies of the system combined, its speed relative to the center of mass of the most massive body.
A Mercury-bound spacecraft launched from Earth must travel 91 million kilometers into the Sun's gravitational potential well. [12] Starting from the Earth's orbital speed of 30 km/s, the change in velocity the spacecraft must make to enter into a Hohmann transfer orbit that
The speed of the planet in the main orbit is constant. Despite being correct in saying that the planets revolved around the Sun, Copernicus was incorrect in defining their orbits. Introducing physical explanations for movement in space beyond just geometry, Kepler correctly defined the orbit of planets as follows: [ 1 ] [ 2 ] [ 5 ] : 53–54
Mercury's elliptical orbit is farther from circular than that of any other planet in the Solar System, resulting in a substantially higher orbital speed near perihelion. As a result, at specific points on Mercury's surface an observer would be able to see the Sun rise part way, then reverse and set before rising again, all within the same ...
Escape speed at a distance d from the center of a spherically symmetric primary body (such as a star or a planet) with mass M is given by the formula [2] [3] = = where: G is the universal gravitational constant (G ≈ 6.67 × 10 −11 m 3 ⋅kg −1 ⋅s −2 [4])
Few missions have targeted Mercury because it is very difficult to obtain a satellite orbit around the planet. Mercury orbits the Sun very quickly (between 24.25 miles per second (39.03 km/s) and 30 miles per second (48 km/s)), so spacecraft must be travelling very fast to reach it.
Approximately four (Earth) days before perihelion, the angular speed of Mercury's orbit exactly matches its rotational velocity, so that the Sun's apparent motion stops. At perihelion, Mercury's orbital angular velocity slightly exceeds the rotational velocity, making the Sun appear to go retrograde.