Quote from: AlexandrKushnirtshuk
How could MRO have lost 11.2-2.9 = 8.3 km/s of velocity during flight through space with zero resistance?Mars is farther from the Sun than the Earth.
- So a rocket launched from Earth will lose speed as it climbs out of Earth's gravitational field
- It will lose more speed as it climbs out of the Sun's gravitational field.
- It will gain a bit of speed as it falls into the gravitational field of Mars - but not as much as it lost climbing out of Earth's gravitational field (since Earth is much more massive than Mars).
In a classic Hohmann Transfer maneuver, a rocket will start from an inner circular orbit (say, at the radius of Earth's orbit), and accelerate to become an elliptical orbit
- This elliptical orbit will have a perihelion at the radius of Earth's orbit, and an aphelion at the radius of Mars's orbit.
- If the rocket did nothing when it reached the radius of Mars, it would fall back towards the Sun, as it's radial velocity away from the Sun has reached zero, and it's angular velocity around the sun is too slow to maintain a circular orbit.
- So when you reach the target orbit, you need to fire the rocket to increase the rocket's speed to keep it in a circular orbit at the radius of Mars's orbit.
This simple Hohmann transfer assumes no mass at the destination.
- In fact, the gravitational attraction of Mars means that the rocket gains some velocity as it approaches Mars.
- By positioning the satellite's arrival relative to the position of Mars, you can cause the satellite to get a gravitational assist, either to increase or to reduce its velocity. This slightly reduces the amount of rocket fuel you need to carry.
- However, if you want the satellite to be an a low orbit around Mars for ground observation, you need to reduce the relative velocity to Mars by a considerable amount, or it will go shooting past (a flyby), or end up in a high orbit.
- So MRO needs to increase it's radial velocity around the Sun to stay in orbit near Mars, and shed most of the velocity built up as it approached Mars, for a Mars Reconnaissance mission.