Once the bullet is free of the atmosphere, which happens a few hundred kilometers up, it follows an elliptical orbit around Earth.

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The perigee of its elliptical orbit (the closest point it comes to Earth) has actually risen slightly, and is just above 200km.

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Its elliptical orbit actually crosses Neptune's path, yet the timing of the two planets' respective motions ensures that, like figures in a clockwork model, they never collide.

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Phobos-Grunt launched successfully on its Zenit rocket from the Baikonur Cosmodrome and was dropped off into an elliptical orbit with an apogee (farthest point from Earth) of 345km.

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It was a delta-v opposite to the direction of travel and that slowed Messenger enough so the relatively weak gravity of Mercury could capture the spacecraft and put it into an elliptical orbit around the planet rather than the Sun.

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The intervening weeks will be spent in a manoeuvre called aerobraking, which involves using the air-resistance of the thin Martian atmosphere to change the elliptical initial orbit into a circular one from which the planet's surface can be examined in detail.

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