Which probe is orbiting mercury

Reducing the part of orbital energy related to the distance from the Sun automatically increases the part related to speed by the same amount.

Consequently, as the spacecraft gets closer to the Sun, it begins to speed up like a car driving downhill. Speeding up is not necessarily a bad thing.

So, our spacecraft heading to Mercury will have to gain some speed to catch up with the tiny planet. Unfortunately, without shedding some orbital energy, the spacecraft will pick up too much speed and reach Mercury moving far too quickly to be captured by its gravity and will fly right past.

The energy our spacecraft needs to shed is on a completely different scale to using brakes to slow down while driving downhill on Earth. Arriving at Mercury travelling slowly enough to be captured by its gravity is a bit like falling down a cliff millions of kilometres high and landing softly on a moving target half way down! To burn off all the necessary orbital energy using only thrusters, our spacecraft would need such a huge amount of fuel, and extra material to hold it, that it would become far too heavy to launch into Earth orbit even with the most powerful rocket.

Instead, the spacecraft can use the powerful gravitational pull of other planets it approaches on the way to its destination to offload the excess energy and alter the path of its own orbit.

Depending on how the spacecraft approaches the moving planet, it can either increase or decrease its speed relative to the Sun. This manoeuvre, known as a gravity assist manoeuvre , flyby, swing-by or gravitational slingshot , allows us to explore a wide range of objects across the Solar System on a tight fuel budget.

MESSENGER will also be studying Mercury's atmosphere and inner structure, as well as its magnetic environment, which changes rapidly due to the planet's close interaction with the sun.

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