“Hello World” from BepiColombo

It’s difficult to reach the Mercury as the planet closest to the Sun is deep within its gravitational well. This means that a lot of energy is needed to reach there: its not as easy as walking down a hill.

One way for a spacecraft to change its orbit is to use its propulsion system - in other words, fire its rockets. But getting to Mercury is, as already mentioned, difficult, in particular for current technology.

The most efficient way to get from the Earth to Mercury solely using rockets is the so-called Hohmann transfer orbit. Using one to go from orbiting the Sun at the distance of the Earth to orbiting at the distance of Mercury would require the spacecraft to change its velocity by approximately 17.1 km/s, and that ignores the need to escape the Earth’s gravitational field and slow down due to Mercury’s. With a rocket with exhaust velocity of 4 km/s that would mean the fuel would be over seventy times the weight of the spacecraft itself.

That is not doable.

So engineers use the gravity assist method, whereby the spacecraft is sent to flyby by one or more planets, each time selecting an approach geometry that results in the spacecraft slowing down. This is much more efficient, as the hard work is done by planets like the Earth and Venus.

This technique is being used by BepiColombo, the Japanese-European mission to Mercury that flew by Earth recently, giving us this lovely video:

“Hello World” from BepiColombo

On that little ball or (for a handful of astronauts) orbiting around it is every single one of us.

Hello World