TIL that it takes less energy to launch something out of the solar system than directly into the sun. Apparently its because the massive gravity of the Sun causes objects to orbit instead of pulling objects into itself. Why?

Question

TIL that it takes less energy to launch something out of the solar system than directly into the sun. Apparently its because the massive gravity of the Sun causes objects to orbit instead of pulling objects into itself. Why?

In: Physics

Answer 1

The Sun’s huge gravity will vastly speed up an object falling towards it. If you try and launch an object deep into the inner solar system, then any initial horizontal velocity will be greatly magnified as it gets close in – it will be carrying far too much (angular) momentum to actually fall into the Sun unless you start with almost zero horizontal velocity respect to the Sun. At the initial launch position from Earth’s orbit, you need to therefore slow the object down from the Earth’s orbital speed (about 29.8 km/sec or 66,700 mph) to almost zero. This requires an enormous amount of energy.

For launching out deep into space, then you ‘only’ need to speed up an object by about 41% – you can work this out by looking at something called Gravitational Potential.

So from Earth orbit – to drop something into the Sun – slow the rocket down by 29.8 km/s / 66,700 mph

To launch something into deep space – speed up the rocket by 12.3 km/s / 27,600 mph.

You can see which of these needs less energy.

Actually, it’s an even more pronounced difference than this because you would usually be starting from low Earth orbit – which already requires you to travel at nearly 8km/sec. When accelerating away from the Earth you lose a bit of energy, but you can use some of your forward momentum to reduce the 12.3km/s requirement to escape into deep space to about 8.8 km/s. In the launch-to-the-Sun direction, you can use the 8km/sec on the opposite side of the orbit to reduce your 29.8km/sec requirement to about 24.0km/sec.