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?



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

Something you have to consider is that when you launch something from Earth, it also carries Earth’s orbital velocity with it. If you just start nudging it towards the Sun, it’s going to maintain that velocity, and it’s going to make the orbital path really wacky.

If you want to put something into the sun, you need to cancel out Earth’s velocity, and that takes a crapton of energy.

Let’s say you’re Inn a car driving down the highway. That car is the earth. If you try to throw something straight out the window it will keep moving at the same speed as the car. Since the earth is orbiting around the sun, anything we throw off the earth will also start orbiting the sun. We would need to first launch it fast enough to cancel out how fast the earth is going.

The sun and every planet Star and particle in our universe are in fact moving.

When we launch something it has to escape our gravity, as a planet. But then we set it down in a specific spot. Then using gravity itself, computers, telemetry data, and small thrusters, we keep it in place.

When we send something to a distant place like Mars, we actually have to calculate where Mars is going to be when our item gets there. Like placing a rifle shot before a moving target.
(When we launched the Hubble space telescope, we basically dropped it off our planet using a rocket and told it to keep moving, using small thrusts, but the natural motion did more for us than those thrusts.)

The Sun’s gravity doesn’t work like a drain.
The Sun doesn’t bring things to itself, we orbit it on an elliptical path, relative to where it is on it’s own path on our arm of the Galaxy. We would need constant forward thrust to get an item to it, more energy necessary.

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.

It is because you are not starting from nothing.

You are starting from earth which is in orbit around the sun.

If you had an object that was as far away from the sun as Earth is but not moving around and you let it go, it would more or less drop straight down without any effort on your part.

However since you are starting from Earth, you are starting with all that orbital speed that Earth has in respect to the sun.

To get down to the sun you need to slow down from that enormous speed. (something like 100,000 Km/h).

It takes as much energy to slow down from going that fast as it would to speed up to that speed from nothing. Escape velocity is less than two thirds of the velocity needed to fall into the sun.

We would have a similar problem when deorbiting for earth orbit back to earth, but we cheat there by using the earth’s atmosphere to slow down returning spacecraft. That is unfortunately not an option for trying to fall down into the sun.

Here is a longer, but hopefully more ELI5 understandable way that teaches you a bit of the whys:

You’ve read from other answers by now that the primary reason is because the Earth is already going so fast. But why can’t you just accelerate towards the sun and hit it anyway? Because of the dynamics of circular motion.

[Image for visualisation](https://www.amesweb.info/Physics/images/Uniform-Circular-Motion-Calculator.jpg)

Imagine, if you will, our classic example – a bucket of water, on a string. As you swirl it around in a circle, you can intuitively realize that to keep it going in a circle at the same speed, you have to constantly exert a pull on the string straight towards you. So even though the bucket is always moving perpendicular to you (following the v in our image), you always pull towards the centre of the circle (the a in our image).
Orbiting objects work the exact same way. To keep something on the same circle at the same speed, you need to exert a constant pull towards the centre of the circle. Luckily, we have just such a thing – the sun’s gravity! It keeps us in a constant orbit.

So now, think again of your bucket. If you pull on the string a bit harder, what happens? Your bucket won’t hit you in the face – it just starts spinning faster. Similarly, in Earth’s orbit (igoring the issue of how it will deform our orbit), you can’t get to the sun by pushing inward – because you’re in a circular motion, in the general sense, pushing yourself towards the middle harder won’t get you much closer, and instead just speed you up. It’s a bit more delicate than that, because we don’t have a fixed-length string, so our orbit can deform, but for ELI5 purposes it will do.

So assuming the suns gravity remains more or less constant (gravity decays surprisingly little over distances such as that between the Earth and the Sun), how do we get closer to the sun? We have to decelerate. By slowing how fast we’re going on the circle, gravity becomes ‘stronger’ than the speed that’s keeping us on the circle. It will pull us closer towards the sun – accelerating us again in the process – until we’re going so fast again that we’re going fast enough again to match the gravity. Again – an orbital scale this works slightly differently and ovalizes your orbit, but ELI5.

That’s why the fact Earth is already going really fast matters – because you can’t just push yourself towards the centre. Counterintuitively, you need to push yourself *backwards*, against the direction you’re going (so opposite the v in our image), to get close to the centre. Push yourself forwards – accelerate – and you’ll get farther from the centre, eventually having enough energy to just fly away. And the amount of energy you need to do that just happens to be smaller than the amount of energy you need to stop and fall into the sun.

Imagine you’re on a frictionless roller coaster stuck between two huge hills. You can make it about 80% of the way up before stalling and then rolling backwards down and up the other hill where you make it 80% up again before coming to a stop and starting the cycle over (in a stable ‘orbit’). If you want to get over the hill, you need a little extra boost. So you strap some rockets on the back (you know, those ones that you always carry around and everyone make fun of you, saying “you’ll never need them!”) and light ‘em up!* This gives you enough extra energy to clear the hill (your gravity well) and move on (exit the solar system).

Now, if instead you wanted to roll around near the bottom of the hill (orbit the sun) or even come to a stop (fall into the sun), you could fire your rockets in the opposite direction to slow down. But since you have a lot of momentum already, you have to fire a lot more rockets to slow down than you did to boost over the hill.

*If you’re smart you’ll light them just when you reach the bottom and you’re going the fastest, as this will give you the most ‘boost’ (see [Oberth effect](https://en.m.wikipedia.org/wiki/Oberth_effect) )

Related (hopefully) question: Could the atmosphere and gravity of Venus be used to adjust the trajectory of the object to get it into the Sun? Or use one of the larger planets ( more gravity=more braking force)?

The distance of the orbit is based on the speed. It takes less energy to go from earth speed to escape speed than to go from earth speed to 0 (compared to sun speed).

Think of those things where coins spin around cone shaped thing until they fall into a hole. If you had two coins together (earth and a rocket) and you released one coin with very little energy the coins would follow similar paths. To get the released coin to fall right into the hole it would need a strong boost off of the other coin to cancel out all of its speed. If that same amount of speed were used to speed the coin up instead of stopping it the released coin would shoot out of the cone.