Where is the edge of Solar System?

today I want to look on the problem of edge of Solar System. The problem is that such a border is not very clear. It is similar problem as the classification of what is planet and what is not, simply because there is no clear distinction in size or so for something to be planet. Biologists also had/have this problem with defining what is life since they discovered that there are lot of small things that are lying on the edge of dead (it was never alive) and alive.

So whole post I will be working with this picture, so take time to acknowledge what is there:

Voyager 1 entering heliosheath region.jpg

Sun is in the center.

So the short answer is that heliosphere is the edge of Solar System. It is defined as region with the border where Sun’s solar wind is stopped by the pressure of interestellar medium.

Solar wind is the flow of plasma from the Sun. It is pretty fast, up to 400 km/s or even more. This plasma is then hitting the interstellar medium which is stuff from other stars and simply dust and so on.

Since this solar wind pushes the stuff out we have smaller pressure inside our bubble. This creates the border called heliopause where solar wind is slowed to 0 velocity.

It is best to define the region like this because otherwise Sun is technically visible anywhere in the space and its gravity also just decreases a little bit by the distance but solar wind just stops.

You may have noticed that I did not mention the termination shock which is another border in Solar System (btw. heliosheath is just the region between heliopause and termination shock).

It is the border where the velocity of the solar wind decreases below the speed of sound in space [1]. This creates a termination shock which I will explain in other post when I will talk about the speed of sound!


PS: also I promised something about orbitals so I should do that too..
PPS: Bow shock probably does not exist and it should be shock wave of interstellar medium.

[1] Oh yes, there is speed of sound space too. It varies from region to region but since there is dust and other stuff lying around the speed of sound is roughly 100 km/s.


Solar sails

have you ever wanted to be space pirate? To just fly through Universe close to speed of light and sometimes stop on some planet to steel some stuff? Well now, your space craft can even look like a ship, with huge sail!

Solar sails are sails like any ordinary sails. They even use wind, wind made of photons!

On the picture above, you can see IKAROS spaceprobe, the first succesful probe to use solar sails.

Those solar sails are powered by nothing more than the electromagnetic radiation coming from the Sun. It then pushes the probe because of photon’s momentum[1].

But photon has no momentum!

Yes according to Newton’s mechanics it should not have a momentum because it has no mass.


p is momentum, m is mass and v is velocity (speed).
If you would use this you would get that photon does not move and it does not have momentum but if you use theory of relativity as I already showed in the post about conservation laws you will get some real value, using what is called “relativistic mass” which is not zero for photon.

The IKAROS’s sail was 20 meters on diagonal and most of it was 7.5 µm thick so it was very light.

Whole probe had 315 kg while the sails had only 2 kg. This is about average mass for probes since New Horizons had about 450 kg while Jason-3 about 250 kg.

The pressure of the solar wind from Sun was 1.12 milli newton and it was able to gain velocity of even 400 m/s or 1440 km/h.

This method is not much useful for fast speedup and also for deep space travel since the radiation decreases with squared distance, but still you can be a local pirate.



[1] The photon’s momentum is transfered to the solar sail and because of conservation of momentum the sail must be pushed a little bit while the photon bounces away, this is the same effect as with a shooting rifle. When you shoot the bullet goes forward while the rifle backwards with the same momentum.

2nd picture
1st picture