# Blocking Solar Radiation with an Eclipse

It seems to me like one of the major obstacles to terraforming a planet is to give it a magnetosphere. In our solar system, this is accomplished naturally either by a hot iron core (for example, Earth), or by a heavy, thick atmosphere (for example, Venus). Neither of these things are easy to produce on a planet that doesn't already have them.

A workaround I've considered for this is to position an object in between the target planet and the star, to block out harmful rays while letting through the necessary light/heat to support human life. I want to think this would be as simple as fitting a large craft with a series of mirrors, and positing it to 'follow' the target planet as it goes around the star. But since I haven't heard about this kind of idea I'm guessing there's something I'm missing.

So, would this idea work? Or if not, what would go wrong?

• The first pass answer would be that an orbit closer to the sun would have a different orbit period, so it'd constantly be out of position. However, you may be able to play some games with lagrange points or geosyncrhonous orbits. – Cort Ammon May 22 '17 at 21:14

The biggest threat to the atmosphere is not the visible light, but the streams of charged particles. To protect against high energy particles, a smaller magnetic field between the planet and the star would work just as well as a planetary one. Placed at the $L_1$ point of the system and sized accordingly as shown in the picture.

The obvious flaw of the idea is timing.

There are 2 possibilities - you are too early, or you are too late.

If a planet has an atmosphere, let's say if mars had an atmosphere, it will take millions(hundreds) of year to noticeably change the composition.

If the planet hasn't significant atmosphere, let's say as mars today, there is nothing to protect and current losses of mars atmosphere are minimal and it will take billions of years to noticeably change the current situation.

So if you discover some planet and it has an atmosphere, it means it has it since about star system formed and the planet was formed and initial processes of forming of the system gone in its stable state. And there will be reasons why the atmosphere is still there and those reasons will not change soon in most cases.

If a planet you discover has no atmosphere and you would like to create one there, then there are 2 important factors to consider. The speed of a planet atmosphere loss is slow compared to the amount/mass of the possible atmosphere. The second, if you able to create the atmosphere at any reasonable rate years, hundred years, thousand years, million years(timescale of optimists and pessimists) then you have no problems to replenish the losses.

However, if you concerned to reduce possible losses, just because you can or other reasons. Yes, a construction in L1 point could help you to do so. However, it less likely to be a set of mirrors, and most likely to be transparent solutions, electrostatic solutions, magnetic solutions.