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The Question

(Submitted January 27, 1997)

Helium particles sound strong. Could they push a solar sail better than sunlight pressure? In other words, what's their energy per sq. meter near earth?

The Answer

We know of two kinds of energetic particles near the Earth, cosmic rays and solar wind. In both cases, the densities are rather low, so even though the individual particles are energetic, they do not create a strong pressure.

Cosmic rays are from outside our solar system, and come from all directions; so they cannot be used for propulsion.

Solar wind (stream of particles from the Sun) is usually around 0.001 joules per square meter per second -- about a million times less than sunlight.

Best wishes,
Koji Mukai & Jonathan Keohane
for Imagine the Universe!

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