Use light as a push, and account for its limits

Light carries momentum. Absorbing or reflecting it transfers momentum to a surface, producing a small force. A solar sail uses this radiation pressure — open definition, rather than the solar wind of charged particles, as its main propulsion mechanism. Reflection and sail orientation affect the force; a sail is not simply a spacecraft with a larger electricity-generating panel.

JAXA reported photon acceleration of IKAROS in July 2010 using orbit determination after deployment. That is a historical flight demonstration of a specified vehicle. It does not establish that every larger sail, destination or control strategy is already demonstrated.

Acceleration is force divided by mass. In a fictional comparison, a 0.002-newton force acting on a 20-kilogram craft gives 0.0001 meters per second squared. If that acceleration kept a fixed direction for 10,000 seconds, the velocity change would be 1 meter per second. Real sail trajectories must also account for gravity and changing geometry.

Sunlight becomes weaker with increasing distance. Under otherwise identical ideal conditions, doubling the distance gives one quarter of the illumination and correspondingly less radiation-pressure force. A larger area can help, but brings mass, deployment, structural and control demands. More sail does not automatically mean a larger acceleration if the total mass grows too much.

A small push sustained over time

Electric propulsion uses electrical energy to accelerate expelled propellant. Solar panels or another source can supply that electricity. A photon sail obtains momentum from incident light and need not consume onboard propellant for that particular push. Both systems still require spacecraft power, control and communications.

For a mission proposal, compare thrust direction, duration, available power or illumination, total mass and the arrival requirement. A method that increases departure speed may lack a demonstrated way to slow down at the destination. Treat diagrams of dramatic journeys as proposals until the required chain of capabilities has applicable evidence.