How Fast is a Light-Sail?
The answer my friend is, oh snap you know the rest!
This information comes from the Light Sail 2 web site, and from the Wikipedia web site.
Because solar sails do not use traditional rocket engines, they do not have a single “maximum speed” or a rapid blast of acceleration. Instead, they rely on the continuous, gentle pressure of photons hitting a reflective Mylar sail. The acceleration of LightSail 2 was incredibly small but continuous. The push against the Lightsail was roughly equivalent to the weight of a single sheet of paper resting on your hand.
In a perfect vacuum with constant, optimal sunlight, this acceleration means that over the course of one month, the spacecraft’s speed would increase by about 549 km/h (341 mph)—roughly the cruising speed of a commercial airliner. And would continuing to increase.
When measuring the “speed” of a satellite in orbit around Earth, it’s important to distinguish between its orbital speed (how fast it circles the planet) and the change in speed (delta-v) provided by the sail itself.
LightSail 2 was launched into Low Earth Orbit (LEO) at an initial altitude of about 720 km (450 miles). At that altitude, gravity dictated that it travels at an orbital velocity of roughly 17,000 mph just to stay in orbit.
The goal of the mission wasn’t to break speed records, but to prove the sail could alter the orbit. By changing its orientation relative to the Sun during each orbit, LightSail 2 successfully raised its orbital high point (apogee) by several kilometers, proving it could fight atmospheric drag using nothing but sunlight.
While LightSail 2 stayed bound to Earth’s gravity, the technology it proved is revolutionary for deep space. Because a solar sail never runs out of “fuel,” a spacecraft traveling away from Earth into deep space could theoretically continue accelerating indefinitely, eventually reaching speeds vastly exceeding traditional chemical rockets.
But I would think that as a Lightsail increases its distance from the Sun the source of photons decreases according to the Inverse Square Law. So, the effectiveness of the Lightsail should decrease leaving the spacecraft to continue coasting through space at whatever speed it had reached.