Earth's atmosphere is a drag

LightSail 2 flies at a higher altitude than most satellites in low-Earth orbit. While the International Space Station orbits Earth at an altitude of about 400 kilometers, LightSail 2 orbits at about 720 kilometers. Since fewer spacecraft orbit at LightSail 2’s altitude, there wasn’t enough data on Earth’s atmospheric density to reliably predict how much atmospheric drag would slow down the spacecraft. We now know for certain that the atmosphere at 720 kilometers is dense enough to overcome the thrust imparted by solar sailing.

The team uses a simple on-off sail control strategy each orbit, turning the sail edge-on to the Sun’s rays when the spacecraft is traveling toward the Sun, and face-on to the Sun when moving away from it. Out of each 100-minute orbit, LightSail 2 spends 67 minutes either in eclipse or moving toward the Sun. Of the remaining 33 "sail-able" minutes each orbit, the spacecraft spends about 5 minutes turning to the desired orientation. Therefore, LightSail 2 enjoys at most 28 minutes of each orbit in an orientation for capturing the momentum of solar photons to change its velocity.

Mansell and his colleagues documented LightSail 2's orbital change during time intervals in which it was actively orienting itself for solar sailing and compared that change to periods in which the orientation was not controlled. When the spacecraft was randomly oriented, its semimajor axis—a measure of the size of the orbit—shrank by an average of 34.5 meters per day. When it was solar sailing, the orbit only shrank by an average of 19.9 meters per day. Yet, the rate is highly variable and the semimajor axis actually increased by as many as 7.5 meters some days when sailing, which means LightSail 2 increased its orbital energy during those periods.