The activity of the Sun increases and decreases in a cycle that lasts approximately 11 years. When the cycle reaches a maximum there are a larger number of sunspots and an increase in solar radiation and charged particles reaching our planet. This can cause the atmosphere of the Earth to expand slightly. A satellite in low Earth orbit experiences a small amount of atmospheric drag despite the low density of air. The exact amount depends on how active the Sun is. Each cycle varies somewhat in duration. A cycle can be as short as 9 years or as long as 14. The average is about 10.7 years. These variations complicate the process of making predictions of future activity which are important for estimating the orbital decay of a satellite.
The magnitude and shape of the peaks in activity is also variable. The maximums in the early 1800s were very small, a time period known as the Dalton minimum. The next figure is a closer look at the Sun’s activity during the space age. The maximum in the late 1950s when Sputnik launched was the largest ever observed. During the Apollo missions the maximum was lower than other recent peaks but greater than the most recent maximum.