The orbital altitude of the Hubble Space Telescope is primarily affected by atmospheric drag. This drag force fluctuates over time, mostly due to atmospheric density changes from solar flux levels. These levels vary periodically over the course of solar cycles, which occur roughly every 11 years. The solar flux levels are currently longer in duration and more elevated than previously anticipated, resulting in an earlier reentry forecast for the Hubble Space Telescope if no reboost mission is conducted.
Due to increased solar flux, current estimates predict that reentry will occur in 5-6 years, but it is difficult to estimate this value due to the variability of future solar flux. In the best case, Hubble may not reenter for 15 more years, around 2040. In the worst case, it could reenter in 4 years, around 2029.
Regardless, if no action is taken, the Hubble Space Telescope will be making an uncontrolled reentry at some point in the next decade. This site, which was initially designed as a quick test project, will be updated and improved continuously until that time.
The graph above shows Hubble's highest and lowest points in its orbit at a given time. As the altitude decreases, density will increase, resulting in an even higher decay rate due to increased drag. When Hubble reaches 400 km, it will likely have a year or less remaining before reentering, and any reboost mission would be unlikely to occur in time. Reentry is generally imminent below 200 km in altitude.
Orbital data is from space-track.org, and the solar flux data is from spaceweather.gc.ca --- Thank you.
For more info on Hubble, visit NASA's Hubble Space Telescope page.
This site is not affiliated with NASA, and is somewhat disappointed that NASA is not more open to a reboost mission. More on that later.
Hubble has been reboosted three times in its history, all by servicing missions using the Space Shuttle. No reboost was performed on SM-3A and SM-4. Unfortunately, NASA has not approved any plans for reboosting the telescope, despite a generous offer from Jared Isaacman. NASA's reason for not currently pursuing a reboost mission is unclear, but Hubble's scientific output is becoming progressively limited due to issues with gyroscopes and, eventually, due to the lower orbit.