If we do nothing, the Hubble Space Telescope will fall to Earth sometime in the in the 2030's.
It is in an orbit roughly 560 km above the Earth and circles the Earth once every 97 minutes. While for most intents and purposes the Hubble Space Telescope can be considered to be in space, it actually lies in what is known as the thermosphere: the largest and most tenuous part of the Earth's atmosphere. The thernosphere is roughly one million times less dense than the atmosphere at sea level, yet it is enough to affect the orbits of satellites that fly within it.
Any satellites in low Earth orbit experience a small but significant resistance as they fly over the planet's surface, slowing them down and decaying their orbits. If not corrected, or periodically 'pushed back up' by a rocket or the Space Shuttle, the satellites at this altitude are eventually doomed to fall to Earth and burn up in the atmosphere.
The Space Shuttle has its own engines and isn't in orbit long enough to be affected by this drag but the International Space Station and the Hubble Space Telescope are affected, and they must be periodically pushed into higher orbits to correct for their orbital decay.
To complicate matters, the amount of drag on the Hubble as it orbits within the thermosphere isn't constant. It varies with the 11 year sunspot cycle. As the Sun becomes more active, the atmosphere of the Earth swells and reaches farther out into space than it otherwise would. This increases the density of the air that Hubble must fly through, slowing it down further, lowering its orbit and ultimately shortening it's lifespan.
Hubble has no jets or engines of any kind for propulsion, so throughout its life, it has relied on the Space Shuttle to grab onto it and move it to a higher orbit.
Now that NASA has suspended the Shuttle program, no more launches are scheduled to service the most powerful telescope ever built. The Hubble Space Telescope is on its own.
That doesn't mean however, that NASA has not planned for its demise. On the last servicing mission, astronauts placed a ring, known as the Soft Capture Mechanism to the back end of the spacecraft. This ring will give future robotic spacecraft an easy place to grab onto.
Because the Hubble is so large and heavy, it will not completely burn up when its orbit decays and it re-enters the Earth's atmosphere. This presents the danger that pieces could fall over populated areas.
To ensure a safe re-entry, the Hubble Robotic and De-orbit mission is building a robotic spacecraft designed to grab onto that ring, attach itself to it and guide the re-entry of the Hubble onto a safe trajectory.
While the details of when this mission will be launched is unclear, one thing IS clear: with the demise of the Space Shuttle program, no more manned missions to boost the Hubble into a higher orbit are imminent and any chances of saving it will probably rest with robotic craft. The last days are in sight for the most important scientific instrument ever constructed.
If we do absolutely nothing, that last day will arrive in 2024. If we have the will, there is plenty of time to arrange an alternative.
The Hubble Space Telescope stands at the pinnacle of a pantheon of great space telescopes. It has done more to advance our understanding of our place in the universe than any that has come before or since, and at a cost that is microscopic compared to other budgetary expenses.
Because of public outcry, NASA reversed a previous position not to service the Hubble and managed one more fix. Is it possible to design a robotic spacecraft to grab Hubble and bring it home?
It seems the least we could do.