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.