Second, there are traces of gas well outside what is regarded as the limits of earth's atmosphere. Geosynchronous orbit is a little above 22,000 miles (Hubble is less that 400 miles) and there's enough gas floating around -- even at that altitude -- that the satellites in Geosync need the occasional boost.
Space junk that's too big to de-orbit gets thrown out to "graveyard" orbit, 22,400 miles, about 180 miles beyond geosync. Even space junk that's in "graveyard" orbit eventually will fall back to earth, but in some cases that isn't predicted to happen for millions of years. Nonetheless, even that high, eventually aerodynamic and gravity wave drag will bring them back to earth.
I guess NASA is counting on Future Man to have a landing net big enough to catch them when they eventually come home.
The deal with Hubble is that NASA was ready to pull the plug and let it de-orbit, particularly with Webb now on station. But Hubble isn't entirely obsolete (and Webb can't be looking everywhere at once) so this in part is as much an experiment for its own sake as it is an effort to keep Hubble in service.
No doubt they'll use a Hohmann transfer to boost Hubble. That means they won't push vertically, they'll push horizontally, sideways. As the satellite gets faster it naturally goes higher (centripetal force).
Orbits by nature tend to be elliptical (Kepler's first law of orbital motion) and special measures have to be taken to make them more nearly circular (a circle is a special case of an ellipse). Hubble's current orbit is very nearly (but not entirely) circular and if they want the new orbit to be circular, too, it will take two boosts because the new orbit initially will be elliptical. It will take a second boost, with firing beginning when the satellite is at the point in the new orbit that's furthest away from earth (at apogee),
The initial boost begins at apogee for the old orbit, which becomes perigee for the new orbit (labeled point P) and establishes an elliptical orbit through a new apogee (point A). If they feel the need, a second firing at Point A can give them back Hubble's original nearly circular orbit.
