Orbital decay occurs when a satellite or other body in orbit begins to fall into lower and lower orbit as time goes on. There are several reasons for this but the most important is loss of orbit due to friction from the upper atmosphere. The earth’s atmosphere actually extends out to 8.5 miles, with the troposphere holding 90 percent of the earth’s dense atmosphere, then the stratosphere where airliners fly, between 8.5 miles and 56 miles, and the thermosphere which extends at least 273 miles above the earth. Most satellites, as well as the International Space Station, orbit in the thermosphere, which changes diameter a according to the amount of heat being produced by the sun.
The sun puts out varying levels of energy and goes through cycles of low and high solar activity. More energy means that the thermosphere expands and more objects in orbit are affected. The thermosphere is actually not very dense and so the energy put into the thermosphere doesn’t actually heat it up much but it does make the thermosphere expand.
Friction between the thermosphere and the object in space slows it down and makes it lose momentum, which in turn means it falls closer to the earth. This brings it into more contact with the thermosphere, slowing it even more, until the object hits the altitude at which the object begins to burn up from excessive friction induced heating. Even large objects burn up in re-entry, as most of the American Skylab in 1979 and Russian Mir in 2001 broke up on re-entry. And then there was the tragic loss of the space shuttle Columbia which broke up at 43.6 miles altitude and 22.8 times the speed of sound, after friction from hot atmospheric gases entered the shuttle’s wing and caused a massive structural failure. In this case substantial portions of the shuttle fell to the ground due to the low altitude at which it began to break up, but this illustrates the power that friction with the air can have.
The way to counter orbital decay is to boost the object back into a higher orbit, usually by means of small thrusters or assistance from another craft, as the International Space Station was boosted by the space shuttle in the early days.
Making the object speed up or slow down using thrusters will not work as all that does is change the shape of the orbit, which is usually elliptical rather than a direct circle. In fact the way that space vehicles meet up and dock together is by dropping down in orbit to catch up to a craft at a higher orbit, or increasing the orbit so the craft slows down and then meets up by finally adjusting the orbit so they meet.