Shooting Down a Satellite: All in the Timing

"If they can't hit this, they can't hit anything," said John Pike, a Washington, DC-based military policy analyst with GlobalSecurity.org.

What is uncertain is the timing. Officials want the satellite to fall through the atmosphere as quickly as possible in order to ensure its debris steers a relatively harmless path to Earth. But there are multiple factors at play.

Earth's atmosphere expands depending primarily on how much extreme ultraviolet radiation is streaming from the sun. Traveling at the speed of light, 186,000 miles per second, photons from the sun reach Earth in about eight minutes.

The energy typically is absorbed, but if there is enough energy, an electron from an atmospheric particle can break off, contributing to the ionosphere. Otherwise, the extra energy becomes kinetic energy, which causes atoms and molecules in the atmosphere to rotate faster, causing a gradual rise in temperature.

Heating up

The warmer the atmosphere, the more friction is placed on objects in orbit. Tugged by Earth's gravity, they lose altitude as they slow, until they no longer have the momentum to remain pinned in orbit and plunge to a fiery destruction in the atmosphere.

The physics of atmospheric heating are well understood, Kent Tobiska, with Space Environment Technologies in Pacific Palisades, Calif., told Discovery News. But only in the last decade have physicists had the tools and computer models to correlate solar flux with changes in Earth's atmosphere.

Several satellites constantly monitor the sun and their data is used to create simulations to predict atmospheric heating.

"Since the start of the decade, we've made some real advances in operationally specifying what the upper atmosphere is doing in real time and predicting it out two or three days into the future," Tobiska said.

Ironically, the best indicator of how the atmospheric predictions are doing come from tracking orbital debris. The North American Aerospace Defense Command, or NORAD keeps tabs on more than 13,000 Earth-orbiting objects, continually refining their paths based on radar and other observational data.

"There's a predicted solar energy that's coming in and those are fed into atmospheric density models, producing a global atmospheric map for a given time and going out into the future," said Tobiska. "You use the junk satellites to recalibrate what the atmosphere is doing every few hours."

The hit on the dead spy satellite is being timed so that hopefully its debris will re-enter the atmosphere as quickly as possible. Still, there are uncertainties.