It’s all about the drag. ‘Drag’ has slowly but surely pulled the Upper Atmosphere Research Satellite (UARS) out of orbit, down towards earth. All satellites experience drag but most rely on onboard propulsion systems to maintain altitude and keep the satellite in its intended orbit. After 19 years in space, UARS propulsion system is no longer operational. Without a propulsion system the satellite cannot counteract the drag and reboost itself into orbit.
The big question on everyone’s mind (including NASA) is when will the satellite re-enter the Earth’s atmosphere and where will the debris spread? If the satellite’s drag could be accurately calculated, then this question would be answered exactly.
Most assume that it must be easier to get a satellite down to an exact landing place on Earth than get it up into space in an exact altitude and orbit. Not so. The drag that continuously pulls at all satellites is determined by multiple constantly changing variables. The unpredictability of those variables makes the calculation of drag almost impossible.
Drag is a force that opposes the direction of the satellite's velocity and retards its movement. Three primary factors determine the amount of drag on a satellite: the satellite's mass, the satellite’s area and the density of the atmosphere at the satellite's altitude. Two of these factors, the area of the satellite and the atmospheric density, constantly change.
How can the area of a satellite vary? When calculating the area of UARS, for the purposes of determining drag, only the frontal, flat surface area perpendicular to the velocity vector is used (sort of like the front windshield of a car). However, the satellite is constantly moving, thus the amount of frontal area is constantly changing.
Richard Strafella, a former NASA aerospace engineer for 33 years, is an expert in flight dynamics and spacecraft orbit decay. Strafella says, “It is difficult to calculate the area for a satellite like UARS, which has a solar array on one side of the main body. The solar array must constantly point towards the sun (to gain power), so the satellite is continuously moving its orientation to stay ‘locked-on’ the sun. During the night-time portion of the orbit, the solar array may be slewed back to another angle so that when the sun is again visible to the satellite, the solar panel is in the correct orientation to point towards the sun.” The satellite’s area will constantly change as the satellite travels in its orbit and maneuvers to stay locked on the sun.
How does the density of the atmosphere vary? Strafella explains, “At any given altitude, the atmospheric density is not constant but varies greatly, more so on the side of the earth in sunlight than the side in darkness. In addition, the sun’s radiation causes the earth's atmosphere to heat up. As the atmosphere heats up the atmosphere expands outward from the earth increasing the atmospheric density at all altitudes. Further, the sun follows roughly an 11-year cycle, known as the solar cycle and in 2011 the sun's activity is increasing towards the solar maximum.” The increase in the sun’s activity causes even greater fluctuation in the density of the atmosphere making drag on the satellite even more erratic as it travels around the earth.
As the satellite decreases in altitude and gets closer to Earth the atmosphere becomes denser increasing drag. Finally the drag force becomes so intense it exceeds the satellite's capacity to control its own orientation and the satellite will tumble through space. Once it begins to tumble it is impossible to calculate the satellite’s area making re-entry estimates uncertain. When the satellite has begun its final plunge, the spacecraft is usually torn apart by the drag force and the heat of the Earth’s atmosphere.
Strafella, responsible for determining orbit decay throughout NASA’s history (from the Apollo missions up to the Hubble Telescope) says conclusively, “It is very difficult to give an accurate long-range prediction for when and where any satellite will re-enter. However, UARS will be tracked and only when it is within a couple of orbits (i.e., within a few hours) of re-entering will there be a precise prediction for the satellite's reentry time and location.”