Forecasting satellite movements

  • Published
  • By Michael P. Kleiman
  • 377th Air Base Wing Public Affairs
Kirtland Air Force Base, NM --  Orbital Drag Environment Program developed to avoid collisions in the cosmos

On Feb. 10, 2009, the first major traffic accident in space occurred when the Iridium 33 and Kosmos-2251 satellites crashed into each other at approximately 26,170 miles per hour 490 miles above the Earth.

Eleven months later, almost 1,800 pieces of debris created by the collision remained in orbit with the potential to impact spacecraft coming into contact with the wreck's waste.

In an effort to reduce the risk of further collisions and to study the impact of the cosmos environment on a satellite's motion, the Air Force Research Laboratory's Space Vehicles Directorate established the Orbital Drag Environment Program two years ago.

"If you want to predict where the satellite is, you've got to do three things," said Dr. Frank Marcos, senior research physicist, Institute for Scientific Research, Boston College, serving as a contractor with AFRL's Orbital Drag Environment Program. "One is you have to know what the solar inputs are and number two, you've got to have a model that takes those inputs correctly and here is what it does to satellite friction in time and space. Number three is somebody has got to go ahead and say that the satellite has got this size and shape and this drag coefficient so it is going to propagate like that. The first part is the solar part; the second part is accurately predicting the satellite drag and then taking the friction coefficient and decide the shape and size of the orbit propagation modeling to know physically where the satellite is going to be."

As Dr. Marcos stated, knowing when and where the next active or defunct space object plunges toward Earth or avoids bumping into another satellite must take into account the sun's role in the process. Solar-generated activity such as sunspots have been forecasted in 11-year cycles. Scientists predicted the current round would have minimal effects and peak in 2013.

Nevertheless, radiation from solar wind and particles heats up the thermosphere, the largest of the Earth's atmospheric layers, situated 50 to 300 miles above the planet and causes it to enlarge, producing increased density and ultimately, friction. For spacecraft operating in this realm, the sun-induced drag prompts a decrease in altitude and potential for collision with other satellites operating in low Earth orbit, which ranges from 100 to 1,240 miles above the earth's surface. It has been estimated that 200,000 debris pieces comprise 95 percent of space objects located in LEO.

"I need to stress the importance of getting the solar predictions right," said Ed Cliver, principal astrophysicist, AFRL's Space Vehicle Directorate. "We're dealing with expensive satellites and replacing them comes down to dollars and cents. How soon we have to replace them and how long they will last - those are important issues for the particular mission. I would like to be able to make shorter-term forecasts on when and where a satellite or other space object will re-enter the Earth's atmosphere."

The program's initial case study involved Tactical Satellite-2, which launched in Dec. 2006 and successfully completed a year-long mission. In the last three plus years, the spacecraft, administered by AFRL, but under the operational control of Air Force Space Command, floated propulsionless in LEO. Due to minimal solar activity, the satellite's life had been extended and its demise had been long overdue. In Sept. 2010, project participants conducted research on TacSat-2's likely end trajectory and upcoming solar impacts and predicted the defunct spacecraft would re-enter the Earth's atmosphere in February 2011.

"Our team used the current knowledge of solar cycle predictions to predict TacSat-2 re-entering last September,' said Dr. Chin Lin, program manager, AFRL's Orbital Drag Environment Program. "We got an excellent prediction at that time."

As the TacSat-2's believed return to the Earth's atmosphere got closer, the program forecasted February 6 as the due date. On February 4, Dr. Lin and his team, which included solar and astrodynamics sections' expertise, predicted the spacecraft would re-enter on February 5 at 9:15 p.m. Mountain Standard Time. TacSat-2 ultimately came down at 10:35 a.m. MST that day and some surviving parts fell on an uninhabited area on Earth. Although only 10 hours and 40 minutes off of the real-time event, the Orbital Drag Environment Program seeks to improve on future forecasts through merging of the other two participating entities, the solar and astrodynamics groups.

"We would like to, in the future, combine AFRL's solar and astrodynamics groups and even maybe NASA with the Orbital Drag Environment Program to improve the accuracy of the satellite re-entry prediction model and to enhance prediction of where and when they are going to land," said Dr. Lin. "Ultimately, we are working to produce prediction models that will assist in making the Iridium 33 and Kosmos-2251 satellites' collision a one-time event and our recent TacSat-2 study is a productive first step in that process."