How and Why Did Two Satellites Collide This Week?

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The collision this week involving an active U.S. commercial Iridium satellite and an inactive Russian Cosmos 2251 satellite in low Earth orbit has, if nothing else, raised public awareness of the growing problem of space debris. But how and why did this collision happen? If NORAD, the U.S. Air Forces’s Space Surveillance Network, NASA’s Orbital Debris Program Office and other entities are tracking space debris, did anyone know the collision was going to occur? Those who analyze data and track satellites say predicting collisions is difficult because of changes in satellite orbits which occur due to solar radiation and the gravitational effects of the Moon and Earth. Therefore, the orbit analysis is only as good as the data, which may be imprecise. “The main problem here is the data quality for the data representing the satellites locations,” said Bob Hall, Technical Director of Analytical Graphics, Inc. (AGI), the company that released video and images on Thursday recreating the collision event. “Given the uncertainty in the accuracy of the TLE orbital data, I do not believe anyone was predicting or necessarily expecting an event.”

AGI has tools that run automatically every day such as SOCRATES – (Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space) which is based on the current space catalog supplied by NORAD to look for close approaches.

“This analysis is performed automatically every day and you can easily go in and search it,” Hall told Universe Today. “Because the analysis is performed with the public two-line element (TLE) set satellite catalog, the analysis is only as good as that imprecise data is. So when it shows conjunctions on any given day (and for Tuesday this Iridium event was not even in the ‘top 10’ close approach predictions!) this has to be taken with some uncertainty.”

Hall said the closest approach predicted for last Tuesday’s Iridium-Cosmos event was predicted to be 584 meters. “Again, as close as that sounds (and it is), there were at least 10 other on-orbit conjunction predictions that day alone with smaller miss distances,” Hall said.

The crash occurred on Tuesday 485 miles above northern Siberia in a crowded polar orbit used by satellites that monitor weather, relay communications and perform scientific surveys.

The International Space Station, as well as most satellites can be maneuvered out of harm’s way to avoid a possible collision, but a defunct satellite like the Russian Cosmos 2251 has no such ability.

Even with the uncertainties of tracking orbiting satellites, one group, the Secure World Foundation, is calling for the need to establish a civil space traffic control system.

“Unfortunately, it appears that there was data warning about the possibility of this collision beforehand,” noted Brian Weeden, Technical Consultant for Secure World Foundation. “However, it must be stressed that close approaches between satellites somewhere in Earth orbit occurs on almost a weekly basis…and until this event, have never before resulted in an actual collision.”

Weeden agreed that in every case it is impossible to give a definite answer on whether or not two objects will actually collide, only probabilities and potential risks.

“Getting the right information to the right authorities in time to make the right avoidance maneuver decision is a very complicated process that doesn’t entirely exist yet,” Weeden said. “The Secure World Foundation is working with many other organizations around the world to try and develop this process.”

The Secure World Foundation endorses the creation of a space traffic control system.

“This collision underscores in a dramatic way the importance of instituting an international civil space situational awareness (SSA) system as soon as possible,” said Dr. Ray Williamson Executive Director of Secure World Foundation.

Williamson said that such a civil SSA system could have been used to warn the Iridium operations managers of the danger of collision and allow them to take evasive action. “In the absence of reliable ways to clear debris from orbit, it will be increasingly important to follow all active satellites to prevent future preventable collisions,” he added.

Before this collision, another collision event happened in 1996, when a French spy satellite called Cerise was severely damaged by a piece of debris from the rocket that launched it.

The United States tracks debris or micro-meteorites down to 10 cm wide, but objects as small as a scrap of peeled-off paint can pose a threat once they start hurtling at orbital speeds through space.

Sources: Email exchange with Bob Hall of AGI, Secure World Foundation press release, Reuters