Spacefaring Nations Pitched In To Pinpoint Phobos-Grunt Re-entry


PARIS — An international organization of spacefaring nations has concluded that the data provided by its members permitted an accurate prediction of the likely landing area of Russia’s Phobos-Grunt spacecraft, which entered the atmosphere in an uncontrolled flight Jan. 15.

The 12-member Inter-Agency Space Debris Coordination Committee (IADC), whose members do not always submit for common assessment all the data they collect on spacecraft whereabouts, said that combined data from the United States, Russia and Europe gave it a reasonably accurate picture of Phobos-Grunt’s last hours.

The data sources include ground radars in Russia, Europe and the United States and what two officials said was the almost certain contribution of U.S. infrared missile-detection satellites.

Phobos-Grunt was launched Nov. 8 on a mission to collect samples from Mars’ larger moon, Phobos, and to place a small Chinese satellite in Mars orbit. The spacecraft failed to leave Earth orbit for reasons Russian authorities have variously blamed on intentional or unintentional radiofrequency interference from U.S. ground radars, space radiation and poor workmanship by its builders.

The 19-nation European Space Agency (ESA) was assigned to coordinate the IADC’s re-entry prediction campaign for Phobos-Grunt.

Heiner Klinkrad, who is IADC secretary and heads the space debris office at ESA’s European Space Operations Centre in Darmstadt, Germany, said the IADC acquitted itself reasonably well for Phobos-Grunt’s re-entry given the limits of its mandate.

In a Feb. 1 interview, Klinkrad said IADC made no claims of having received all the Phobos-Grunt data available to its members. But using those inputs that were provided on the spacecraft’s orbit, the IADC campaign correctly predicted that Phobos-Grunt would enter the atmosphere near the southern coast of South America Jan. 15 at 17:45 Greenwich Mean Time.

U.S. Strategic Command, whose network of ground and satellite assets for tracking spacecraft is the most extensive, subsequently said Phobos-Grunt re-entered the atmosphere one minute later than the IADC prediction, at 46 degrees south latitude and 87 degrees west longitude.

Klinkrad said that as usual with U.S. data on re-entries, the identified time and place correspond to a spacecraft at an altitude of 80 kilometers. Using well-established models for what happens to objects at that altitude — well inside the atmosphere — ESA and the IADC produced a prediction that Phobos-Grunt would reach an altitude of 10 kilometers some seven minutes later.

Several amateur satellite watchers have speculated that IADC was too quick to dismiss the possibility that Phobos-Grunt debris could have landed on Chilean or Argentine territory.

Klinkrad did not reject these suggestions. He said that as Phobos-Grunt began to burn on contact with the atmosphere, it maintained a lateral velocity that can be modeled but not with pinpoint precision. “We think the likelihood is small” that debris fell on land, he said. “But this cannot be excluded. In any event, since Jan. 15 we have heard no reports suggesting that this occurred.”

Klinkrad said the data on Phobos-Grunt’s final hours were substantially aided by Russian radars, which he said were well-placed to track the spacecraft during its final orbit. French and German ground radars also provided data, he said.

U.S. Strategic Command’s data inputs to the IADC are delivered through NASA’s Johnson Space Center, where the agency’s orbital debris office is located.

The IADC does not ask for details on the sources of data provided by its members. Klinkrad declined to speculate on where the U.S. 80-kilometer data came from.

But two space debris experts said 80 kilometers corresponds to the point at which an object like Phobos-Grunt would give off the maximum amount of heat before it begins to break up. It is at this moment that the object would be most easily visible by U.S. missile-detection satellites carrying infrared sensors.

While Phobos-Grunt’s crash landing made international headlines with speculation of where it might fall — the satellite’s relatively circular orbit made it particularly difficult to predict a re-entry point until the day before it occurred — its breakup in the atmosphere means it will not add to the orbital debris field circling the Earth in low Earth orbit.

That piece of good news follows a year in which the total population of orbital debris decreased, in large measure because of increased solar activity, which expands the atmosphere and pulls more orbital objects from orbit.

NASA’s Orbital Debris Program Office, in its mid-January report on the state of the debris population, said the overall population should continue to decrease in 2012 and 2013, when the multiyear cycle of solar activity is expected to reach its peak.

At the end of 2011, NASA’s tally showed about 16,000 catalogued objects orbiting the Earth, a figure that does not include the smaller pieces of satellites and rocket upper stages that have broken up in orbit. Some 9,000 of these tracked objects were fragments of rockets or satellites large enough to be tracked by the U.S. Space Surveillance Network of ground radars. The figure was close to 10,000 a year earlier, according to NASA data.