Why it's hard to predict where failed Russian craft will fall

Where will the remains of Russia's derelict Phobos-Grunt tumble back to Earth?

That's this weekend's 5-billion-ruble question as space agencies and other satellite trackers around the world try to gauge when and where the spacecraft is likely to begin its fiery descent into Earth's atmosphere.

Estimates on when reentry starts range from about 11:20 a.m., Eastern time, on Sunday to 8 p.m. Monday. But they can change several times a day, lending forecasting efforts an air of confusion.

The uncertainty highlights the challenge in forecasting a track for an object no one can control, specialists say. And it underscores a need for a freer flow of more accurate information on satellite positions and tracks to avoid collisions that would add to the space junk already on orbit, others add.

For its part, Phobos-Grunt appears highly unlikely to add to the space-junk problem. Nor is it deemed much of a threat to people on the ground.

Phobos-Grunt launched Nov. 8. The mission aimed to return soil samples from Mars' moon Phobos. The spacecraft also carries a small Chinese probe designed to orbit Mars and gather data on its atmosphere. And it is acting as cosmic sherpa for a small experiment sponsored by the Planetary Society in Pasadena, Calif. The experiment is designed to test whether microbes could survive a prolonged trip from one planet to another housed, in this case, in an artificial meteoroid. The meteoroid was to have returned to Earth along with soil samples from Phobos.

Phobos-Grunt failed to leave low-Earth orbit as planned, however. Efforts to communicate with the 13.5-metric ton craft failed, leaving ground controllers with no way to guide it on a predictable path to a controlled reentry.

Initially, concerns centered mainly on whether the craft's tanks bearing 11 tons of toxic fuel could survive the plunge and then present a hazard to people on the ground.

But engineers with Russia's space agency Roscosmos and with the National Aeronautics and Space Administration have calculated that the aluminum fuel tanks will burst before the craft falls below about 60 miles in altitude, allowing the fuel to dissipate harmlessly.

"This, combined with a relatively low dry mass of just 2.5 tons, means Phobos-Grunt is not considered to be a high-risk entry object," said Heiner Klinkrad, who heads the European Space Agency's Space Debris Office, in a statement released Jan. 12.

Still, tracking the craft is important, notes Joshua Horwood, a research scientist with Numerica Corp., a company based in Loveland, Colo., that develops orbital tracking and forecasting software for the US military, in addition to other forms of specialized computer-based tools for civilian aerospace and biomedical clients.

In addition to the need to know where and when a spacecraft will end up after reentry, information on where an uncontrolled craft may be headed next is vital to other satellite operators, who may need to move their craft out of the way. But forecasting the track of an uncontrollable craft – especially one in low-Earth orbit, or between 100 and 800 miles up – is fiendishly difficult, Dr. Horwood says.

Even in low-Earth orbit, he notes, the significantly thinned atmosphere can exert drag on a spacecraft, slowing it and allowing Earth's gravity to tug it out of an intended orbit. The sun's activity can change the atmosphere's density with altitude, increasing or decreasing drag.

In this case, the drag is acting on a very irregularly shaped craft, which makes its effect very hard to calculate, Horwood says.

Working backward from the most recent observation of a derelict's position to reconstruct an orbit from past observations is no problem, he adds. But drag and other issues can throw curves into projections of where in the sky one can next expect to spot the object.

"We're not dealing with cannonballs here," he quips.

Still, one potential benefit from Russia's woes is that the information gained from tracking Phobos-Grunt could help improve track-forecasting models, Horwood says.

Indeed, the international community is doing that now.

The Inter-Agency Space Debris Coordination Committee, made up of representatives from 12 national space agencies, is using Phobos-Grunt's problems as an opportunity to figure out more-accurate ways of estimating the reentry time and place for incoming spacecraft.

Then again, the estimates are only as good as the data that feed them. The best data come from the United States, Russia, and Europe. But while the US cooperates with space agencies in alerting them to possible hazards to their craft, it keeps the most accurate of its tracking information under wraps.

Some data need to be held close to the vest for national-security reasons, acknowledges Brian Weeden, a former Air Force officer who was deeply involved in the US military's efforts to track objects in space.

"But we think there is a batter balance" that can be struck in making more accurate information publicly available, says Mr. Weeden, currently the technical advisor to the Secure World Foundation, which advocates the sustainable use of near-Earth space.