PARIS — The Russian space agency Roscosmos said Thursday that its stranded Phobos-Grunt spacecraft could re-enter Earth’s atmosphere on Sunday over a patch of the Indian Ocean off the coast of Tanzania.

However, the prediction comes less than 24 hr. after the agency said the point of re-entry was most likely to occur over the ocean west of Indonesia.

“Dynamics of the deceleration of the spacecraft in Earth’s atmosphere depends on a variety of technical and space factors, including those that cannot be controlled by man,” the agency said in a statement. “Chief among these is the ambiguity of the behavior of the atmosphere,” which expands and contracts relative to the Earth’s solar cycle.

“The exact spot, the date and time of their fall is possible to predict, but not earlier than one day beforehand,” the agency says.

The 13,200-kg (29,040-lb.) Phobos-Grunt was left stranded in low Earth orbit after its Nov. 8 launch atop a Zenit 2-SB rocket when the spacecraft failed to boost itself into an interplanetary trajectory after separation. Subsequent efforts to salvage the mission failed and the spacecraft’s orbit has been gradually decaying ever since.

Tough call

Brian Weeden of the Secure World Foundation, a nonprofit group based in Washington, says an accurate prediction is unlikely even hours prior to re-entry.

“At the moment it’s really beyond our abilities,” says Weeden, a former U.S. Air Force officer with expertise in orbital debris and space situational awareness. “It’s not a conspiracy, it’s just a really hard problem. There’s a lot of things that need to be taken into account.”

For example, if an object is traveling in a mostly circular orbit, “it can re-enter just about anywhere along that orbit,” Weeden says. “There’s a lot of unknowns in terms of upper winds in the atmosphere, size and shape, angles and velocity, whether it’s rotating or not, when it breaks apart, what comes off and what doesn’t come off; all of that plays into it and makes it very difficult to say.”

As of Thursday, Phobos-Grunt was traveling in a near-circular orbit between 163 km (100 mi.) and 193 km above the Earth at an inclination of 51 deg.

Weeden says the accuracy of predictions will improve as the spacecraft nears re-entry, but that “even a couple hours ahead of time, you’re still looking at, in a lot of cases, plus or minus 20 or 30 minutes re-entry time.”

In a Wednesday statement, Roscosmos said between 20-30 pieces of debris weighing a combined 200 kg are expected to survive atmospheric re-entry, raining down over a strip of the Earth’s surface between latitudes 51.4 deg. north and south. But the agency downplayed the risk to populated areas, including the re-entry of toxic hydrazine fuel on board the spacecraft, which the agency said is expected to burn up in the atmosphere at an altitude of about 100 km.

Likewise, a small amount of radioisotope used in one of the scientific instruments aboard the spacecraft, a miniaturized Mossbauer Spectrometer, constitutes a mass of less than 10 micrograms. However, the isotope’s small half-life “will not cause dangerous contamination,” Roscosmos said.

One piece

Still, at least one piece of the spacecraft, a small canister weighing less than 10 grams that was intended to return soil samples from Mars’ moon Phobos, is expected to fall to Earth intact.

“It was designed to survive re-entry and return samples from Phobos,” Weeden said. “So it’s going to land on the ground somewhere.”

Still, Roscosmos said the danger from falling debris is minimal.

“Many years of international statistics shows that in leaving orbit, spacecraft are almost always completely burned in the dense atmosphere and their residual fragments, as a rule, do no harm,” Roscosmos said.