A 1400 lb tank of liquid toxic ammonia, dubbed the Early Ammonia Servicer (EAS), which was used early in the life of the international space station to provide backup cooling for the onboard electronic equipment, but was rendered obsolete with the installation of a more efficient, permanent cooling system. In a move uncommon for such large pieces of debris, the EAS was deliberately jettisoned from the space station’s robotic arm in 2007, and has been travelling on a slow decaying orbit ever since.

The refrigerator-sized tank was supposed to be flown back to Earth in a space shuttle and be safely disposed of there, but NASA scientists concluded that its structural integrity was not good enough to permit this, and rather than risk the loss of a shuttle, they carefully detached it and let it float into space, taking great care so that it doesn’t collide with the ISS or other space craft. There was also a 212 lb obsolete camera which was dumped along with the EAS but it has long since burned up due to friction with the extreme upper atmosphere of the earth.

The lumbering ammonia tank is too big for that to happen, however, and the friction merely served to slow it down, decaying its orbit slowly for the past year. It is expected to fall out of orbit sometime Sunday afternoon and re-enter Earth’s atmosphere proper. IT is not precisely known where it will hit, but NASA say there isn’t cause for alarm.

"This has got a very low likelihood that anybody will be impacted by it," said Mike Suffredini, NASA's space station program manager, in an interview. "But still, it is a large object and pieces will enter and we just need to be cautious."

Specialists from NASA are expecting up to 15 pieces of the EAS to weather the extremely hot temperatures caused by re-entry fiction. The pieces range in size from 1.4 ounces to almost 40 pounds. If any hit land, the largest of the pieces will hit Terra’s surface at 100 mph, but a splashdown is more likely, as two thirds of the planet are covered by oceans.

"If anybody found a piece of anything on the ground Monday morning, I would hope they wouldn't get too close to it," Suffredini said.

The Early Ammonia Servicer is the largest piece of orbital debris hand jettisoned from the international space station. One-shot Russian and European cargo spacecraft are also destroyed in Earth’s atmosphere over the Pacific Ocean as a matter of routine. But in their case the disposal is much more tightly controlled.

The EAS has served as a backup coolant reservoir which supplemented the space station’s main coolant system in the eventuality of leaks or other failures. Recent upgrades to the coolant system, however, have made the EAS obsolete and it had to be dumped for reasons expounded earlier.