The Progress M-27M (59P) cargo space vehicle, which was launched April 28 on a routine resupply mission to the International Space Station, reentered the Earth’s atmosphere at 9:04 p.m. CDT (02:04 GMT May 8) while over the Pacific Ocean, Russia’s space agency reported.

“Progress M-27M ceased to exist,” Roscosmos stated on its website, adding that the spacecraft met its end on the 160th orbit around the planet. It was not immediately clear if the report was based on observed data or predictions. U.S. Strategic Command gave a re-entry time of 9:20 p.m. CDT (0220 GMT) over the South Pacific to the west of the southern tip of South America.

Generally, experts maintain that the risk of anyone being hit by falling space debris is extremely remote. But what does it mean by ‘uncontrolled’, and is it true that the probability of casualty on ground is so remote?

There has been so far only one occasion in which a human came in contact with a falling space debris. In 1997, in Tulsa, Oklahoma (USA), a woman was walking her dog before sunrise when she watched a fireball streaking through the sky from north to south. Half an hour later a slowly falling fragment brushed the lady’s left shoulder and hit the ground with a metallic sound. The debris was from the re-entering second stage of a Delta II rocket that was launched 8 months before. Around the same time, a large 250 kg stainless steel propellant tank from the same rocket landed in the front yard of a farmer near Georgetown, Texas, and outside the town of Seguin, also in Texas, a titanium sphere embedded into a field. Reports of similar space debris landings appear regularly in the press in particular for large conspicuous pieces. Luckily, up to now, no one has been injured or killed, YET.

For decades, spacecraft and rocket upper stages in Low Earth Orbit (i.e. orbiting between 160km and 2000km) were designed to be left decay naturally from orbit and to reenter uncontrolled. They were not viewed as a major hazard because the tremendous heat generated upon reentry was assumed to completely destroy them. Only very large space structures, like Skylab and MIR station, and satellites containing radioactive or other toxic materials were deemed significant threats. This viewpoint appeared to be supported by the fact that many satellites and rocket upper stages have been re-entering the Earth’s atmosphere, but very few pieces have actually been recovered. However, the reason for this is not necessarily that the objects completely burn up in the atmosphere, but rather that the probability of impacting a populated area on the Earth’s surface is very low, and the probability that debris are found is also low. When detailed search for surviving debris was done on a large scale, as following the Shuttle Columbia accident, the amount of recovered debris far exceeded the expected 10-30% of dry mass.

Number and mass of surviving fragments directly affects casualty expectation. If the risk of casualty is larger than 1 in 10,000 (risk threshold initially defined by NASA in 1997), a controlled reentry must be performed. Essentially the reentry has to pinpoint an area of the globe that is uninhabited and that can be cleared in advance from air and sea traffic. It should be noted that aircraft are highly vulnerable even to small fragments of few hundred grams. For making a controlled reentry specific maneuvers need to be planned and executed, otherwise a random or uncontrolled reentry takes place.

Heavy spacecraft, as the 7 tons Russian Progress M-27, much exceed the risk threshold for uncontrolled reentry. The estimate is between 1 in 1000 and 1 in 2000 compared with the 1 in 10,000 threshold. Unfortunately the timing of an uncontrolled reentry cannot be predicted. As a spacecraft grazes the outer fringes of the atmosphere it generates drag that slows the craft down by a small amount. However, the vehicle also generates some lift that causes it to bounce back possibly several times as it skips along the upper reaches of the atmosphere much like a stone skipping across the surface of a lake. This skipping gradually slows the spacecraft until it starts re-entering the atmosphere. Because of uncertainties about atmospheric density, and orientation and dynamics of a reentering spacecraft the time of an uncontrolled reentry can at best be predicted with an error of approximately 10 percent of the estimated time to reentry. That means an uncertainty in the reentry point of approximately ±2740 km one hour prior to reentry.

Now that Progress has reentered, are we safe from falling space debris? Yes for 5 days, until May 12 when a massive Ariane 44LP rocket body is expected to reenter. And next? Every week there is a major space system reentering uncontrolled. You can follow reentry forecasts at http://www.aerospace.org/cords/reentry-predictions/upcoming-reentries-2-2/