Tiangong-1 Is Expected to Crash Tomorrow. The Odds of It Hitting Anyone Are One in a Trillion

Defunct spacecraft all burn up in the atmosphere. A few leave debris behind, but none has ever hit a person.

The North American Aerospace Defense Command (NORAD) predicts that China's spacecraft Tiangong-1 will re-enter the atmosphere at 21:30 UTC on April 1 — 5:30 a.m. Beijing time on April 2 — somewhere near Chile, in the Southern Hemisphere. Other agencies put it at around the same moment, give or take about 24 hours.

For the past six months to a year, the Beijing Aerospace Control Center, the European Space Agency (ESA) and NORAD have all been tracking Tiangong-1's flight path. Ever since its equipment failed and its mission ended on March 21, 2016, drag from the thin atmosphere of low Earth orbit has made one outcome certain: it will fall into the atmosphere and burn up. That is the standard way every defunct spacecraft is disposed of.

Over the past month, space agencies have issued forecast after forecast of Tiangong-1's descent path. ESA has said several times that the craft would re-enter the atmosphere in an uncontrolled state in late March or early April. A research institute under ESA used the TIRA radar to produce radar images of Tiangong-1 that show it tumbling as it flies — out of control, in other words.

China's space authorities have avoided the word "uncontrolled." Their statements say Tiangong-1's mission ended in 2016 after it outlasted its planned service life, that it is now re-entering the atmosphere and burning up on schedule, and that re-entry is expected in early April. In fact, once its fuel and power ran out, Tiangong-1 has been flying uncontrolled. But a spacecraft this size should burn up completely in the atmosphere, and the chance that any surviving debris hits someone on the ground is, mathematically, tiny. Tens of thousands of spacecraft have re-entered the atmosphere over the years, and not one has hit a person.

Tiangong-1's main job was to test docking, the technology you need to build a space station

Tiangong-1 is a large spacecraft China launched on September 29, 2011. China calls it a "target vehicle," because its main function is simply to fly in orbit; you can think of it, roughly, as a small space station. In 2011, 2012 and 2013 China launched the Shenzhou 8, 9 and 10 spacecraft, each of which docked with Tiangong-1 in turn, proving out orbital docking technology. Astronauts also passed from the Shenzhou craft into Tiangong-1 and carried out tasks such as teaching a lesson from space.

Orbital docking is the technology you need to build a space station that can operate for years. Today the only spacecraft in the world that can carry people into low Earth orbit are China's Shenzhou series and Russia's Soyuz-TM series. Both are small, truck-sized vehicles designed to stay in orbit for about a week. Craft like these have little room inside; they are transport, not housing.

The only station in space with people living aboard permanently is the International Space Station (ISS), built and run jointly by the United States, Europe, Russia, Japan and other countries. Several stations flew before it, among them the United States' Skylab and the Soviet Union's Salyut and Mir. Mir stayed in orbit for 15 years and was the only long-running station before the ISS went up.

The ability to house people for long stretches is the core difference between a space station and a spacecraft. A station needs a complete air- and wastewater-recycling system, enough room to keep the crew's work, sleep and private time apart, separate bathing and toilet facilities, and space to store water and food.

A station's docking ports also have to take both crewed and cargo craft, so that crews can rotate, food and water can arrive, rubbish can be hauled away and the orbit can be maintained.

So far China has launched two "target vehicles," Tiangong-1 and Tiangong-2, and plans to launch a similar core module called Tianhe between 2018 and 2020. More modules will be docked onto it later to build a permanently crewed "China Space Station."

After Mir was retired, it too re-entered the atmosphere and burned up

Between 1986 and 2001 the only long-term crewed station in Earth orbit was the Soviet, later Russian, Mir. Starting with the launch of its core module in 1986, Mir was assembled over many launches, its modules docking together in space like building blocks until the station was complete.

Seven launches later, in 1996, the finished Mir weighed 129 tonnes and measured 20 to 30 meters in each dimension — as tall as a 6- to 10-story building. Its pressurized interior came to 350 cubic meters, about the entire inside of a 120-square-meter apartment. Mir could house three expedition crew members permanently and up to six for short stays.

Over those years Mir used Soyuz craft and the American Space Shuttle to ferry people, and Progress freighters to bring supplies and carry rubbish away.

The International Space Station, Mir's successor, is bigger and better equipped. When construction finished in 2011 the ISS weighed 419 tonnes, spanned more than 100 meters and enclosed 930 cubic meters of pressurized volume — nearly three Mirs. What it can do goes well beyond Mir too.

Mir was designed to last five years, a life extended several times once international partners came aboard. In the end it was decided that after its final mission ended on April 4, 2000, Mir would re-enter the atmosphere and burn up under control: the station was too big to be sure it would burn up completely on the way down, so to keep it from hurting anyone on the ground, the last Progress-M freighter served as a braking rocket, firing repeatedly to lower Mir's orbit. At 05:08 UTC on March 23, 2001, the engine burned for 22 minutes straight to slow the station, bringing it into the atmosphere at 05:44 UTC over the thinly populated South Pacific, where whatever survived would fall into the sea.

Tiangong-1 is small, so it will most likely burn up clean

Tiangong-1 weighs 8 tonnes and is 10 meters long, about the size of a heavy truck. In theory, if Mir — 16 times heavier — could burn away almost entirely, the chance that Tiangong-1 leaves debris is very small.

The hard part of re-entry, in fact, is usually making sure a spacecraft does not burn up.

For Earth, the atmosphere is an excellent shield. Nearly everything that enters it is torn apart by drag and aerodynamic heating and burns to ash at an accelerating rate. The meteors you can see with the naked eye are the trails left by meteoroids burning up in the air.

Man-made objects can hold together better, because parts such as engines are denser than natural ones. Statistics suggest that roughly 10-40% of the mass of a re-entering satellite may reach Earth's surface. But most of that surface is water, and much of the land is thinly populated, so the chance of a person being hit by man-made debris is less than one in a trillion.

When people do need a craft to reach the ground intact — a ballistic missile warhead, a crewed spacecraft, the return capsule of a research craft — it takes a whole set of techniques.

Take a ballistic missile. Once it launches into space, flies to the neighborhood of its target and its rocket engine trims the trajectory, it enters the atmosphere at a nearly vertical angle. The warhead is shaped as a hyperbolic cone to minimize the atmosphere's effect, and is wrapped in heat-insulating materials — plastic, resin, carbon fiber — so that it can pass through the air and reach the spot it is aimed at.

A crewed spacecraft is harder. It is bigger than a warhead, and beyond keeping it from burning up, engineers have to give it an aerodynamic shape that uses the atmosphere to brake, so that by the time it nears the ground it is slow enough to open a parachute and land.

Ordinary crewed craft such as Shenzhou and Soyuz have can-shaped hulls with an ablative layer of carbon fiber and resin on the bottom, and are built to a particular profile. Coming down, the craft enters the atmosphere at a set angle, its broad, blunt end leading, to slow itself. The end of the film Gravity shows a return capsule's descent in detail, along with the other modules — the ones never meant to come back — burning up in the atmosphere.

The strangest case is the American Space Shuttle. Far larger than an ordinary spacecraft, it carried thermal-protection tiles on its underside and also had to hit the atmosphere at a precise angle of -5.5 to 6.9 degrees to the atmospheric tangent. The Shuttle turned its belly to the air with its nose pitched up, skipping across the sky like a stone across water.

Big spacecraft can drop debris on the ground, but none has ever hit a person

The worst spaceflight accident in history during re-entry was the loss of the Space Shuttle Columbia. On February 1, 2003, Columbia broke apart on its way home from the STS-107 mission.

At launch on January 16, a piece of foam had come off the fuel tank and struck the heat-shielding tiles on a wing, damaging them. Foam came off routinely during launches, but it had never damaged a shuttle before.

When the mission ended on February 1, the shuttle left orbit and began re-entry at 8:15 a.m. Eastern time. At 8:48, sensors on the leading edge of the left wing recorded temperatures that were too high. Normally aerodynamic heating drives the outer surface of the tiles to 1,370 degrees Celsius while the tiles protect the aluminum-alloy airframe beneath. The damaged tile let searing gas into the wing structure.

At 8:53 the shuttle reached the U.S. West Coast, 70 kilometers up and moving at more than 23 times the speed of sound, the wing by then above 1,540 degrees Celsius. People on the ground saw debris starting to come off it.

Between 8:54 and 8:59, observers on the ground kept seeing debris fall, and unusual flashes on the shuttle's body. Normally a shuttle enters the atmosphere glowing red-hot at a steady brightness. At the same time its sensors began returning abnormal readings.

At 8:59:32, mission control received its last word from the crew — the commander said only "Roger" — and then the signal was gone. After that the main hydraulic system began throwing errors and the craft started yawing beyond control.

From 9:00, people on the ground watched the shuttle break up from the nose, turning into a great many fragments. In theory the astronauts could have survived a few dozen seconds more, but nothing could be done. At 9:05 the shuttle gave off a loud bang and broke into still smaller pieces.

In the end Columbia and its seven astronauts became more than 40,000 pieces of debris that fell inside the United States. None of it caused a casualty on the ground. Columbia left so much debris because it passed through the upper atmosphere still intact; it came apart below 70 kilometers, by which point its speed had dropped from a peak of 23 times the speed of sound to under 20, and the atmosphere's burning effect had already weakened.

Because Tiangong-1 is uncontrolled, when it comes down depends entirely on its contest with the upper atmosphere. NORAD predicts it will begin re-entering near the coast of Chile at about 5 a.m. Beijing time on April 2; ESA puts it later, near Australia's east coast at about noon Beijing time the same day.

People on the ground should be able to see it glow as it comes in. All the regions predicted so far are in the Southern Hemisphere, so nothing will be visible from China. But if Tiangong-1 re-enters over the Northern Hemisphere, its orbit would pass close to Chinese airspace, and the glow might be visible to the naked eye.

Cover image/A still from Gravity