The Heavenly Palace falls

Chinese space station Tiangong-1 crashed into the Pacific April 1

The Chinese space station Tiangong-1 (Tiangong means “heavenly palace”) de-orbited from space and crashed into the Pacific Ocean Sunday, April 1 at around 5 p.m. PST. No casualties have been reported.

Tiangong-1, China’s first prototype space station, launched in 2011. China’s space agency announced the end of its service in March 2016, and admitted a few months later that the agency had lost control of the station.

Without control over the space station, the agency could not guide the time and landing path of Tiangong-1. The majority of its 8.5 tons burned up during reentry into Earth’s atmosphere.

Large space debris can become a concern when it de-orbits without control and communication, like Tiangong-1. But as technology improves, these rare incidents are likely to become more limited. Small orbital debris, on the other hand, will burn up before entering the atmosphere and thus have little effect on life on Earth, but it poses a threat to space objects within lower Earth orbit.

According to NASA, man-made, orbital space debris consists of nonfunctional spacecrafts, abandoned launch vehicle stages, mission-related debris, and fragmentation debris. Orbital debris travels at 17,500 mph. At this velocity, even small paint flecks can cause severe damage to a space shuttle or satellite.

The Tech spoke with former NASA astronaut Jeffrey A. Hoffman, who is also a professor of Aeronautics and Astronautics at MIT. Hoffman has flown five space missions and was on the team sent to repair the Hubble Space Telescope in 1993. He noted that while there is little debris the size of Tiangong-1, there is a much larger number of smaller debris. According to NASA, 500,000 debris pieces are being tracked. Smaller pieces of debris are estimated to be in the millions. This small orbital debris poses a risk to functioning space shuttles, satellites, and space stations.

The size of the debris is important. “We have the ability, using radar, to track things down to the size of about five centimeters, two inches. Below that, we can’t really track them, so it’s just an estimate.” Hoffman said. This estimate can be performed using a Long Duration Exposure Facility, which is launched into space to record the number of hits over time to give an estimate for the abundance of orbital space debris.

“If radar tracking shows that any trackable object is going to pass within a critical distance, the station or shuttle will change its orbit,” Hoffman said, adding that space stations will generally maneuver to an orbit further from Earth, while space shuttles will maneuver toward Earth.

Spacecraft can be shielded against the small debris. “When a small piece of space debris hits the space station, it will vaporize before it actually damages the aluminum shell of the pressurized modules,” Hoffman said. The technology behind shielding consists of multiple layers of aluminum — most small debris will vaporize within the top two.

The debris that is too small to track but large enough to cause serious damage is the most dangerous type of orbital debris. “You’re actually much safer with one large object like Tiangong, because that's easy to track. What you’re worried about is when one large object collides with another object. Then instead of two objects, now you have 20,000 small objects going all over the place,” Hoffman said, recalling China’s 2007 anti-satellite missile test, for which the country was widely criticized.

The resulting phenomenon is known as the Kessler syndrome, or collisional cascading. Every collision can generate space debris and increase the probability of further collisions, thus creating an exponential runaway. “If that happens, lower Earth orbit would become unusable,” Hoffman said. 

Efforts to prevent the scenario of a Kessler syndrome are being discussed, but Hoffman explained the current challenges in realizing them: “There are two problems. Probably what you want to do, that would be most effective, is to bring down the large dead satellites, the things that would create the most debris if they got involved in a collision,” he said.

But the costs involved are enormous and it remains unresolved who would pay for them. “Space orbits belong to everybody,” Hoffman said. “So should the U.S. pay the money to clean up space so that the Chinese can have safer satellites?”

“The second problem is [that] by the Space Law Treaty, once a satellite is launched, it is considered the responsibility and the property of the launching nation. The U.S. does not have the legal right to remove even a dead solar Russian satellite from orbit,” Hoffman said.

Hoffman noted that technology has progressed so as to avoid a Kessler syndrome event. In the past, the upper stage rockets taking satellites into orbit posed a problem. “In many cases they had residual propellant in their fuel tank and over the years the pressure would build up and then they would explode and create many pieces of debris.” Today, pressure relief systems are built to prevent these explosions.

According to Hoffman, NASA is currently the only space agency with a policy requiring launched objects without propulsive capability to de-orbit within 25 years by atmospheric drag. The policy mostly concerns smaller satellites, which are often not equipped with propulsive capability.

The number of objects in lower Earth orbit is expected to rise and the probability of collisions will increase along with it. With advances in microelectronics, new policies will be called for to address the issue of trackability. “Currently, the size of a cube satellite is about 10 centimeters and that’s big enough to track, but there are people talking about launching even smaller satellites and the Air Force, which is responsible for tracking, is concerned about that because if you get smaller than five centimeters it is no longer trackable,” Hoffman said.

Hoffman also mentioned numerous proposed solutions, such as laser satellites, which use repeated laser detonations to decrease the orbital velocity and alter the orbital altitude of the debris. The atmospheric drag would then eventually draw the satellite into Earth’s atmosphere.

Thus far, the number of incidents caused by orbital space debris has been limited, and efforts to reach international agreements are slow. “So far we’ve been lucky,” Hoffman said.