Look up on a clear night, away from the city lights, and you might just spot a spacecraft falling out of the sky. The Chinese space station Tiangong (Hanyu pinyin for “Heavenly Palace”) 1 is in an ever-closing orbit around Earth and is expected to enter the atmosphere as a spectacular fireball any time in the next few days. The stricken station currently swings by over northern latitudes in minute-long passes and can be seen in Delhi skies, too, as a faint dot of magnitude 4 to the unaided eye.
The 8.5-tonne-Tiangong-1 has been orbiting Earth since 2011 at an altitude slightly lower than the International Space Station (ISS), helping Chinese scientists study rendezvous and docking technologies. It was also used as a base for China’s manned Shenzhou 9 and 10 missions. But ever since its power systems failed a couple of years ago, engineers have been counting down to the demise of Tiangong 1 and its eventual fall Earthwards.
A spacecraft drops from orbit when it loses its velocity due to the constant tug of Earth’s gravity. Although the outer layer of the atmosphere is very thin, it is still thick enough to drain orbital energy from spacecraft. This orbital decay happens to all satellites (including the ISS) as their orbits are disturbed by more than a kilometre every year. Space engineers use rocket thrusters on board the spacecraft to realign their orbits at periodic intervals. As a spacecraft enters the upper reaches of the atmosphere, it falls faster, squashing the air molecules below it, and they heat up. Most of the time, the spacecraft burns up completely in the upper reaches and all we get to see on the ground are ‘shooting stars’: the fuselage, solar panels and so on burning up brightly as they hurtle down.
The most famous example of orbit decay was the US space station Skylab, which burned up in the atmosphere in July 1979. It was the largest uncontrolled reentry by spacecraft. Debris from the 77-tonne Skylab fell harmlessly into the Indian Ocean and across a remote region of western Australia. In March 2001, pieces from the 130-tonne Russian space station Mir splashed down in the south Pacific.
The larger the spacecraft, the more the likelihood of at least some parts surviving the fiery descent and making it to the ground. Even then, the chances of any spacecraft debris falling over populated regions are extremely slim as much of the planet is covered by water. In the entire history of spaceflight, there has been only one incident of a human being struck by space junk. On January 22, 1997, a small metal piece from a Delta rocket grazed the left shoulder of Lottie Williams of Oklahoma, leaving her more surprised than injured.
Although the Chinese publicly dismiss Tiangong 1’s re-entry as simply “the end of a mission”, there is the theoretical possibility of some of the craft’s remnants surviving reentry. The leftover hydrazine fuel on Tiangong 1 is highly toxic and adds a dangerous dimension to the station’s fall. But these concerns pale before an alarming aspect of space exploration that the kamikaze dive of Tiangong 1 highlights: the increasing space junk in low Earth orbit (LEO).
In addition to the 2,000-odd satellites currently orbiting Earth, there are thousands of derelict satellites and spent rockets circling the planet along with countless metal pieces, solar cells, flecks of paint, etc. (Sorry, Elon Musk, but your car just added to this orbiting jetsam.) In such a crowded space environment, the chances of collision rise dramatically as space trash moves at speeds touching 30,000 kph. At such speeds, even minuscule scrap pieces can rip holes in a spacecraft’s fuselage or its solar panels, or disable satellites by causing electrical shorts.
To clear orbital trash, space agencies have toyed with several ideas. This includes the use of ground-based lasers to alter spent satellites’ orbits, powering them with ion engines so they would reenter the atmosphere faster and deploying electrodynamic ‘trusses’ to increase drag on space trash and nudge them into the atmosphere.
The best way to make the LEO a litter-free zone, however, may be to not litter in the first place. There are several ways in which this can be done – from ensuring no fuel is left behind on discarded satellites to prodding expended rocket stages to lower orbits so that they burn up during reentry.
After the Tiangong reentry, a 1 tonne Indian rocket stage is probably next. PSLV-C19 fourth stage, launched in 2012, currently in a 205 x 218 km orbit.
— Jonathan McDowell (@planet4589) March 30, 2018
The so-called ‘25-year law’ is a voluntary rule that major spacefaring countries follow. It makes it incumbent upon space agencies to ensure that anything launched into LEO comes back to Earth within 25 years once the mission is over. Never mind if some agencies find it cheaper and convenient to just abandon spent satellites.
In any case, space experts believe that all these efforts may count for too little too late. For even without any new launches, the amount of space junk created by collisions in orbit would exceed the amount that falls back to Earth in the next 40 years. In fact, the rate at which the LEO is getting crowded, getting into orbit may soon become too dangerous. Imagine a time when you can no longer launch rockets!
So are we prepared to go the extra mile by skipping Earth orbit altogether whenever possible so as to not add to the junk in orbit? Terrestrial fibre-optic systems, for instance, can easily handle many functions of orbiting satellites such as providing for phone calls, television and other communication links. And given the speed of technological development today, it shouldn’t be difficult for space agencies to work with private companies to come up with viable solutions. The only alternative, of course, is to watch helplessly as the blanket of trash spreading across LEO steadily closes the only door we have to the final frontier.
Prakash Chandra is a science writer.