Without determined efforts on an international scale, it may be only a question of time before the most useful regions of space become too hazardous to operate in. The need of the hour is to avoid creating any more space debris.

After just half a century of spaceflight, there’s so much ‘junk’ up there that it is becoming a major hazard. When the United States launched Vanguard I in 1958, its makers could scarcely have imagined that it would survive many decades. In fact it remains the oldest artificial item still in orbit around the earth. Out of the known population of 19,000 large objects and about 30,000 total objects ever launched, a little over 1,000 are operational satellites; the other surviving objects are, by definition, space debris.

Most objects orbiting the earth cannot remain circling forever. Slowed down by the outer fringe of the atmosphere they eventually burn up as they return to earth. A few larger objects may reach the ground practically intact and constitute a potential risk to life and property. However, since space exploration began no one has ever been hurt by falling debris. In September, when the out-of-control bus-sized Upper Atmosphere Research Satellite was expected to plunge back to earth it did create a bit of a scare. But NASA claimed that the chances that a human being might be hit were just one in 3,200. Considering that earth’s population exceeds seven billion, the chances of a specific person “P” being hit were less than one in 21 trillion. In comparison, the chances of “P” being struck by lightning are as high as one in 60,000.

Kessler’s Killer Syndrome

Of far greater concern is the threat of damage to satellites. Several confirmed and suspected impact events have already taken place in space. According to a recent US study, the amount of debris orbiting the earth has reached “a tipping point” for collisions, which could in turn generate more debris. The vast majority of debris consists of small objects, one cm or less across, numbering perhaps tens of millions of separate fragments. Between one and 10 cm size, there are estimated to be over 5,00,000 items. What of the large items of junk? There are already 22,000 objects in orbit that are big enough (10 cm or more) to be tracked from the ground. They could cause severe damage to human-carrying spaceships and expensive satellites especially in low earth orbit (LEO).

A worrying possibility is that if the existing junk does not decay from orbit before impacting other objects, the number of debris items will continue to grow even if there are no further rocket launches. A one kg object, for example, at a closing velocity of 10 kmps (36,000 kmph), can be more destructive than the explosion of one kilogramme of TNT. Such a ‘hypervelocity impact’ can catastrophically break up a 1,000 kg spacecraft if it strikes a high-density portion. In the event of a breakup, numerous fragments larger than one kg would be created; these could, in turn, collide with other objects creating yet more debris. A single satellite failure, therefore, could lead to cascading failures of many satellites over a period of a few years, perhaps months—a possibility known as the Kessler Syndrome. If the Kessler Syndrome comes to pass, the threat to human spaceflight missions may be too great to plan operations in LEO.

On April 2, 2011, for the fifth time in three years, the International Space Station (ISS) started its engines to dodge a piece of debris that was closing in rapidly and was feared to be on a collision path. Other spacecraft are also regularly forced to scurry out of the way of approaching rocket and satellite debris. Such action to evade large objects may need to be resorted to more frequently in future.

Mitigation Measures

How is collision risk reduced? Space tracking, by organisations like the US Space Surveillance Network, holds the key. Such networks maintain a database of all known rocket launches and keep the various components under constant surveillance. Computer programmes then predict possible collisions between large space debris objects and high-value spacecraft. When they detect the likelihood of such a collision, the threatened spacecraft is manoeuvred (by small onboard rockets) out of harm’s way. However, such drills are expensive and can disturb delicate experiments. Besides not all satellites have the ability to manoeuvre.

Most detection systems like radar and lidar (optical detection device) can only track space objects larger than about 10 cm, with typical masses of the order of one kg or more. But the damage due to smaller debris is rapidly becoming a significant threat. And there has always been the possibility of being hit by meteors and micrometeorites—leftover material from the formation of the solar system. Since the 1990s, chipping of satellite windows and other surfaces has become all too common. Even a one-cm object can cause severe damage to a satellite, so collision avoidance will never be 100 per cent effective. Statistically, it is more likely that spacecraft will be lost to things too small to track. The main way to avert this is to improve tracking techniques for potential spacecraft-killing objects.

The other main method is to provide a debris shield for enhanced protection. A spacecraft can be given enhanced resistance to collisions simply by increasing the thickness of its walls. However, this also increases the mass of the satellite and makes it a lot more expensive to launch. Specially designed shields, known as Whipple shields, take advantage of the fact that two thin walls separated by a small space are more resistant to debris penetration than a single thick wall. The outer wall absorbs much of the debris impact energy and so the inner wall is not ruptured. This design is named after the astronomer Fred Whipple who came up with the idea in the 1950s. Whipple shields and modified versions of it are installed on the ISS. However, this method does not offer 100 per cent protection either.