Space Junk

Space is composed of different celestial bodies. Due to some scientific phenomenon and human intervention there has been a formation of space junk etc. Although outer space is often imagined to be a desolate, empty place, the region around Earth swarms with millions pieces of man-made debris that create potential hazards for their functioning neighbors.

Space debris, junk, waste, trash, or litter is the collection of defunct man-made objects in space – old satellites, spent rocket stages, and fragments from disintegration, erosion, and collisions – including those caused by debris itself. As of December 2016 there were 5 satellite collisions with space waste.

As of July 2013, more than 170 million debris smaller than 1 cm (0.4 in), about 670,000 debris 1–10 cm, and around 29,000 larger debris were estimated to be in orbit. As of 5 July 2016, the United States Strategic Command tracked a total of 17,729 artificial objects, including 1,419 operational satellites. They cause damage akin to sandblasting, especially to solar panels and optics like telescopes or star trackers that can not be covered with a ballistic Whipple shield (unless it is transparent).

Below 2,000 km (1,200 mi) Earth-altitude debris are denser than meteoroids; mostly dust from solid rocket motors, surface erosion debris like paint flakes, and frozen coolant from RORSAT nuclear-powered satellites. For comparison, the International Space Station orbits in the 300–400 kilometres (190–250 mi) range and the 2009 satellite collision and 2007 antisat test occurred at 800 to 900 kilometres (500 to 560 mi) altitude. The ISS has Whipple shielding, however known debris with a collision chance over 1/10000 are avoided by maneuvering the station.

The Kessler syndrome, a runaway chain reaction of collisions exponentially increasing the amount of debris, has been hypothesized to ensue beyond a critical density. This could affect useful polar-orbiting bands, increases the cost of protection for spacecraft missions and could destroy live satellites. Whether it is already underway is debated.The measurement, mitigation and potential removal of debris are conducted by some participants in the space industry.

The Source of Space junk:

With the launch of the Soviet satellite Sputnik in 1957, mankind began its journey to reach the stars. But although the first probe in space returned to Earth after only three short months, it kicked off a series of launches that not only inspired people around the world but also filled the region with large chunks of inert metal.

Inactive satellites, the upper stages of launch vehicles, discarded bits left over from separation, and even frozen clouds of water and tiny flecks of paint all remain in orbit high above Earth's atmosphere. When one piece collides with another, even more debris is released. Over 21,000 pieces of space trash larger than 4 inches (10 centimeters) and  half a million bits of junk between 1 cm and 10 cm are estimated to circle the planet. And the number is only predicted to go up.

There are also millions of pieces of debris smaller than a third of an inch (1 cm). In Low Earth-orbit, objects travel at 4 miles (7 kilometers) per second. At that speed, a tiny fleck of paint packs the same punch  of a 550 pound object traveling at 60 miles per hour. Not only can such an impact damage critical components such as pressurized items, solar cells, or tethers, they can also create new pieces of potentially threatening debris.

For fifty years, the primary source of all of the junk came from objects that exploded by accident. However, in 2007, the intentional destruction of the Chinese weather satellite Fengyun-1C as part of an anti-satellite missile test created a significant field of space debris. Two years later, a defunct Russian military satellite struck an operational American Iridium satellite over northern Siberia, blowing even more trash into space.

source: http://www.space.com/16518-space-junk.html
             https://en.wikipedia.org/wiki/Space_debris