6. International Space Station
> Type of object: Space station
Constructed between 1998 and 2011 with the cooperation of 15 countries, the 420-ton International Space State (ISS) is the largest human-made object in orbit, big enough to be seen from Earth without a telescope. Built and maintained mainly by the United States, the ISS has hosted various missions and experiments conducted by astronauts from around the world, and it is expected to do so into the next decade. After that, its fate is uncertain.
The U.S. could spend hundreds of millions of dollars to either boost the space station into a much higher orbit or to deploy rocket thrusters to control its descent into the atmosphere, in the manner that the Russian space station MIR was destroyed in 2001. A less likely possibility is that a budget-conscious NASA would simply stop employing the boosters used to keep the ISS in orbit. In this case, the ISS would fall into the atmosphere in an uncontrolled descent, like Skylab in 1974, posing a risk of injury and destruction on Earth.
7. Hubble Space Telescope
> Type of object: Space telescope
In 1990, NASA, in cooperation with the European Space Agency (ESA), placed the first space observatory into orbit, beyond the haze of Earth’s atmosphere. In the years since, and with the help of five service missions, the Hubble telescope has produced over 1.3 million images of the solar system and the stars and galaxies beyond.
However, the 27,000 pound solar-powered observatory is nearing the end of its useful life, meaning that NASA will have to either engineer a controlled return to Earth, boost the craft into higher orbit, or design a new mission that would increase Hubble’s longevity. Until a plan is put in place — and succeeds — Hubble’s orbit will continue to decay due to Earth’s drag, potentially resulting in disastrous uncontrolled reentry some time between 2028 and the mid-2030s.
> Type of object: Earth-observing satellite
For a decade — twice the length of its projected usefulness — the satellite launched by the ESA monitored the environmental health of the Earth, from natural disasters to the threats caused by humans to our planet, such as pollution, and the effects of climate change. Known as Envisat, the satellite sent back huge caches of information that has formed the basis for thousands of scientific papers and a better understanding of the ominous changes to our environment, particularly the loss of arctic ice.
Without warning, Envisat stopped communicating in April of 2012, and its status and location are now unknown. Given its size, 85 by 32 feet, and weight, about 18,000 pounds, its uncontrolled descent to Earth, predicted to take place over 150 years, poses a risk of injury and damage. In the meantime, scientists believe there is a 15% to 30% chance the satellite will collide with other space junk, creating a major fragmentation event.
> Type of object: Comet
The Swift-Tuttle comet, discovered in 1862, orbits the sun every 133 years, leaving a trail of dust in its wake. Earth’s annual passage through the comet’s trail produces the famous Perseids meteor shower every August. Since its discovery, some astronomers have predicted the comet’s collision with Earth, a particularly startling prediction given the comet’s immense size, about 16 miles across. The comet does come relatively close to Earth — in its pass in 3044 it would come as close as a million miles to the Earth, only twice the distance from Earth to the moon. Still, more accurate calculations provided assurance that Swift-Tuttle poses no real threat to Earth.
10. 14,000 pieces of space debris
> Type of object: Space debris
There are perhaps millions of pieces of debris from objects humans launched into space. The debris pieces, known as space junk, are orbiting the Earth. Over 14,000 of these pieces measuring 4 inches and larger are being tracked by the U.S. Space Surveillance Network. Astronauts can employ debris avoidance mechanisms when tracked objects threaten a space mission, but even the smallest fragments, measuring a fraction of an inch, can damage spacecraft because of the high speeds in which they travel. The risk of a disastrous collision between space debris and the International Space Station was determined to be 1 in 300, and with the Hubble Telescope, 1 in 184.
Scientists have raised the possibility of a single collision between a satellite and space junk setting off a chain of collisions and creating clouds of debris so dense as to make Earth’s orbits unusable.
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