There are at least 500,000 objects orbiting the Earth today. Some estimates bring the number closer to 700,000. More than 21,000 are larger than 10 centimeters and these objects pose a threat to future space travel and life on Earth. Many are fragments of artificial satellites that were destroyed when they collided with other satellites.
Today there are more than 1,700 artificial satellites in operation and another 2,600 that are no longer working. Most of these satellites have either completed their missions or been dysfunctional. At least 30 of these inoperable objects were eventually nuclear powered. They still contain – and in some cases, nuclear waste to this day.
The following list discusses 1
Tiangong-1 is a prototype space station launched in 2011 by the Chinese government. She originally had a two-year mission to test the effects of space travel on astronauts and the docking capabilities of other spacecraft. The mission was extended beyond its original plan before it was finally abandoned because the operators of the station in China claimed that they were no longer in control.
Tiangong-1 was tall and weighed about 8,500 kilograms (19659002) Although most of the station was burned in the atmosphere on reentry over the Pacific Ocean in early April 2018, it was expected that the rocket engines were made from materials that not burnt. Although once feared that these intact pieces could severely damage structures, animals and humans, no catastrophic events were reported. 
9 SNAP 10-A
In 1965, the United States launched SNAP 10-A into space at Vandenberg Air Force Base. SNAP 10-A is the only nuclear-fission satellite launched into space by the United States. It was designed as an experimental spacecraft that can generate 500 watts of electrical energy. Its main purpose was to observe how nuclear fission reactors behave in space.
Unfortunately, the nuclear reactor only lasted 43 days, and then the voltage regulator of the power supply failed. The satellite began to disintegrate in the late 1970s, resulting in about 50 debris pieces. 
During this ejection process, it was very likely that some radioactive material had been released into space. The nuclear reactor currently orbits the Earth 700 nautical miles above the Earth's surface. It will remain in orbit for the next 4,000 years unless additional discarding or collision with another object shortens its orbital life.
8 Cosmos 1818
In 1987, the Soviet Union launched the Cosmos 1818, which was a TOPAZ 1 (or thermionic) nuclear reactor. The purpose of Kosmos 1818 was as a maritime surveillance satellite or RORSAT (Radar Ocean Reconnaissance Satellite). Unfortunately, the nuclear reactor on Cosmos 1818 only worked five months before it was shut down.
In 1978, a similar satellite re-entered the atmosphere and plunged into the earth, radioactive material being spread across Canada. Cosmos 1818 was put into high orbit to avoid a similar catastrophe. Its high orbit, however, also means a high probability of collision.
Any collision could accelerate the descent of potentially contaminated material to Earth. It is believed that some of the objects and the liquid emitted from the spacecraft are radioactive and still in orbit. 
7 Cosmos 1867
Cosmos 1867 was launched in 1987 by the Soviet Union, in the same year as its twin Cosmos 1818. It had a similar purpose as Cosmos 1818, but Cosmos 1867 worked for 11 months before he stopped.
Since Kosmos is in a high orbit like his twin in 1867, he succumbed to the pressure of repeated solar warming. As a result, the coolant pipes on board the nuclear reactor of the satellite have cracks and allow the release of liquid metal into space. 
6 Cosmos 1900
Cosmos 1900 is a US-A or controlled active satellite used for RORSAT missions. Launched in 1987 by the Soviet Union, the satellite was plagued from the start and never reached the trajectory for which it was designed.
After several rocket accelerations to correct its orbit, the satellite continued to lose altitude. In addition, the nuclear reactor did not get into its storage orbit. Sometime before 1995, NASA discovered that a cloud of liquid radioactive material came from the Kosmos 1900 satellite. NASA claimed the leak was likely due to a collision with another satellite. 
5 Satellite Rubble
With all the collisions with satellites, there is now a large debris field orbiting the Earth. This debris field may be more dangerous than any single intact object due to the increased probability of possible collisions of multiple debris objects. Several large satellite collisions have already been recorded, and these events have exacerbated the problem of space debris.
In 2009, the Iridium 33 and Kosmos 2251 satellites collided at a speed of 42,000 kilometers per hour (26,000 mph) in a low Earth orbit (approximately 800 kilometers above the planet's surface). Both satellites were destroyed by the collision.
So, instead of two large objects orbiting Earth, we now have about 1,000 objects larger than 10 centimeters (4 inches) that threaten many other satellites. (There are also many smaller pieces.) 
Although about half of the debris from the 2009 accident has been burned in the atmosphere, several other collisions have occurred. Scientists estimate that the 2007 Iridium-Cosmos accident, together with China's intentional destruction of a satellite by long-range missiles, has doubled the number of dangerous and potential collision objects in orbit.
4 Black Knight
Whether or not Black Knight is dangerous depends on who you ask. Conspiracy theorists argue that the object is a 13,000 year old extraterrestrial satellite of the star system Epsilon Bootis, which Nikola Tesla discovered in 1899. NASA claims that the object in question is nothing more than a thermal blanket that dissipated during a spacewalk.
This object is especially dangerous for the time wasted by conspiracy theorists. Unfortunately, conspiracy theorists and speculations about this object have wasted more time than all the time lost to those who prematurely died as a result of falling space debris. 
The International Space Station (ISS) is not a nuclear or probable collision hazard we know, but due to its size it remains one of the most dangerous objects in orbit. Collisions are possible with any space object, but any such space station crash could create a doomsday scenario of space debris proposed by Kessler syndrome.
In simple terms, this means that an object that encounters the ISS will cause a cascading effect of other such accidents on all resulting debris. At some point there would be too much debris for us to continue certain space activities, possibly for generations. It was not until 2017 that objects have left the station and now have the potential to crash into the ISS.
The station is also a danger to the astronauts working on board. There were several problems with the oxygen generators, carbon dioxide removal systems, environmental controls, the central computer, electrical and energy systems, ruptured solar cells and ammonia leaks.  If any of these problems became catastrophic, the ISS could quickly become a serious threat as it fell to the ground and collided with other satellites and debris.
2 Hubble Space Telescope
The Hubble Space Telescope is not as big as the ISS. But Hubble is still one of the largest objects in orbit and a threat to its collision potential. If Hubble hit another satellite or piece of debris, the amount of additional wreckage would greatly increase the space debris problem.
Hubble was originally launched in 1990 after a multi-year delay following the destruction of Challenger aboard Space Shuttle Discovery. Hubble is currently not in a controlled orbit and is sinking towards the Earth.
Since Hubble's materials are so strong and dense, the Space Telescope is unlikely to burn in the earth's atmosphere during the descent. After entering the atmosphere Hubble would then fall uncontrollably on the earth's surface. This will probably occur between now and 2040. Envisat
Envisat is a large satellite launched in 2002 to monitor the Earth's environment and geography. Although the European Space Agency (ESA) was five years ahead of its original plan, it lost contact with it in 2012. Envisat now poses the greatest threat posed by Kessler Earth orbit syndrome.
Two objects pass through Envisat and could cause a collision. Considering Envisat's mass of about 8,200 kilograms, any collision between it and other satellites or parts of space junk would be catastrophic and create a large debris field that would be almost impossible to clean.
The wreckage of Envisat would be so powerful that the potential chain reaction of collisions suggesting Kessler's syndrome poses the real threat and Envisat poses the greatest risk.
It is currently expected that the satellite will remain in orbit for approximately 150 years before falling to Earth, greatly increasing the likelihood of an accident. For this reason, special considerations have been made to develop a spacecraft that can remove Envisat from orbit. 
Envisat is perhaps one of the greatest ironies in our space program: a satellite that has been hailed to help us understand the health of Earth's environment is now one of the greatest risks to its orbital field ,