10. The Dead Cosmonaut
The Russian satellite Sputnik I gets some finishing touches before its launch in the fall of 1957.
During the 1960s, the Soviet Union and United States raced to become the world leader in space exploration.
The winner would be able to claim technological superiority over the other.
The Soviet Union had the early edge: In 1957, it launched Sputnik, the first man-made satellite.
In 1961, the Soviet Union dealt the American space program another blow when cosmonaut Yuri Gagarin became the first human in space.
But, according to the Judica-Cordiglia brothers from Italy, Gagarin wasn't the first.
The brothers set up a listening station in Italy to intercept communication transmissions between ground operations and spacecraft for both American and Soviet missions.
Weeks before Gagarin's successful flight, the brothers claimed to have detected and recorded radio transmissions of a cosmonaut slowly dying while adrift in space.
The Soviet Union denied the brothers' claim. Supporters of the theory believe the Soviet government hid the cosmonaut's death to preserve the country's reputation as a leader in space exploration.
The truth remains a mystery, though the recordings are available online, if you're curious to hear for yourself.
9. Shifting Poles
One interpretation of the polar shift hypothesis.
Some people believe that the Earth's North and South poles weren't always located where they are now. They believe the Earth once rotated on a different axis.
Others say that the Earth always rotated about its polar axis, but that the Earth's crust shifts so that the land located at each pole changes.
Climate change, earthquakes and volcanic eruptions result from stresses on the Earth's crust during the shift.
Some claim that the Earth will soon experience another dramatic polar shift, and that, as a result, entire continents might sink while new ones emerge from the sea.
The polar shift hypothesis enters the world of conspiracy theories when some claim that top governments and scientists know of the impending shift but refuse to share the information with the public.
Supposedly, they're keeping it a secret to avoid causing a panic.
Sceptics dismiss polar shift hypotheses, saying that there's no real scientific basis for them.
The Earth's true poles aren't the same as its magnetic poles, which are definitely shifting.
The North Magnetic Pole is moving slowly northwest across the northern stretches of Canada.
This means that if you took two trips 10 years apart to the North Magnetic Pole by following a compass, you'd end up at a different destination each time.
8. The Secret Lunar Base
Sketch of what a commercial lunar base could look like (does not look like).
Our Moon factors heavily into a few different space conspiracy theories.
One such theory is that not only have we been to the Moon, but someone has built a fully functional lunar base on the far side.
The Moon is in tidal lock with the Earth.
That means the rotation of the Moon has slowed down over millions of years to the point where the same side of the Moon always faces our planet.
It's called tidal locking, and it means that whenever you look up at the Moon you'll always see the same features.
It also means the far side of the Moon -- sometimes mistakenly called the dark side -- is out of view.
You have to admit, a base located on the far side would be safe from prying eyes. But who could have built such a thing?
The theories postulate everything from the Nazis in World War II to the United States to aliens.
We've certainly had a look at the far side of the Moon a few times -- many of the Apollo missions passed the other side and took amazing photographs.
The Chinese have landed on the far side of the Moon in early 2019 and nothing appears to be there.
But images and videos from these passes by the Apollo missions and the Chinese spacecraft show no signs of secret Moon bases.
It doesn't help the conspiracy theorists that some of the evidence they've pointed to in the past turned out to be promotional efforts for movies like "Iron Sky" or "Transformers: Dark of the Moon."
So far, it appears that science fiction is the only place you're likely to find a Lunar Base.
7. Illuminati and Majestic-12
Were Harry S. Truman and NASA director James Webb part of a secret government agency called Majestic-12?
Secret governments are a staple in conspiracy theories.
According to William L. Moore, a "UFOlogist," Majestic-12 was a code name for a secret government department headed by U.S. President Harry Truman.
The purpose of this department: to deal with extra-terrestrial beings.
Sceptics say that the documents Moore uses to support his claims are forgeries and that no such department exists.
Supposedly, the Illuminati is another secret organisation with ties to space.
There are hundreds of different theories about the Illuminati, some of which say it's the organisation to which all other secret governments report.
In what is possibly the strangest theory on this list, some people claim that the Illuminati are the descendants of the union between humans and an extra-terrestrial reptilian race.
According to this theory, these hybrid descendants have infiltrated the highest levels of government in countries around the world.
The supposed goals of the Illuminati range from simple world domination to preparing the Earth for extra-terrestrial colonisation.
Sceptics say there's no evidence to support the existence of the Illuminati, but conspiracy theorists say that the lack of evidence actually proves the group exists.
Believers say the Illuminati is so effective that the group has erased almost all evidence of its existence.
6. Roswell/Men in Black/Area 51 Conspiracy
Did the U.S. government hide evidence of a crashed alien spacecraft in Roswell, N.M.?
No discussion about space conspiracy theories would be complete without mentioning the Roswell incident.
In July 1947, rancher Mac Brazel discovered some strange debris in an area northwest of Roswell, New Mexico. Brazel couldn't identify the debris, and a nearby military base released conflicting information about the material's origin.
A few local newspapers picked up the story.
While people thought the incident was unusual, several years passed before the first theories about alien activity really took off.
At that time, dozens of UFO enthusiasts published stories and books about the Roswell incident.
The basic theory was that the debris came from an alien spacecraft and that the United States government confiscated the material for later study.
According to some theories, the government transported the debris to an Air Force base in Roswell.
Others say they took it to Area 51, a secret military base in Nevada where the government tested experimental aircraft.
There were stories of autopsies performed by government officials on dead aliens.
Some people claimed that anyone trying to uncover the government's secrets about aliens would receive a visit from a group of intimidating officials in dark suits: the so-called Men in Black.
The U.S. government repeatedly denied having any information about extra-terrestrials, but many theorists dismissed the government's responses.
Officially, the Air Force says that the debris found in Roswell came from a crashed surveillance balloon that was part of a top-secret project called Project Mogul and that it has no information about alien life forms.
Another nail in the theory's coffin came when the United States CIA acknowledged the existence of Area 51 in a document about the U-2 spy plane.
The document didn't suddenly blow the lid off of alien technology.
Instead, the paper confirmed what many writers have said about the military base -- it was a testing ground for top-secret military aircraft.
5. Planet X
Does our Solar System contain a giant, unnamed planet?
Does our Solar System contain a giant, unnamed planet? It does, according to the Planet X theory.
This theory says that our Solar System has a 10th planet (if you count Pluto as a planet, otherwise the mystery planet is no. 9).
The planet is supposed to be enormous and on an orbital path that will bring it close to Earth soon.
Proponents of this theory cite earthquake and weather data as evidence of this planet's growing influence on Earth.
They also claim that the government is forcing observatories to close to keep the planet's approach secret and prevent panic.
Several astronomers have said that there's no evidence for the Planet X theory, and that if the planet did exist, humans would be able to see it even without a telescope.
Even if you did need a telescope to see this planet, very little would stop amateur astronomers from seeing it.
Keeping countless amateur stargazers quiet about such a massive discovery would be impossible.
Unless the world's governments have collaborated in secret planet-sized stealth technology, this theory seems pretty flimsy.
4. The USA 193 Spy Satellite
A 24-storey Delta IV-Heavy rocket is poised for launch on Cape Canaveral's Complex 37B in July 2012. The payload? A classified spy satellite for the National Reconnaissance Office dubbed NROL-15.
On Dec. 14, 2006, Vandenberg Air Force Base launched a rocket carrying a satellite called USA 193.
The satellite was part of the National Reconnaissance Office's (NRO) intelligence programme. In other words, it was a spy satellite.
The satellite had trouble maintaining orbit, and by late 2007 the Air Force determined that it would soon crash to Earth.
The U.S. government announced in January 2008 that it intended to shoot the satellite down with a missile.
Why shoot down the satellite?
The government said it was because the satellite's fuel tank contained hydrazine, which is toxic.
If the fuel tank survived re-entry, it could land on a populated area. On Feb. 20, 2008, the U.S. fired a missile and struck USA 193.
Some people believe the government didn't tell the entire story.
They say that the government also wanted to prevent the satellite from falling into another country's possession.
Others go even further and suggest the United States only wanted to show the world that it's capable of shooting down a satellite with a missile.
In 2007, China destroyed a weather satellite with a missile, calling it a test.
That test spread debris in an area around the Earth filled with other satellites, endangering those machines.
By contrast, the United States' approach was to fire on USA 193 when it reached a low enough orbit to minimise danger to other satellites.
Was the U.S. government using USA 193 as an excuse to respond to China's test missile? Some people think so.
3. Flat Earth Society
For decades, there was a whole society devoted to the idea that the Earth is flat.
In 1956, Samuel Shelton founded a society that subscribed to a theory about the shape of the Earth -- namely, that it's flat.
Shelton based his theory upon what he called common sense and personal observation.
He called the scientific evidence for a globe-shaped Earth "dogmatic," meaning scientists were making this claim without adequate evidence.
Later, when people showed him pictures of the Earth taken by satellites, Shelton claimed the photos were fake.
He and the members of his society continued to support the idea that the Earth is flat and that those who disagree are part of a conspiracy to keep the truth about the Earth hidden.
Shelton died in 1971, and leadership of the society passed to Charles Johnson. Johnson led the society until his death in 2001. Without leadership, the society fell apart.
Critics of the Flat Earth Society say that its members simply deny any evidence that conflicts with their world view without offering real alternative hypotheses.
Keep in mind that humans have known Earth was round for centuries.
The ancient Greeks learned by measuring shadows that the planet's surface must be curved.
That led to astronomers concluding that the world was round.
The myth that people during Christopher Columbus's journeys believed Earth to be flat has no real basis in fact -- the argument was really about how big Earth was, not whether or not it was flat.
2. The Face on Mars
The Red Planet. Do you see a face?
In the 1970s, NASA launched two spacecraft called Viking 1 and Viking 2 on missions to Mars. Both spacecraft photographed the surface of Mars and sent images back to NASA.
In one photo, a hill on Mars' surface looked a little like a face.
Author and conspiracy theorist Richard Hoagland went a step further and said that it didn't just look like a face, it was definitely a face.
Hoagland theorised that an alien civilisation colonised Mars and that the face was evidence of an alien city in the Cydonia region.
When other satellites photographed the surface of Mars, the pictures of the Cydonia region revealed that the face was just an eroded mesa.
Hoagland argued that the equipment NASA used to take the subsequent photos wasn't as accurate as the Viking equipment.
He also claimed that NASA doctored the images, making them look less like a face.
We've since had a very good look at Mars's surface and the "face" turns out to be an illusion created by hills and shadows.
Humans tend to recognise distinct shapes and patterns even from vague or indistinct shapes and objects.
If you've ever seen a cloud that reminds you of a particular animal, that's an example.
We call this tendency pareidolia, and there are several examples on Mars.
1. The Moon Landing Was a Hoax
This is Armstrong, back inside the lunar module, at the conclusion of the Apollo 11 Moon walk. Or was he just mugging for the camera on a soundstage somewhere?
The granddaddy of all space conspiracy theories has to be that the Moon landings were faked on a soundstage.
Those who believe the Moon landings were hoaxes say that the United States lacked the technology necessary to transport humans to the Moon and back.
They claim that NASA faked the landings in order to make people believe the U.S. had fulfilled President John F. Kennedy's promise to land a man on the Moon before 1970.
What evidence do these sceptics cite? For one thing, on the lunar landing videos, you can't see any stars in the sky.
NASA says that's because the Moon's surface and the astronauts' suits were so reflective that it was too bright for the camera to pick up the comparatively faint stars.
Another point theorists make is that while planting the American flag in lunar soil, the flag appears to wave.
With no air in space, how is that possible?
NASA says that the astronauts rotated the flag's pole back and forth while planting it so that it would remain upright.
The rotation of the pole caused the flag to move back and forth as if rippling in the breeze even though there's no air on the moon.
NASA says that there's plenty of evidence that men landed on the Moon.
There are photos, videos and testimonials from the dozen astronauts who have set foot on the Moon's surface.
The astronauts returned with soil and rock samples, which NASA also cites as physical evidence of our presence on the Moon.
Some hardcore conspiracy theorists maintain that all of this evidence is fake or came from unmanned missions to the Moon.
That's the real problem with conspiracy theories in general -- there's no real way to convince people who believe in them that they might be wrong.
The theorists may claim that any evidence contradicting their ideas was fabricated in an effort to cover up the truth.
They may also argue that the lack of evidence to support their beliefs is due to the government (or some other responsible party) taking great pains to remove all evidence from view.
In other words, arguing with some theorists is like saying "heads you win, tails I lose."
But even if these conspiracy theories don't reflect reality, it's no secret that there are boundless mysteries waiting for us out in space.
We might even know two or three of them already -- but we aren't telling.