The moon will turn green.
In spring 2016, there was a rumor that the moon would turn green because several planets had aligned and caused an eerie glow. This was supposed to happen on April 20 and again on May 29 for the first time since 1596, the rumor alleged.
The moon never actually turned green, although it can appear red during a lunar eclipse, when the moon passes through Earth's shadow. In the same way sunsets often appear red, sunlight is scattered as it passes through Earth's atmosphere, casting a reddish shadow on the moon's surface.
Skywatching columnist Joe Rao debunked this green-moon myth. He pointed out that a full moon actually took place on April 22, 2016, and speculated that the April 20 date of the "green moon" might have to do with "National Weed Day," popularly known as 4/20. Considering that the last green moon supposedly happened 420 years ago as well, this doesn't appear to be a coincidence.
Earth will go dark for two weeks
In July 2015, a website called "NewsWatch33" wrote an article claiming that Earth would have 15 days of complete darkness that year. The website, which is actually a fake news site, was borrowing from an older version of the tale that has been circulating for years, according to debunking website Snopes.
As we all know, Earth did not actually experience that much darkness that year. (The article claimed that the alleged darkness was partly due to a Jupiter-Venus conjunction, which actually took place more than 500 million miles apart.) Darkness occurs when the Earth rotates, causing the sun to "set" on the local horizon. Brief periods of darkness can also happen when the sun is totally obscured during total solar eclipses, which occur rarely in any particular spot on Earth. But even during an eclipse, Earth is never completely in the dark.
Zero-gravity day will make you weightless
If you ever wanted to leap into the sky and soar like Superman, this hoax is for you. In late 2014 and early 2015, a widely shared story claimed that on Jan. 4, 2015, everyone on Earth would experience weightlessness due to a rare alignment of the planets. A doctored image of a purported tweet from NASA's Twitter account that went around on social media fooled a lot of people into believing the hoax.
But, of course, nobody floated off the surface of Earth that day. Earth's gravity is too strong for people to become weightless. The only way to experience weightlessness without going to space is to ride aboard a plane that performs parabolas, with some including a few seconds of weightlessness. This is sometimes nicknamed the Vomit Comet.
Alien spacecraft caused a mysterious explosion
Back in 2004, an expedition of Russian researchers working in Siberia claimed to have discovered "an extraterrestrial device" close to where the mysterious Tunguska explosion occurred. Scientists still aren't sure exactly what it was that blew up in the sky over Siberia that day in 1908, but the leading theory is that it was a large meteorite or an asteroid.
The Tunguska incident flattened hundreds of square miles of forest, and signs of the destruction were visible even decades afterward. At the time, news reports claimed that evidence of aliens was found at the site, but this claim was never substantiated. "The Russian team stupidly stated long before they went to Siberia that the main intention of their expedition was to find the remnants of an alien spaceship," Benny Peiser, a researcher at Liverpool John Moores University in the U.K., told Space.com. "And bingo! A week later, that's what they claim to have found."
UFO was caught "refueling" at the sun
NASA has a fleet of sun-gazing spacecraft that keep an eye on space weather, especially during solar eruptions. In 2012, telescopic images appeared to show something in the shadows. On YouTube, some viewers said this could be a UFO that was refueling by using the solar plasma.
However, NASA pointed out that the feature is actually something called a "prominence," which has cooler and denser plasma than the outer atmosphere of the sun, or the corona. Scientists are still trying to figure out how solar prominences develop, but they're pretty sure it has nothing to do with aliens.
There is a ______ on Mars!
With NASA's Opportunity and Curiosity rovers regularly taking pictures of the Martian surface, viewers have the chance to check out what they're doing in almost real time. NASA puts the raw images online for the public to see. But over the years, some weird shapes have cropped up. In 2008, for example, the Opportunity rover appeared to photograph a female figure. Other photos have shown things shaped like animals, spoons or other items.
You can imagine that, with all of the rocks available on Mars, some of them would happen to look like familiar objects. In fact, the human brain tends to perceive meaningful images in random patterns — a phenomenon known as pareidolia.
When evaluating the claims, consider that the Martian environment is extremely harsh to life as we know it; the surface is baked with radiation, the "air" is mostly carbon dioxide and there's not much atmospheric pressure.
I just saw a bright UFO!
It's a familiar trope for police stations and astronomy writers. From time to time, somebody will call (or write) in to say they just saw a UFO in the sky. They spotted a bright light around sunset, or saw a light moving around in an unfamiliar way.
While every situation is different, one common explanation for "UFOs" is actually another extraterrestrial object: Venus. Venus can be extremely bright when it's at its closest, because it's relatively near Earth. The planet is also extremely reflective because the sun's light bounces off the clouds. So before calling to say you've spotted a UFO, check your sky charts! [7 Things Most Often Mistaken for UFOs]
NASA can travel faster than light
If you've seen the "Star Trek" clips that show the Enterprise spaceship warping into another sector, you might have wondered how fast NASA is making progress on being able to move at the speed of light. The EmDrive has created years of speculation, with some breathlessly saying NASA must be on the verge of breaking the famed barrier.
In reality, NASA is downplaying the reports. The engine in question is a prototype that is producing some interesting results, such as appearing to create thrust when there was no reason for this to happen – and thereby violating Newton's Third Law of Motion. That said, NASA has not yet verified the results from these tests, and the engine has not been widely discussed in peer-reviewed research.
We've launched balloons into space!
With the advent of high-resolution, miniature cameras, several people have decided to strap these cameras on to high-altitude balloons and take pictures from up high. They've caught glimpses of blackness and, at times, taken interesting tiny passengers along (such as these Lego pieces and a Canadian flag in 2012). So they must be in space, right?
There's no way a balloon can get into space, and simple physics explains why. When a balloon rises into the sky, the air inside will expand in response to the dropping atmospheric pressure and eventually pop. Even Felix Baumgartner's stunning high-altitude balloon jump in 2012 was not actually from space, but from the stratosphere, which extends to roughly 31 miles (50 kilometers) above the Earth's surface. At that altitude the air is thin enough to see the blackness of space, but thick enough to support special high-altitude balloons. The boundary between Earth's atmosphere and outer space is about twice as high as the upper limits of the stratosphere.
There are canals on Mars
Author Percival Lowell became one of space's first popularizers when he wrote many books for the general public back in the late 1800s and early 1900s. In these books and other writings, he said there were canals on Mars built by an intelligent civilization, perhaps to move water into desert-stricken areas. He claimed to have seen the canals in his own telescope, and produced several sketches that are still available on the internet today.
There are no artificial canals on Mars. Several spacecraft have flown by the planet or orbited it, and not one has caught signs of aliens from orbit. What they have seen, however, are smaller channels that were created by nature – likely from water, ice or other processes that cause erosion.