This Monday Mercury passes between the sun and Earth. For seven hours, the planet will be visible as a little black dot - but never look directly at it or the sun! Here's what else you need to know.
Image: Reuters/NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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Messenger - Mission to Mercury
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This Monday, May 9, 2016 Mercury passes right in front of the sun. Between roughly 11:12 a.m. and 6:24 p.m. UTC the planet will be visible as a small black dot on the surface of the sun. The last such "May transit" took place 13 years ago. Other transits occur in November.
Solar eclipse glasses are not enough!
First of all a warning: As with solar eclipses, you should never look directly at the sun without proper protection. If you do, you risk serious damage to your eye sight.
But in the case of the Mercury transit special glasses made for watching a solar eclipse are insufficient protection - it is not advised to use them. In contrast to a solar eclipse, which is triggered by the moon, a Mercury transit allows the sun to continue shining with its full intensity.
This also means the tiny planet can not be seen without the correct protection and a little help.
Planet Mercury in 1975 - as seen from the NASA spacecraft Mariner 10Image: picture-alliance/dpa/Nasa
You'll need a telescope
Mercury is visible in transit with a special telescope - don't use an ordinary one, it is not safe. Watching the sun through a telescope is even more dangerous for the retina than looking at it with your bare eyes. So you need expert knowledge and special equipment.
If you want to view the transit, we suggest contacting a local amateur-astronomers' association for help and more advice. They exist worldwide.
Or watch the event as it unfolds on the Internet, via the larger space agencies or astronomical observatories.
And don't forget: dw.com will bring you the most beautiful pictures too. So follow us on twitter @dw_scitech.
Tiny planet
Mercury is the smallest of all the planets in our solar system. It's just 4,880 kilometers in diameter. The diameter of Earth is almost three times Mercury's at 12,700 kilometers. And the largest planet of our solar system - Jupiter - is huge by comparison at 143,000 kilometers. That's almost thirty times the diameter of Mercury.
The front and back of Mercury as seen by spacecraft MessengerImage: Reuters/NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Close to the sun
Mercury is the planet closest to the sun. Its distance from the sun is about 60 million kilometers. Earth 149 million kilometers from the sun, and Neptune at the far outer reaches of our solar system orbits at 4.48 billion kilometers from the sun.
As Mercury is so close to the sun, its orbit is fairly fast. It takes only 88 days for it to orbit the sun - so one Earth year is four years on Mercury. Mercury orbits at a high speed. It moves at 47 kilometers per second. Earth travels only at just under 30 kilometers per second.
Hot and cold
Temperatures on Mercury can reach up to 427 degrees Celsius. It's hardly a surprise, given its proximity to the sun.
But on the side that faces away from our star, it can get really cold - as cold as minus 173 degrees. The reason for the massive difference in temperatures is Mercury's ultra-thin atmosphere. The atmosphere is thinner than even a vacuum here on Earth under perfect laboratory conditions. If there is no atmosphere, there is nothing to shield Mercury's surface from sun rays, and there is nothing to trap the warmth and transport it to the dark side of the planet.
Volcanoes and rocks
The surface of Mercury consists of porous and rough rocks. It is covered with craters and has changed over millions of years - shaped by the impacts of meteorites. While there is no indication of any tectonic activity, there are signs of volcanic eruptions.
Messenger orbited and explored Mercury for four years. It crashed in 2015Image: picture-alliance/dpa
A life in the shadows
Its being so close to the sun has made it difficult to observe Mercury throughout the history of astronomy. The bright sun light simply outshines everything. A fly-by in front of the sun is a rather rare event. On average, it occurs every 13 to 14 years. Legend has it, the astronomer Nicolaus Copernicus regretted on his death bed in 1543 that he had never seen Mercury.
The next opportunity for astronomers will be in 2019.
Dare to fly so close
Due to its proximity to the sun, space agencies have been hesitant to send even unmanned spacecraft to Mercury. The threat of coronal mass eruptions damaging the sensitive technology is simply too high.
So far only two spacecraft have made it there. In 1975, the last of NASA's ten Mariner planet-discovery spacecraft reached Mercury. And in 2011 the special Mercury-exploration spacecraft Messenger entered an orbit around the planet and explored it until 2015.
Meet the planets
The first close-ups of Pluto awed the world this week. A couple of decades ago, photos of Venus or Saturn taken from space had a similar effect on scientists. Join DW on an interplanetary photo safari!
Image: Reuters/NASA/APL/SwRI/Handout
Our solar system
Depending on who you ask, there are eight or nine planets in our solar system - some experts still count Pluto, while the International Astronomical Union (IAU) took away its planetary status in 2006. People were still excited when NASA presented the first high-res images of Pluto this week. Its neighbors all had their portrait taken as early as the 1960s.
Mercury
The spacecraft Mariner 10 left for the planet closest to the Sun in 1973. It took this picture of Mercury's moon-like surface in March 1974. The planet's distance to the Sun varies between 28.5 million miles (46 kilometers) and 43.5 million miles (70 kilometers), because its orbit isn't a perfect circle. Scientists were surprised to discover that Mercury had a small magnetic field.
Image: picture-alliance/dpa/Nasa
Venus
Mariner 10 took this first close-up of Mercury's direct neighbor on February 5, 1974. The picture was color-enhanced by NASA to bring out Venus' cloudy atmosphere - the planet is perpetually blanketed by a thick veil of clouds rich in carbon dioxide. Mariner 10's journey to Venus was a rocky one: the spacecraft's high-gain antenna developed problems and a mechanical issue caused a large fuel-loss.
Image: picture-alliance/dpa/Nasa
Earth
The first full-on photo of our planet as seen from outer space was taken by Lunar Orbiter 1 in August 1966. That was three years before a human being had ever set foot on the Moon, which can be seen in the foreground of this picture as a shadow. The now-iconic photo was one of a series of pictures taken in preparation for the Apollo missions that would eventually put a human on the Moon.
Image: picture-alliance/dpa/Nasa/Loirp
Mars
This close-up of Earth's neighbor is the first picture ever taken of another planet by a spacecraft. Mariner 4 snapped it on July 15, 1965. Scientists who had expected to see lakes, valleys and mountains were disappointed - instead of an Earth-like planet, they were treated to craters similar to those on the Moon. The New York Times wrote: "Mars is probably a dead planet."
Image: picture-alliance/dpa
Jupiter
Spacecraft Pioneer 10 took the planet's first close-up from roughly 80,780 miles (130,000 kilometers) away on November 19, 1973. Jupiter is our solar system's largest planet. At its equator, Jupiter's diameter is a whopping 88,846 miles (142,984 kilometers). Its mass is two-and-a-half times larger than the masses of all other planets combined.
Image: picture-alliance/dpa/UPI
Saturn
This first shot of the ringed planet was taken on Pioneer 10's follow-up mission, Pioneer 11, on August 31, 1979. It was a perilous adventure: as the spacecraft flew through Saturn's outer rings, it almost crashed into one of two new moons it discovered. Visible at the upper left-hand corner in this photo is Saturn's moon Titan.
One of the first glimpses scientists got of Uranus was of its rings. Voyager 2 took this shot of them in 1986. Scientists had to remote-fix the spacecraft's camera for it to be able to photograph the planet with the coldest atmosphere in our solar system (as low as -366 degrees Fahrenheit or -221 degrees Celsius). The device had malfunctioned while Voyager 2 was passing Saturn.
Image: picture-alliance/dpa/Consolidated
Neptune
Voyager 2 also took the first picture of Neptune in August 1989. The planet has four cloud features that scientists know about. For those who don't count Pluto, Neptune is the planet in our solar system that's furthest away from the sun: at an average of 2.8 billion miles (4.5 billion kilometers), that distance is 30 times greater than the one between the Sun and Earth.
Image: picture-alliance/dpa
Pluto
The fact that Pluto is not officially a planet anymore didn't detract from the excitement scientists and lay-people all over the world experienced when NASA released this first close-up of the copper-colored (dwarf-) planet taken by New Horizons on July 13, 2015. The spacecraft traveled 3 billion miles (4.88 billion kilometers) to the solar system's farthest reaches for this shot.
