With eight planets, five dwarf planets, at least 146 moons, more than half a million known asteroids and about 4,000 comets, the solar system is more crowded than you might think. Come join us on our cosmic voyage. And don’t forget to turn on the Sound.
“The solar system is an insignificant bunch of dust. It also happens to be where we live.” (Gene Shoemaker)
Our solar system is four-and-a-half billion years old. Science is only gradually gaining insights into its endless expanse. NASA’s New Horizons spacecraft recently paid a visit to Pluto. For the first time since its discovery 85 years ago, mankind now knows what the planet looks like. This tour through the solar system takes you to Pluto and beyond.
Our journey starts at the sun, which makes up 99.8 percent of the mass of our solar system. The sun’s gravitational force ensured that the planets, including Earth, were able to form.
By Milky Way standards, the sun is a perfectly normal, average star. With surface temperatures of about 5,000 degrees Celsius (9,032 degrees Fahrenheit), and up to 15 million degrees Celsius at its core, the sun derives its energy from the process of nuclear fusion. Hydrogen atoms melt into helium, fueling the solar fire.
The good news is that this will continue for about another 5 to 6 billion years. The bad news is that at the end of this period, the sun could expand to the point that it will swallow Earth.
From the hottest place in the solar system, we now go to somewhat cooler realms. The first stop is a planet of extremes.
With temperatures ranging from minus 180 to plus 430 degrees Celsius,Mercury has the largest temperature fluctuations of all planets as a result of its proximity to the sun. The side that happens to be facing the sun heats up while the other side cools off. Did you bring your long johns? They might come in handy now.
Mercury’s surface, with its many craters, resembles the surface of Earth’s moon. There is tons of water ice in craters on the planet’s poles.
Given its proximity to the sun, Mercury has seen relatively few visits by spacecraft to date. In 1974 and 1975, the US spacecraft Mariner 10 flew past Mercury three times and once even came within 320 kilometers (199 miles) of its surface. In March 2011, Messenger became the first spacecraft to orbit the planet. It was then deliberately crashed into Mercury in April 2015.
From Mercury, we continue to our cosmic neighbor, a true beauty, but one that packs a punch.
Next to the moon, Venus is the brightest object in the terrestrial night sky. As far as size goes, Venus is pretty similar to Earth. Other than that, they have little in common — in fact, this place is the opposite of cozy.
The planet’s dense atmosphere wouldn’t be much fun for human beings. It consists of 96 percent carbon dioxide, with air pressure that is 90 times as high as on Earth’s surface. If you were hoping to travel in comfort here, we hope you brought a diving bell along. Your equipment should also be good enough to withstand the pressure.
The average ground temperature on Venus is 462 degrees Celsius. At those temperatures, metals like lead become liquid. Sulfuric acid rain and volcanic activity also help make Venus a pretty unpleasant place.
The US Mariner 2 spacecraft completed the first successful Venus flyby in December 1962, at an altitude of 35,000 meters (114,830 feet). The Soviets achieved the first hard landing with Venera 2 in March 1966, but it was unable to collect and send data back to Earth. Venera 7 achieved this in August 1970, for an impressive 23 minutes. Remember, it’s hot up there, so maybe it’s a good idea to keep your distance from Venus. Since April 2006, the European Venus Express spacecraft has been transmitting data from its orbit around the planet.
So-called Aten asteroids, of which more than 900 are known, can be found on the route toward Earth. In principle, some of them could even cross Earth’s orbit and pose a threat to the planet. But there is currently no evidence that any specific asteroid is about to crash into Earth anytime soon.
Astronaut Thomas Reiter Peers at Earth from Space:
The most recognizable feature of Earth from space is its blue color. Oceans cover about 70 percent of its surface. With an average depth of 3,500 meters, many parts of our oceans remain as unexplored as space.
When it comes to land surface, human beings are of course deeply familiar with their home planet. This applies to the 7.2 billion people who explore their world on the ground every day, the three to six residents of the International Space Station and countless earth observation satellites, which keep a constant eye on all signs of life.
Before we go from Earth to the next planet, let’s take a little detour to our neighbor.
Thomas Reiter Looks Skyward to the Moon
So far, the moon is the only celestial body in space on which humans have landed. Of course, it’s also the one we are most familiar with, given its visibility in the night sky. Although some of the craters on its surface can be seen with the naked eye, the moon’s largest, at its south pole, is not visible from Earth.
The first spacecraft to fly past the moon was the Soviet Union’s Lunik 1, in January 1959. Its successor, Lunik 2, made a landing in the same year, but it was a hard landing. The Soviets accomplished the first soft landing with the Luna 9 spacecraft in February 1966.
More than three years later, in July 1969, two human beings landed on the moon for the first time, as part of NASA’s Apollo 11 mission. A total of 12 people — all men, and all Americans — have set foot on the moon. But that happened more than 40 years ago. Johann-Dietrich Wörner, head of the European Space Agency (ESA), is currently campaigning for an international, manned moon base. Let’s see where that leads.
“Don’t tell me that man doesn’t belong out there. Man belongs wherever he wants to go — and he’ll do plenty well when he gets there.” (Wernher von Braun, the famous German-American aerospace engineer who designed the Saturn V launch vehicle that propelled the Apollo spacecraft to the moon.)
Better shake off that dust! The nearest dry cleaner is a long, long way away. On the way to Mars, our flight passes the so-called Apollo asteroids, of which more than 7,000 are known. Some of them could cross Earth’s orbit, but there is currently no indication of an impending collision. Let’s get a move on it. There’s nothing to see here.
When Will Men be able to travel to Mars?
Mars is the second-smallest planet in the solar system after Mercury. Because a significant portion of its surface is covered with dusty iron oxide, it is known as the Red Planet. Although its atmosphere is very thin, there are still storms on Mars. It’s also pretty chilly there, with temperatures ranging from minus 153 to plus 20 degrees Celsius, depending on the place and time. Although Mars seems cold and barren today, it used to be a lot more pleasant, and there was even liquid water on the planet.
There is two-thirds less gravity on Mars than on Earth. Even with a bulky space suit on, you could still jump three times higher than you could at home. If your old gym teacher could only see!
The Olympus Mons on Mars is the tallest volcano in the solar system. The mountain, known as a shield volcano, is about 25 kilometers high, or almost three times as high as Earth’s Mount Everest. The Red Planet also has the deepest canyons in the solar system. The Valles Marineris, for example, is steep and runs up to seven kilometers deep.
Mars is a popular destination for research spacecraft from Earth. The Soviet Union achieved the first flyby in June 1963, with Mars 1. In July 1965, the US’ Mariner 4 spacecraft delivered the first images of Mars, from a distance of about 10,000 kilometers. The Soviets accomplished the first successful landing in December 1971, with their Mars 3 craft. In December 2003, the Europeans made a hard landing on Mars with the Beagle 2. Meanwhile, the Mars Express has been in orbit since December 2003, together with four other active spacecraft.
NASA currently has two robotic vehicles on Mars, Opportunity and Curiosity. The agency is officially pursuing a plan to accomplish a manned Mars landing, sometime after 2030. Some astronauts are keen to make the trip, including Alexander Gerst of Germany.
On the way to the edge of the solar system, our route now passes through the asteroid belt. It contains half a million known asteroids, and more are constantly being added to the list. These rocks of various sizes are remnants from the early days of the solar system. Rapidly growing Jupiter once ensured that they could not come together to form a planet. This is one reason asteroids are so interesting to scientists — as a sort of cosmic time capsule.
Asteroids are constantly colliding with each other or — if they are thrown out of their orbits — with planets. Asteroids have collided with Earth before, sometimes with dramatic consequences. We don’t have to worry about an asteroid entering a collision course with Earth in the near future, but sooner or later it will happen again – an eventuality we are still poorly equipped to deal with. NASA is thinking about a manned asteroid mission. But so far only thinking about it.
There is a very special dwarf planet orbiting within the asteroid belt.
The especially fascinating features of the dwarf planet Ceres are its white spots, the origins of which still remain a mystery.
The goal of NASA’s Dawn spacecraft is to unlock Ceres’ secrets. It has been orbiting Ceres since March 2015 and has already sent back large numbers of images.
The European Space Agency’s Rosetta spacecraft has been orbiting the comet 67P/ Churyumov-Gerasimenko since August 2014. Like asteroids, comets are also leftovers from the early days of the solar system. The comet, affectionately known as Churi, which Rosetta and its small Philae landing robot are studying, is only one of 4,000 known comets. Never before has mankind learned as much about this type of object.
We are now about to enter the region of the so-called gas planets. The biggest one is the next station on our journey through the solar system.
Jupiter is the giant of the solar system — larger and with a greater mass than all of its relatives, making up 70 percent of the combined mass of all planets in the solar system. It presumably has a solid core, and it is surrounded by giant layers of gas, consisting mainly of hydrogen and helium. Unimaginably powerful storms rage within those layers. The layers are also responsible for Jupiter’s rings. Another known feature of Jupiter is the Great Red Spot, a storm system larger than Earth.
At the end of 1973, NASA’s Pioneer 10 became the first spacecraft to fly past Jupiter, transmitting data back to Earth from an altitude of 130,000 kilometers. In December 1995, the NASA Galileo spacecraft dropped a probe into Jupiter’s atmosphere, where it burned up in 2003. NASA’s Juno probe is currently en route to Jupiter, where it is expected to arrive in just under a year.
In addition to the planet itself, Jupiter’s moons, of which there are at least 50 (more than any other planet), are also of interest to scientists. For instance, they believe that there is an ocean of water under thick ice on giant Ganymede, which is larger than Mercury.
The Jupiter moon Europa, a veritable flying snowball, is also fascinating to scientists, who speculate that simple life forms may exist there as well. The conditions for life there are not bad, prompting repeated discussion about a possible Europa landing mission. In 2022, ESA plans to launch its spacecraft Juice, with which it hopes to at least conduct a flyby of the Jupiter moon.
“Can we actually “know” the universe? My God, it’s hard enough to find your way around in Chinatown.”(Woody Allen)
The next gas giant is Saturn, the second-largest planet. Its rings, the planet’s most salient feature, are made up of water ice and rock. What most people probably don’t know is that there are more than 100,000 individual rings, some with a diameter of almost a million kilometers.
As on Jupiter, there are storms raging on Saturn of almost unimaginable dimensions. They spin through the atmosphere for months, generating giant bolts of lightning 10,000 times stronger than those found on Earth. And like Jupiter, Saturn appears to have a solid core.
Mankind has known what Saturn looks like up close since September 1979, when the Americans successfully completed the first flyby, at a distance of 22,000 kilometers, with Pioneer 11. The Cassini spacecraft has been orbiting Saturn since July 2004 and is still transmitting data today.
Scientists are also interested in Saturn’s moons, the biggest of which, Titan — like Jupiter’s moon Ganymede — has a larger diameter than Mercury. In January 2005, ESA’s Huygens lander managed to gather data for 70 minutes on the moon’s surface, which is usually concealed by a thick atmosphere. Titan also has giant hydrocarbon seas. It’s a fascinating but uncomfortable world.
The next stop is Uranus, also a gas planet. It has a bluish-green shimmer, because methane gas in its atmosphere swallows some of the incoming sunlight. The reflected remaining light creates the planet’s characteristic Color.
No one has ever seen Uranus’s small core beneath its atmosphere. The gas planet has a uniquely tilted rotational axis, probably the result of a collision long ago. It’s also pretty cold there, so let’s keep going!
So far only the US’ Voyager 2 spacecraft has flown past Uranus, coming within 71,000 kilometers of the planet in January 1986. Another mission is unlikely to take place within the next two decades.
The icy planet Neptune is even farther out in the solar system. This planet also has a dense, gaseous atmosphere, in which storms churn at speeds of more than 2,000 kilometers per hour.
There are seasons in Neptune’s atmosphere, but they last 40 years.
Voyager 2 has also been the only visitor from Earth to Neptune to date. In August 1989, the spacecraft approached the planet’s north pole at a distance of about 5,000 kilometers, only to continue flying farther out into space.
Neptune’s Triton moon is considered the coldest place in the solar system visited to date. Temperatures as low as minus 223.5 degrees Celsius have been measured on Triton, where ice geysers spew nitrogen.
In the past, we would have been flying on to the next planet. But according to a vote by the International Astronomical Union, Pluto is no longer considered a planet. Pluto, now classified as a dwarf planet, is probably the best known object in the so-called Kuiper belt.
Given Pluto’s location far out in the solar system, light from the sun is very weak on the dwarf planet. It takes more than four-and-a-half hours to reach it, which is how long the trip would take if one were to fly there at the speed of light.
Pluto isn’t at the end of the solar system by a long shot, but man still knows very little about the darkness beyond. Scientists cannot rule out the possibility that yet another, still unknown planet is orbiting the sun beyond Pluto.
Astronomers know even less about a structure called the Oort Cloud. They believe it contains an unimaginably large number of objects made of rock and ice, left behind when the solar system was created. No one has actually observed the cloud, but many believe it exists, as a sort of garbage dump of the solar system. Gravitational effects occasionally propel objects from the Oort Cloud into the inner solar system, where they become long-period comets.
The edge of the Oort Cloud could be 1.6 light-years away from Earth, or almost half the distance to the nearest star, Proxima Centauri. It is significantly smaller than the sun and has only an eighth of its mass. Traveling at the speed of light, it would take more than four years to reach Proxima Centauri. Put differently, if the distance between the sun and Earth were only one meter, Proxima Centauri would be about 270 kilometers away. But we’ll save this part of the trip for another journey.
“Today’s science fiction is tomorrow’s science fact.”(Science fiction writer Arthur C. Clarke)
Note: The numbers used in the article on individual celestial bodies were obtained from solarsystem.nasa.gov. Moons whose existence has not yet been confirmed were not taken into account. The distances quoted for the approach videos are based on the shortest distances from the sun .
Author: Christoph Seidler. Videos: Anne Martin. Editor: Holger Dambeck. Research and fact-checking: Almut Cieschinger, Maximilian Schäfer. Copyediting: Sarah Omar. Design: Hanz Sayami. Coordination: Jule Lutteroth. Translation: Christopher Sultan.