The great challenge for any climber is to conquer each continent's tallest mountain. But tomorrow's climbers can consider a far more extreme challenge: leaving Earth to climb the tallest peaks of the solar system. Here's how they'll do it.

Just under 300 climbers have completed the Seven Summits list, climbing the tallest peak on every continent: Mount Everest in Asia, Aconcagua in South America, Mount McKinley in North America, Kilimanjaro in Africa, Mount Elbrus in Europe (with a tip of the hat to Mont Blanc), Vinson Massif in Antarctica, and either the Carstensz Pyramid or Mount Kosciuszko, depending on how you define the Australian continent.


Climbing all seven is the ultimate mark of achievement for a climber, and each provides a hands-on crash course in pretty much every possible ecosystem and natural phenomenon this planet has to offer.

In the same way, imagining a tour of the solar system's seven summits - the Seven Space Summits, if you will - provides a chance to examine the wildly different environments on our neighbor planets. Reaching the summits of Earth's tallest mountains was one of the great adventures of the 20th century, and it made legends of climbers like Sir Edmund Hillary and Tenzing Norgay. With Earth rapidly running out of records to break, the adventurer of the 21st and 22nd century is going to have to look skyward for new climbing challenges. Let's take a look at the extraterrestrial mountains just waiting to be summited.


To do that, we spoke to three experts on this rather unusual subject. Joe Romig is a space scientist and the coauthor of the lecture "Seven Summits Of The Solar System", along with renowned climber Glen Porzak. Keith Cowing is a former NASA scientist and the current writer of NASA Watch, OnOrbit, and SpaceRef, as well as an amateur mountaineer. William Hartmann is a senior scientist at the Planetary Science Institute and the author, along with Ron Miller, of The Grand Tour: A Traveler's Guide to the Solar System.

So, with these experts here to help us, let's assume you've got the completely unlimited funds needed to make such a journey and figure out just how you would do it!


Getting Ready for the Climb

First of all, we might as well try to understand the fundamental challenges of any extraterrestrial mountaineering. Unlike climbing on Earth, you can't rely on the elements for any assistance whatsoever, and you'll be forced to bring an entire life support system with you at all times. That's a basic fact of space travel, of course, but it's hugely different from climbing on Earth, so much so that Keith Cowing wonders whether it could really be considered climbing at all. He suspects serious climbers would just come up with a special term for this type of mountaineering - something along the relatively simple lines of "space climbing" - and be done with it.

So what are the basics of any extraterrestrial hike? You're going to need oxygen, food and water, shielding from radiation, and protection from the temperature, whether it's extremely hot or extremely cold. (And make no mistake - wherever you go, the temperature will be extreme.) Unless you have a suit that's 99% efficient at recycling your oxygen, Cowing says you will need to set up a supply infrastructure before you attempt the climb, just like in many serious climbing expeditions here on Earth.


Space suits themselves will need to be seriously redesigned. They are currently optimized for use in the zero gravity of space, but they're not really well-suited for environments with any significant gravity, so a more lightweight solution will be required. But they can't be too thin - many environments will be so cold that they will suck the heat right out of your suit and your tools, and that's why proper temperature shielding is so important.

The suits are also entirely inadequate for any sort of complex physical activity, which definitely includes climbing. The gloves, for instance, are huge and bulky - not a problem in outer space, but a significant issue when trying to make precision moves with your tools while climbing up a cliff face. There is a little bit of good news - on most of the environments we're visiting, gravity will be less than that of Earth, which will cancel out most of the added mass of your suit. Just don't count on being able to move around any more easily - the suits will still be plenty big and heavy.

That's another point - humans are so completely adapted to living on Earth that we can't really deal with environments with different gravity. It's going to take time to reorient one's body and get used to the different conditions on the planets and moons you visit, which means you won't be bounding up the mountain on day one. And remember that without Earth's thick atmosphere to offer protection, you will be constantly exposed to various levels of cosmic radiation. On places like the Moon and Mars, shielding won't be too difficult, but this becomes a serious problem when you get out to Jupiter.


0. Warming up on the Moon

Although our new Seven Summits refers just to the tallest peaks on the solar system's planets, we might as well start with the only extraterrestrial body humanity has ever visited. The Montes Apenninus range, named for Italy's Apennine Mountains, is home to the Moon's tallest peaks, including what's generally considered the highest mountain, the 4.7 kilometers tall Mons Huygens. (It's thought there are higher points on the dark side of the moon, but not higher mountains.)


Lunar tourism is already being discussed in something approaching seriousness - one company is trying to take two peoples for a flight around the Moon (but not landing) by 2020, assuming the passengers have a couple hundred million dollars to spare. So it's actually within the realm of possibility that people will actually try some climbing on the Moon within the next few decades. This is only strengthened by the fact that extreme thrill seekers are already looking to leave Earth's atmosphere with orbital skydiving. Keith Cowing makes this prediction: "Orbital skydiving is the harbinger. If that happens, you'll see climbing on the Moon."

It's possible climbing on the Moon could start for more practical reason. Cowing speculates that the crew of a lunar base might want to put radio repeaters and antennae on the tops of tall hills to help guide approaching spacecraft into landing. Or the crew might just get bored and decide to try a little lunar hiking.

The Moon also offers the perfect place to outfit the rest of your journey. Joe Romig explains that you can land near craters in the polar regions and start mining the Moon for water. You can save some of the water for drinking purposes, but a lot of it will need to be split into its component parts: the hydrogen for fuel, the oxygen for breathing. You can also set up solar collectors on the mountains to gather additional energy supplies. If you blast off from the Moon with the power of something like the old Saturn V rocket, Romig estimates that you could reach Mars in a month, Jupiter in nine months, and Saturn in fifteen months.


1. Mars: An Easy Stroll Up Olympus Mons

Mars is the perfect place to start your expedition. It's home to Olympus Mons, the tallest peak in the solar system, which is actually probably the easiest of the seven summits you'll climb. The mountain is 27 kilometers high, three times the height of Mount Everest, but it's not nearly as steep - it's just a gentle upward slope the whole way, no more difficult than climbing the similarly gentle Mauna Kea in Hawaii. In fact, William Hartmann suggests that Olympus Mons is so flat that you'd barely realize you're climbing a mountain until you reach the summit. You wouldn't even need a lot of specialty tools - most current alpine mountaineering equipment could work just fine on Mars without much redesign.


There's also the added bonus that Mars is easily the most hospitable place in the solar system other than Earth. It's a little colder on Mars - the average temperature is 210 Kelvin, or about -63 degrees Celsius. That's definitely cold, but not all that much colder than the sorts of temperatures one might endure climbing some of Earth's tallest peaks, and your suit will be able to warm you right up. Honestly, you'll be quite a bit toastier climbing Olympus Mons than the average climber trekking up Everest. There's some radiation on Mars, but the thin atmosphere will protect you from the worst of it, and it's nowhere near as severe as what you'll experience later on.

Olympus Mons is a welcoming mountain, but its scope is almost impossible to imagine. The mountain is the width of the state of Missouri, and it's actually impossible to see the summit from the base because it's beyond the horizon. Once you get to the top, you'll want to explore the caldera that's the last reminder of the mountain's volcanic past. The caldera is 85 kilometers wide and 60 kilometers wide, and most amazingly it's 3 kilometers deep. And Olympus Mons isn't the only monster in the area - there are three other dormant volcanoes in the Tharsis Region of Mars, and they're all nearly as big as Olympus Mons.

While you're on the Red Planet, you might want to make a side-trip from mountain climbing to explore Valles Marineris, the biggest canyon in the solar system. It's 4,000 kilometers long, 200 kilometers wide, and 7 kilometers deep, utterly dwarfing any canyon on Earth. Nothing we see in the rest of our tour can compare with Olympus Mons or Valles Marineris in terms of sheer scope, but it's going to get a whole lot more difficult. So let's head to the first planet from the Sun and get this expedition started in earnest.


2. Mercury: Scarps and Terminators

From a simple climbing perspective, Mercury isn't too tough. We haven't explored the planet's geology as extensively as that of Mars or Venus, so we don't know exactly what's the highest peak on Mercury, but it likely isn't much higher than three kilometers. That's a bit taller than the Appalachians, but nowhere near the same league as the Rockies. Most likely, you'll be heading up a cliff rather than a mountain, so be sure to bring your pickax and ropes.


Oh, and remember to get them heat-shielded. It's going to be a toasty 700-800 Kelvin at the best of times, and up to 900 Kelvin in the heat. That sort of temperature is difficult for us to imagine. It's not just that you and your suit that will be feeling the heat. The rocks on Mercury (and Venus) are permanently semi-melted, with a consistency similar to gooey concrete. Solid ground is a very relative term on a planet like Mercury.

Mercury is locked in a 3:2 orbital resonance with the Sun, meaning it rotates three times every two orbits around the Sun. Your best bet will be to climb one of the cliffs when it's in the terminator, the twilight region between night and day. At that point, you'll be in the middle between the blazing heat of the Sun-facing side and the freezing cold of the dark side of the planet. It's still going to be plenty hot though, so shimmy up and down that cliff quickly so we can get to our next, even more serious challenge.


3. Venus: The Acid Peak

The next two summits are going to be the toughest on our trip, hands down, and Venus's Maxwell Montes might just win the prize for the most brutal mountain in the solar system. At 11 kilometers high, it's about three kilometers taller than Mount Everest. Its western side is very steep, but its eastern side is a reasonably easy climb. But every step of the way will be fraught with peril, as Venus is the most inhospitable spot in the entire inner solar system. William Hartmann points out probes have only worked for about twenty minutes after landing on the surface of Venus before shutting down, and humans aren't nearly as hardy as your average probe.

So why is Venus so tough? Like Mercury, it's going to be hot, about 700-800 K. Due to a supercharged greenhouse effect, anything that pierces Venus's dense sulfuric clouds remains trapped near the surface, helping to make the atmospheric pressure 93 times what we find on Earth. The good news is that the closer you get to the top of Maxwell Montes, the colder it gets, although you're still going to need massive amounts of coolant just to keep it within the sort of temperature range humans can endure.


Here's the bad news: as you climb higher and higher, you'll get near the cloud deck. Remember how those are sulfuric clouds? Well, it rains sulfuric acid on Venus, which will eat through all but the most heavily reinforced suits. When you're near the surface, the rain won't reach you because it evaporates in the intense heat and pressure before it can get to the ground. But near the summit of Maxwell Montes, the sulfuric acid is a very real threat, so be sure to pack the thickest suit you've got and pray that you're climbing during the Venusian equivalent of the dry season.

4. Jupiter: Fun With Radioactive Volcanoes

As we leave the inner solar system, we've done the last of our planetary mountain climbing. Now we're in the domain of the gas giants, which means we're going to have to climb on their rocky moons instead. For Jupiter, we're headed to Io, the third-largest of Jupiter's moons and home to about 100 to 150 big mountains. These peaks average about five kilometers in height, but the biggest is Boösaule Montes at close to 11 kilometers tall.


Like Maxwell Montes, the actual paths up the mountain aren't too difficult, but the conditions are absolutely lethal unless you plan very carefully. Because Io is so close to Jupiter, you're in the middle of the planet's radiation belt. The moon gets hit with about 3,600 rems of radiation every day, more than triple the lethal dose. You would be safe for about 10 minutes before the radiation kills you, so shielding is going to be a major issue. Io is also the most volcanically active place in the solar system, so be on the lookout for nearby eruptions. But hey, at least the climb itself is easy enough, right?

5. Saturn: Heading Down to Go Up

Now things start getting a lot easier, but they also start getting a lot colder. Back on Jupiter, the Sun would have looked about 1/5 the size it does on Earth, but by the time you get to Saturn it's just a tenth of its Earthly size, and the solar intensity is 1/100 what we experience on Earth. The temperature here is in the 60s Kelvin, which is equivalent to more than negative 200 degrees Celsius. In a sense, this is almost good news - it's a lot easier to protect against intense cold than intense heat. Radiation is still a problem, but not nearly as ridiculously lethal as on Jupiter.


So which moon are we headed to? Although massive moon Titan is the most famous of Saturn's moons, but Joe Romig says it's not the most interesting for climbers. That honor goes to Mimas, a relatively tiny satellite only about 1/8 the size of our Moon. The closest moon to Saturn, it's home to the gigantic Herschel Crater, which is about 10 kilometers deep. Why the sudden interest in a crater? Because there's a six kilometer high mountain right at its center, making it the highest peak in the Saturn system, even if it is four kilometers underground.

The crater itself is about 130 kilometers across, which is particularly huge when you consider how small Mimas is. To put it in perspective, a crater of proportionate size on Earth would be 4,000 kilometers across, which is about the width of the continental United States. So it will be a trek to get to the center of the crater and the top of the mountain, but well worth it - if nothing else, you can tell all your friends you got to stand at the center of the Death Star's main cannon, as the above photo of Mimas will attest.


6. Uranus: The Ice Cliffs of Miranda

As we head into the outer reaches of the Solar System, we come to maybe one of the most awe-inspiring sights in the entire solar system. Uranus's moon Miranda is home to a massive cliffs of frozen ice near its southern pole. These cliffs run about 20 kilometers long, but their height is what's really amazing - these cliffs are roughly as tall as Mount Everest. All our experts agreed this would be the highlight of the trip - as Keith Cowing told me, these cliffs are just begging to be climbed, and William Hartmann calls them "really spectacular."


The cold is going to be a real problem here - it's just 60 K on the Miranda surface, which means the ax and boots you'll need to climb the cliffs will freeze into the ice fast if you don't have a way to melt them back out again. The sheer distance of the Sun from Miranda will be pretty mind-blowing at this point. The Sun appears only 1/20 the size it does on Earth, and it's 400 times fainter. You will still be able to tell the Sun apart from the other stars in the sky, but it's getting awfully point-like by now.

7. Neptune: Ending With A (Very Cold) Whimper

Unfortunately, especially after the ice cliffs, Neptune's moons don't have much to offer the now-seasoned solar system mountaineer. Whatever we'll find, it'll almost certainly be on Triton, the planet's largest moon that accounts for 99.5% of all mass orbiting the planet. Bigger than Pluto, Triton was likely part of the Kuiper asteroid belt before it was captured by Neptune in the early days of the Solar System.


Based on Voyager 2's 1989 flyby, the planet's surface elevation varies very little, with nothing much higher or lower than one kilometer. It's an anticlimactic end for our voyage, but there are some features worth exploring, including extensive ridges and complex valleys. Even though the hiking will be easy, remember to wear some layers - the average temperature is about 40 K, which is actually slightly colder than Pluto. Speaking of which...

8+. Pluto and Beyond

I've consistently talked about there being seven summits, one for each of the non-Earth planets. That, of course, leaves out the minor planets, including the recently demoted Pluto. Part of the reason for my exclusion is pragmatic - we don't know much about the geology of Pluto, although the New Horizons probe that's due to reach Pluto in 2015 should provide us a better idea of what to do on the icy rock.


Further out, there are even more dwarf planets in the Kuiper Belt that might offer some climbing challenges, but again we don't know enough about their geology either way. There might be even greater challenges further out, particularly if there really are Earth-sized planets frozen in the Oort Cloud. And let's not forget the planets beyond our solar system - I imagine the sorts of hardy souls who would be the first colonists on Zarmina might want to check out the local trails.


So why, at the end of the day, would anyone want to go to such exorbitant lengths to climb these peaks? Perhaps because, while most of our focus on space is quite rightly scientific in nature, there still should be some room for adventure, for pushing the limits of human resourcefulness and endurance. Really, there's no better explanation for why one would want to climb Olympus Mons and the rest of the Solar System's seven summits than this famous quote attributed to legendary climber George Mallory: when asked why he wanted to climb Mount Everest, he is said to have replied, "Because it is there." The fact that Mallory died in his attempt shouldn't really enter into it.

(Artist's conception of Herschel Crater on Mimas by Ron Miller, ice cliffs on Miranda by David Hardy, and Triton surface by Don Dixon.)