American Scientist

Extraterrestrial Vacation

Imagine that it’s possible to cross vast gulfs of space-time on journeys to amazing locations in our Solar System, the Milky Way, and deep space, that you’ll survive the extreme conditions you’ll find out there, and that you’ll be able to see the invisible and discover a panoply of wonders through magical, multiwavelength goggles.

For me, it’s just as remarkable that we have the curiosity and technology to explore and study those places without needing to go ourselves. We’ve sent robot avatars close to the Sun, to planets, moons, asteroids, and comets, and to the edge of interstellar space. Beyond that, our telescopes capture light from across the electromagnetic spectrum, allowing us to see new stars and planets forming, the event horizons of black holes, and even the birth of the universe 13.8 billion years ago.

As an astronomer, I’ve been privileged to use some of the largest telescopes on Earth and off it, and to travel to other worlds vicariously through the eyes of robotic spacecraft. For this article, I have compiled some must-see destinations in our Solar System, so pack your protective gear and imagination into your spacecraft. Pick a destination . . . and go!

Miranda

Miranda is one of the most fascinating moons of Uranus, all of which are named for characters in works by Alexander Pope or William Shakespeare, in this case the latter’s comedy The Tempest. Discovered in 1948 by astronomer Gerard Kuiper, Miranda is just 470 kilometers in diameter. It’s the smallest moon that’s roughly spherical in the whole Solar System, which means it has enough mass to keep it flexible internally and allow gravity to pull it into a ball, whereas smaller moons tend to be irregular. But once you get here, you’ll find that it’s all the departures from spherical that make Miranda so interesting.

From above, you’ll see rugged and fractured terrain, a patchwork quilt delineated and crossed by faults, gorges, ridges, and craters. Some regions look as though they’ve had a giant garden rake pulled through them. One possible explanation for the rough surface is that Miranda suffered one or more huge impacts in its early history, causing it to break up and then badly reassemble under its own gravity. Or it may have been kneaded, heated, and reshaped over billions of years by tidal forces, thanks to Uranus and some of its other moons. It’s even possible that a subsurface water ocean was involved and is still liquid.

Descend to the surface, and you’ll discover perhaps the most remarkable feature on Miranda: the giant cliffs of Verona Rupes. Although estimates of the height of this escarpment vary wildly from 5 to as much as 20 kilometers, they’re probably the highest sheer cliffs in the Solar System and are sure to attract gawkers. Future space BASE jumpers might also want to visit this moon after conquering the Cliffs of Hathor on Comet 67P/Churyumov–Gerasimenko (below). Falling from 10 kilometers up, you’ll take more than eight minutes to reach the surface. But Miranda is large enough that you’ll land at about 140 kilometers per hour, so you’ll need to activate a giant airbag to cushion your landing.

Caloris Basin

As you descend toward the scorching surface below, the vast scale of the Caloris Basin will quickly become apparent. Spanning over 1,500 kilometers across—10 percent of the circumference of Mercury—this giant crater was created almost four billion years ago when an asteroid at least 100 kilometers in size struck the closest planet to the Sun.

The impact created two broken rings of mountains and cliffs up to 2 kilometers high, while the basin itself was flooded with lava escaping from the interior of the planet. In the billions of years since the impact, the region has been struck again by numerous smaller asteroids and meteorites, making other craters. Several of the younger craters have groups of strange, irregular depressions with bright floors and rims known as hollows, which are probably caused by sublimation of sulfur compounds brought closer to the surface by the more recent events.

Now turn on your gas sensors, and you’ll discover that Mercury has an extremely thin atmosphere that, as on Earth, includes hydrogen, helium, and oxygen. There are also sodium, potassium, and calcium, volatile elements that should have been removed by the intense heat of the Sun long ago. So where are they coming from today? The Caloris Basin is a rich source for these elements, and it’s thought that they emerge from the hollows in the young craters, as well as from material brought to the surface in the past few hundred million years in explosive volcanic events similar to the one that buried Pompeii.

As you start your journey home, take a look back at Mercury. If the conditions are right, you’ll see a giant yellow tail extending up to 24 million kilometers from the planet. Atoms in the thin atmosphere are blown into this tail by the strong sunlight, which then causes the sodium in particular to glow, giving it that characteristic streetlight hue.

The Cliffs of Hathor

Standing on a promontory jutting from the top of the Cliffs of Hathor, you’re nervous. It’s 900 meters down to the boulder-strewn floor of Hapi Valley below, the same height as El Capitan in Yosemite National Park. You reach to check your parachute one last time and momentarily panic when you remember that you don’t have one. But what good would it do anyway? There’s no atmosphere here to slow you down. Help!

Relax. You’re on 67P/Churyumov–Gerasimenko, the comet explored by the European Space Agency’s Rosetta spacecraft from 2014 to 2016. It’s a loosely packed ball of ice, dust, and organic molecules, and at just 4 kilometers across, the gravity at the top of the cliffs is less than 0.001 percent of that on Earth. So just follow your guide’s instructions: gently fall forward and enjoy the ride. After all, it’s going to take a while—about 90 minutes—to reach the bottom.

That leaves you plenty of time to take in the spectacular scenery offered by the Cliffs of Hathor, named for the Egyptian sky deity, and also to contemplate the oddities of BASE jumping on a comet. Its center of gravity isn’t directly below you, so you’ll be drawn across the valley. Also, the gravity decreases by 50 percent as you descend and the comet rotates every 12.4 hours around an axis through Hapi, further complicating your timing and trajectory. All quite head spinning.

But the valley floor is approaching now, and you need to get ready to land. It’s easier than you think. After falling all that way, your touchdown speed will be only 1.3 kilometers per hour, a slow amble. The next question is whether you’re up for the expert challenge: leaping back to the top of the cliffs again. The feat involves no more effort than jumping onto a paperback book on Earth, but jump too fast and you’ll escape altogether from the comet’s weak pull, leaving you to drift inexorably out into the Solar System.

Olympus Mons

It’s sunrise and there’s a light frost of water ice on the ground. You’re standing in the middle of what appears to be a large crater with high cliff walls that rise to 3 kilometers and span a distance of 30 to 40 kilometers around you in all directions. But looks can be deceiving, as you’re actually in the caldera—volcanic depression—at the top of Olympus Mons, the largest volcano in the Solar System.

Olympus Mons is located just off the western edge of the Tharsis Plateau on Mars, home to several other enormous shield volcanoes, and everything about it is gargantuan, including the caldera at its summit. Spanning over 600 kilometers in diameter and covering a region roughly the size of Poland, the volcano gradually but inexorably rises to 21 kilometers above the plains around it. That’s more than twice the height of Maunakea, Earth’s tallest volcano when measured from its ocean floor base. Even parts of the escarpment at the outer edge of Olympus Mons reach heights almost as tall as Mount Everest.

To the north and west, the terrain is chaotic, thanks to debris from huge lava-fueled landslides off the volcano that extended as far away as 1,000 kilometers. Scientists believe that Olympus Mons was surrounded by an ocean when the first landslides occurred billions of years ago, and that other landslides have happened more recently. The continuous activity has grown the volcano progressively over eons and is one explanation for its enormous size.

Mars lacks Earth’s plate tectonics, so the mountain has stayed fixed over the same hot spot under the martian crust, growing ever larger with every eruption. The volcano might still be active but is dormant these days. Analysis of lava flows on its flanks suggests that they emerged between 115 million and just 2 million years ago, a mere blink of the eye in geological terms. So, watch for new activity as you plan your hikes on the giant volcano.

North Polar Hexagon

Visitors to Saturn often focus on its equatorial regions to view the planet’s magnificent ring system. Beneath the rings at these latitudes, Saturn’s upper atmosphere is a set of belts and zones in a bland, beige palette, and the uppermost clouds made of ammonia ice at around −250 degrees Celsius are battered by ferocious winds reaching 1,800 kilometers per hour. Warmer cloud decks of ammonium disulfide and water ice lie hundreds of kilometers deeper.

The rings might receive most of the attention, but the real excitement takes place in Saturn’s polar regions, particularly in the north. As your eye moves to higher latitudes, you’ll see small storms drifting poleward, and there will be clearer stripes and dark spots. And then suddenly, at around 78 degrees north, you’ll notice something quite bizarre surrounding the pole: an enormous hexagon spanning 29,000 kilometers, each of the six sides larger than Earth’s diameter. Inside the hexagon are many storm systems, large and small, and as you move closer to the pole itself, ragged clouds hurtle around a giant vortex at speeds of up to 600 kilometers per hour at the edges. Imagine yourself in the calm, 9,000-kilometer-wide center of this cyclone, looking at its eye-wall descending more than 100 kilometers into Saturn’s atmosphere.

But what about that weird hexagon? The most likely explanation is that there’s a powerful jet stream around the pole that creates a strong gradient in the wind speed. Combine that with rotational forces from Saturn’s short, 10.5-hour day, and you get atmospheric waves that meander up and down in latitude. The same thing happens to Earth’s jet streams, but on Saturn the waves settle into a stable, six-sided polygon around the pole. Try to time your visit during the northern summer solstice, which happens every 29 years, to see the hexagon change in color from blue to golden as seasonal hazes accumulate over it.

Far Side of the Moon

If you’re searching for some peace and quiet away from our home planet, then look no further than the far side of the Moon. It’s less than 400,000 kilometers away and yet completely hidden from noisy Earth.

We see only one side of our nearest companion. Soon after it formed, the Moon became tidally locked, meaning that its orbital period around Earth matches the rotational period around its axis. As a result, one side of the Moon is permanently facing Earth (apart from slight wobbles, called librations), and the other is concealed. But Pink Floyd notwithstanding, it’s not dark on the far side, at least no more so than on the near side. Both sides experience a fortnight of scorching daylight that reaches up to 120 degrees Celsius followed by a frigid two weeks of night that plummets down to −171 degrees.

The far side is more heavily cratered and has few of the large, dark maria, or “seas,” found on the near side. When the Moon formed, it was at just 5 to 10 percent of its current distance from Earth. This proximity to our hot, young planet affected the composition of the Moon’s near side, resulting in a thinner crust that was more easily penetrated by asteroid impacts. The deep punctures released hot lava that smoothed craters and created maria. The far side’s thicker crust prevented asteroids from reaching the Moon’s interior but left the surface pockmarked with craters.

The only humans to have seen the lunar far side are 24 of the Apollo astronauts, and then only from above as all crewed landings were on the near side. In 2024, the Chinese spacecraft Chang’e 6 landed and returned samples from the far side, and there’s discussion about building a radio observatory there. Because the far side is shielded from the blare of Earth’s incessant technological chatter, the faintest cosmic whispers would become audible.

Thanks to tidal locking, the Moon is receding from Earth at 3.8 centimeters per year, so every day you delay, the longer the journey will get.

This article is adapted from the book 111 Places in Space That You Must Not Miss © Emons Verlag GmbH 2025.