Ranking the 25 coolest things in space so far during the 21st century

There is, of course, no quantitative way to rank amazing things that happen in space. We cannot measure their absolute magnitude nor assign them a numerical value, and trying to do so is something of a fool’s errand. Are we fools? Maybe. But we're fools who like to have a little fun.

Understandably, then, this list is completely subjective. How else could one rank events that span the cosmos, from gravitational wave detection to the launch of humans into suborbital space? What follows is a list of the most incredible, surprising, impressive, and demanding achievements and developments in space and spaceflight during the first 25 years of the 2000s—as selected by the editors of Ars Technica.

You will probably disagree with some choices and their ranking, and that’s totally fine. That’s what the comments section is for. But the point here is simply to bring some of these incredible moments back onto the front burner so we can all bask in their glory once again. Each of these achievements deserves our celebration and appreciation.

Without further ado, here's our list counting down the most incredible space achievements in the 2000s so far.

25. Artemis I

When I asked readers on social media for suggestions about the coolest things they’ve seen in spaceflight during the last 25 years, I received hundreds of suggestions. To the best of my knowledge, not a single person mentioned the Artemis I mission, in which an uncrewed Orion spacecraft flew around the Moon in late 2022.

This is somewhat surprising, as NASA invested more money into the Artemis Program than anything else on this list. And it strikes me as notable that with the Artemis I mission, NASA set down a marker for the program that will finally return humans to deep space after more than half a century. It deserves a place on this list for that reason, even if it did not excite the passions of my followers.

24. Parker Solar Probe

This NASA flagship mission, more than five decades in the making, was literally built to touch the Sun. It is the fastest spacecraft ever built and required a breakthrough in hypersonic materials to become feasible. After launching in 2018, the spacecraft built by the Applied Physics Laboratory will make its closest approach to the Sun on December 24, coming to within less than 10 solar radii of the star's surface.

At its closest approach, the spacecraft will be traveling at something like 690,000 km/h (430,000 mph), or 0.06 percent the speed of light. The primary goal of the mission is to understand the flow of energy through the Sun and better discern the nature of the Solar wind, as well as study other properties of our star. For all of that, the mission was delivered on time and on budget, a rarity for a lot of the projects on this list.

23. ʻOumuamua

What is this object, and where did it come from? The mystery of ʻOumuamua is fascinating to think about. Discovered in 2017 by Robert Weryk using a telescope at Haleakalā Observatory in Hawaii, ʻOumuamua is the first interstellar object humans have discovered moving through our Solar System. At the time of its detection, ʻOumuamua was already moving away from the Sun.

ʻOumuamua is a small object, likely shaped in the manner of a cigar, and is perhaps up to 1 km (3,000 feet) long. It has a reddish hue, similar to objects in the outer Solar System. Unlike most objects from the outer Solar System, however, ʻOumuamua had no coma, or tail, as it passed near the Sun. It is most likely an interstellar comet of some sort, but astronomers can only speculate about its nature, age, and where it may have originated.

22. Falcon 9 and Starlink cadence

How has the Falcon 9 rocket revolutionized the launch industry? Since its origin in 2010, the Falcon 9 has flown 400 times. In 2024 alone, the rocket will launch more times than its would-be competitors—including Russia, Arianespace, and United Launch Alliance—have each flown in the last decade. Next year, the Falcon 9 rocket will likely launch more times than NASA’s space shuttle did in 30 years. All of this is made possible by reusing the rocket’s first stage and payload fairing and through SpaceX’s relentless push to control costs and streamline operations.

The high flight rate of the Falcon 9 rocket has enabled SpaceX to deploy its Starlink constellation at a rapid pace over the last five years, putting more than 7,000 satellites into low-Earth orbit. Starlink has raised serious concerns from astronomers about its impact on their observations, and it has also increased the risk of collisions and debris near Earth. However, there is no arguing with the amazing feeling of plugging in a Starlink dish and pulling down high-speed, low-latency Internet from the heavens moments later.

21. Dark matter and dark energy

These are arguably the greatest astronomical mysteries of the day. Dark matter has not been observed directly, but its existence is suggested by the need for large amounts of hidden mass to explain observations in the cosmos. Dark energy’s existence is implied by an unknown force behind the expansion of the Universe, which is accelerating rather than slowing down after the Big Bang. The observations that hint at the existence of dark matter (1980s and 1990s) and dark energy (1998) were made prior to this century. However, scientists have been working feverishly to understand the nature of both since then.

Remarkably, astronomers estimate that the mass–energy content of the Universe is just 5 percent ordinary matter (such as stars and planets), 26.8 percent dark matter, and 68.2 percent dark energy. So what are dark matter and dark energy made of? We don’t know. Theoretical physicists have all sorts of wonderful ideas for both, but none have ever been directly observed in space or in laboratories. Nobel prizes await these fundamental discoveries.

20. Genesis and Stardust

In the 1960s and 1970s, Apollo astronauts returned rocks from the Moon, and some robotic Soviet missions did likewise. Sample return missions then stopped for a quarter of a century until the launch of two NASA missions, Stardust in 1999 and Genesis in 2001. Stardust flew through the coma of comet Wild 2, collecting samples there as well as those of cosmic dust. Genesis sought to collect and return samples of the Solar wind.

Genesis was first to return to Earth in 2004, but its parachute deployment system failed, and it crashed into the Utah desert traveling at a speed of 310 km/h (190 mph). Miraculously, scientists were still able to recover some of Genesis’ samples and complete the mission’s science objectives. Stardust had a happier return in 2006. Both spacecraft set the stage for more complex sample return missions this century.

19. Cassini finds plumes on Enceladus

NASA’s Cassini spacecraft launched in 1997 and arrived at Saturn in 2004. There, it made many fascinating observations, including of the intriguing moon Titan (more on this later). However, the highlight for many planetary scientists and space enthusiasts came during some of Cassini’s flybys of Saturn’s tiny, ice-encrusted moon Enceladus.

In 2005, Cassini observed water ice geysers erupting from the south pole of Enceladus, and three years later, the spacecraft made an even closer flyby, within 50 km (31 miles) of the moon’s surface. This time, Cassini passed through the plumes extending from these geysers and detected water, carbon dioxide, and various hydrocarbons with its mass spectrometer. NASA later confirmed that there is a subsurface ocean below the ice, making Enceladus a candidate for extant life beyond Earth. Hopefully, we will return soon.

18. The Moon makes a comeback

After NASA and the Soviets visited the Moon frequently during the Cold War, humans—and even robotic spacecraft—stopped visiting our celestial neighbor. It was thought to be a cold, dead world, and scientists were more interested in Mars, Venus, and some of the outer planets. This began to change around the turn of the century with the discovery of (probably) large deposits of ice at the lunar poles.

As a result, in the last decade, interest in the lunar surface has risen dramatically. China has flown a successful series of robotic landers to various locations on the Moon and has flown two sample return missions. NASA has sparked a nascent industry of private companies developing lunar landers, with Intuitive Machines making its first, largely successful landing earlier this year. And NASA and China are engaged in a competition to put humans back on the Moon, this time with the aim of permanence and establishing habitats at the South Pole. The Moon is back, baby!

17. Demo-2

The retirement of the Space Shuttle in 2011 marked a painful moment for NASA. The space agency had to spend several years convincing Americans that the space program was not shutting down. NASA leaders also had to go, hat in hand, to Russia’s space corporation and barter for seats to fly its astronauts to the International Space Station. And in 2020, along came SpaceX and its Dragon spacecraft.

The vehicle’s first mission, carrying Doug Hurley and Bob Behnken, came during a summer of unrest when America needed a win. The country was still largely shut down by COVID-19, and its politics were fractured by the killing of George Floyd in Minneapolis on May 25. America’s return to human spaceflight marked a significant technical achievement by SpaceX, which became the first private company to launch humans into orbit, and it allowed NASA to fly more astronauts to the space station and take full advantage of that facility's research capabilities. Dragon has flown more than a dozen times since.

16. Detection of gravitational waves

Although he theorized about their existence a century ago, eminent physicist Albert Einstein was not sure humans could ever detect the faint echoes of gravitational waves traveling across the vastness of space. Nevertheless, experimental physicists strived to do that for decades. Then, in February 2016, two LIGO observatories announced that they had detected gravitational waves coming from two merging black holes.

This marked a triumphant moment for experimental physics and confirmed a key tenet of Einstein’s theory of general relativity. Now, these observatories have given physicists a powerful new tool to observe violent astrophysics from afar. Since the initial discovery nearly a decade ago, physicists have detected eight additional gravitational waves from a variety of astrophysical phenomena.

15. Rise of space tourism

In April 2001, an engineer and businessman named Dennis Tito launched into orbit on board a Russian Soyuz spacecraft. He was the first private space tourist. A little more than three years later, the experimental spaceplane SpaceShipOne, designed by Burt Rutan and flown by Mike Melvill, reached an altitude of 100.1 km. This was the first privately funded human spaceflight.

Then, for nearly two decades, private space tourism floundered. There were a handful of commercial flights on Soyuz. But it wasn't until the summer of 2021 that things took off, first with Virgin Galactic’s VSS Unity spacecraft and then Blue Origin’s New Shepard vehicle flying private citizens to suborbital space. Shortly afterward, in September 2021, entrepreneur Jared Isaacman commanded the first private orbital mission, Inspiration4, on Crew Dragon. In December of that year, the daughter of the first American in space, Laura Shepard-Churchley, followed in her father's footsteps by flying a similar trajectory on board a spacecraft carrying his name. Private space travel is not yet ubiquitous, but the era is finally opening.

14. Continuous presence on Mars

In early 2004, the Spirit and Opportunity rovers touched down on opposite sides of Mars. Initially designed for 90 days of operations, Spirit functioned until it got stuck in 2009, and it halted communications in 2010. Opportunity proved more tenacious, operating 57 times longer than its planned lifetime. By the time of its final contact with NASA in June 2018, Opportunity had traversed 45 km (28 miles).

Because of Opportunity’s longevity and the ongoing operations of Curiosity (as well as the more recently landed Perseverance), humanity has had eyes on the surface of Mars for more than two decades. This has given NASA an incredible ability to study the planet’s weather and climate continuously for long enough to understand patterns and develop an average climate state. This type of knowledge is essential if humans want to live on the red planet one day.

13. Exploring and moving asteroids

For 4.5 billion years, the surface of the Earth has been vulnerable to killer asteroids, and its inhabitants have been unable to identify threatening objects. That has finally begun to change as humans have become asteroid hunters. In 2003, the Japanese spacecraft Hayabusa became the first mission to land on an asteroid and collect samples. A few years later, NASA’s Dawn spacecraft spent several years orbiting and studying the largest asteroids in the Solar System, Ceres and Vesta. In the mid-2010s, Japan and the United States sent the Hayabusa2 and OSIRIS-REx missions to asteroids to collect larger samples and return them to Earth successfully.

Then, in 2022, the NASA-built Double Asteroid Redirect Test, or DART mission, successfully collided with the small asteroid Dimorphos, about 11 million kilometers (6.8 million miles) from Earth. The collision shortened Dimorphos' orbit around the larger asteroid Didymos by 32 minutes. This successfully demonstrated the capability to redirect an asteroid. In October of this year, a European spacecraft, Hera, was launched to study the aftermath of this collision. These efforts, combined with better monitoring of the heavens, have increased the ability of humanity to prevent killer asteroids from striking the planet.

12. Rise of China’s lunar program

One of the major stories of this century is the rise of China’s space program and its efforts to challenge the United States for preeminence. This has been most apparent in the country’s efforts to study and land on the Moon with its Chang’e project. In 2007, China flew its first orbiter to the Moon, and in 2013, the Chang’e 3 spacecraft successfully landed on the Moon and deployed the small Yutu rover.

These early successes set the stage for even more ambitious missions. In 2019, the Chang'e 4 mission landed on the far side of the Moon, the first time this had been accomplished by anyone. The country’s space program subsequently flew robotic sample return missions, including the Chang’e 6 mission at the South Pole of the Moon this year. China aims to land humans on the Moon by 2030, setting up a competition with NASA and the United States.

11. Starship tower catch

On October 13 of this year, SpaceX launched its massive Starship rocket for the fifth time, but this flight profile was different in that the company sought to recover the Super Heavy first stage. Remarkably, the rocket returned to the launch site, hovered adjacent to the launch tower, and was plucked from the air by a pair of “chopsticks” and subsequently set down back on the launch mount.

This technical achievement demonstrates a number of important things, including verifying the radical approach to catching a rocket (obviating the need for landing legs and reducing launch turnaround times). It also allows SpaceX to accelerate the development and testing of Starship. The visually arresting tower grab also captured the public’s attention and brought wider recognition of Starship’s potential to change spaceflight forever.

10. Voyagers are alive and kicking

Nearly 50 years have passed since NASA launched the iconic Voyager spacecraft into deep space, and for those of us alive in 1990, the “pale blue dot” image may have felt like a coda for the program. However, the Voyagers have continued to return important science this century as they have explored the outer Solar System and beyond. In 2012, data from Voyager 1 indicated that it had entered interstellar space, followed seven years later by Voyager 2.

They still weren't done, though, and have continued to fly in the interstellar medium between stars to collect data, including the recent discovery of an unexpected increase in the density of space. Voyager 1 has now reached a distance of more than 160 astronomical units from the Sun (1 AU is the distance between the Earth and the Sun), and Voyager 2 is more than 135 AU away. Thanks to careful management by NASA’s Jet Propulsion Center, the Voyagers continue to periodically phone home, our silent sentinels in the great void.

9. Kepler and exoplanets everywhere

Prior to this century, astronomers had identified a handful of planets around other stars, but we really had no idea whether planets were rare or common and if Earth-size worlds existed beyond our own star. That all changed in 2009 with the launch of the Kepler space telescope into an Earth-trailing heliocentric orbit. It operated for nearly a decade and transformed our understanding of exoplanets.

Kepler monitored the brightness of approximately 150,000 main sequence stars in our neighborhood of the Milky Way Galaxy, looking for periodic dimmings to identify transiting planets. To date, Kepler has detected more than 2,700 exoplanets and found that far from being rare, there are probably at least as many planets in our galaxy as stars, if not many more. The telescope and subsequent observations have confirmed that Earth-size worlds are also common, providing some promising targets in our search for life.

8. James Webb Space Telescope

The effort to design, build, test, and deploy the Webb telescope was a saga that spanned the entire 21st century. Initial concept studies were completed in the late 1990s, and a major redesign of the space-based infrared observatory was undertaken in 2005. Construction was finally completed in 2016, setting the stage for a lengthy series of tests to verify the observatory’s instruments and its capability to deploy in space.

Along the way, the price tag ballooned from $1 billion to $10 billion by the time the telescope finally reached the launch pad. After lifting off on an Ariane 5 rocket on Christmas Day 2021, the telescope spent half a year unfolding and deploying in space before finally beginning operations. But the astronomical results have been worth it. The saga of Webb has been a story of persistence and perseverance by NASA, Northrop Grumman, and other partners who strived to bring this magnificent instrument online. A happy ending was far from certain, but we got one anyway.

7. Huygens probe lands on Titan

The Huygens probe flew to Saturn as a passenger aboard the Cassini mission, which launched in 1997. Built by the European Space Agency, Huygens was deployed to the moon Titan in 2005. This large moon is one of the most intriguing bodies in the Solar System, with a surface pressure not dissimilar to that on Earth but with temperatures cold enough to support lakes of methane. What's amazing is that a probe built during the 1990s made it all the way down to the surface.

Huygens touched down on land, although the mission designers planned for the possibility that it would touch down in a methane lake. Intended to gather data for a few hours during its descent through Titan’s thick atmosphere and possibly briefly while on the surface, Huygens continued to communicate for about 90 minutes from the moon’s surface. More than two decades later, no other human-made object has ever landed on a body in the outer Solar System. That will hopefully change with NASA’s Dragonfly mission to Titan within the next decade.

6. Human habitation in space becomes routine

The International Space Station is remarkable for a number of reasons, but perhaps no more so than for its longevity across a quarter of a century. Although the first element of the International Space Station was launched in 1998, the first long-term inhabitants, Expedition 1, did not arrive until November 2000. Since then, humans have lived continually in space. More than 280 people have visited the station, including a handful for longer than a year.

During that time, astronauts and cosmonauts conducted countless science experiments and provided human health researchers with a trove of data about the perils of living in microgravity. The space station has also been a beacon for peace and human cooperation at a time of geopolitical turmoil. Finally, the station has nurtured the growing US commercial space industry, and it's likely that companies such as SpaceX would not exist today without the station.

5. Orbcomm-2, the first Falcon 9 landing

If left to myself, I would have put this mission at the top of the list. In writing the book Reentry, I came to understand all of the toil and turmoil behind the scenes of SpaceX’s first successful attempt to land a rocket vertically back on Earth. The mission happened just days before Christmas, on the night of December 21, 2015, at Cape Canaveral in Florida. It felt otherworldly to see a rocket that had just launched minutes before return and land near the launch site.

So much was on the line. The company’s previous launch, a cargo mission to the space station, had blown up on the way to orbit. For Orbcomm-2, SpaceX was debuting a brand-new and much-changed version of the Falcon 9 rocket known as “full thrust.” Its engineers and technicians had to learn how to produce copious amounts of densified liquid oxygen and load it onto the rocket just minutes before launch. SpaceX also had to convince the Air Force to let a rocket land near sensitive facilities. In the end, the company pulled it off, inaugurating the era of modern reusability. Launch will never be the same.

4. New Horizons flies by Pluto

Launched in 2006, the New Horizons spacecraft made a stunning flyby of the Pluto and Charon system in 2015. For the first time since the Voyagers, a spacecraft revealed an unknown planet in great detail, and it was thrilling to see new images and data from Pluto and its largest moon in almost real time as they came in from New Horizons. The excitement was so genuine because we simply had no idea what to expect.

After nearly a decade of travel across more than 5 billion km (3 billion miles), New Horizons managed to fly within 12,500 km (7,800 mi) of Pluto’s surface. During this brief passage, New Horizons revealed vibrant colors and incredible features such as ice volcanoes. After passing by Pluto, New Horizons also became the first spacecraft to visit a Kuiper Belt object, 486958 Arrokoth. New Horizons is now more than 60 AU from Earth, bound for the interstellar medium.

3. Philae touches down on a comet

Built by the European Space Agency, Philae was a small robotic lander that traveled to a distant comet, 67P/Churyumov–Gerasimenko, aboard the Rosetta spacecraft. After a journey of about a decade, Rosetta reached orbit around the comet and released Philae. In November 2014, Philae touched down on the comet, but harpoons designed to anchor the spacecraft failed to deploy. Accordingly, Philae bounced a few times before coming to a soft landing on the nucleus.

This marked the first time a spacecraft had ever landed on a comet. Although lying on its side in the shadow of a steep cliff, Philae still returned rich data about the comet’s nucleus over the next half year or so. Why does Philae rank so highly? Because the spacecraft was so plucky and lovable, and along with Rosetta, it returned images and video from a comet that were at once familiar but also entirely alien. This was truly an inspirational mission.

2. Ingenuity flies on Mars

Almost everyone reading this article remembers the seven minutes of terror associated with the landing of the Curiosity rover on Mars in 2012. A similar thing happened nine years later when the Perseverance rover landed on Mars (this time, with some amazing video of the dynamic experience). Yet as cool as these landings were, and as impressive as the capabilities of Curiosity and Perseverance are, a tiny payload named Ingenuity carried by Perseverance stole the show on Mars.

The small helicopter had a mass of just 1.8 kg (4 pounds), and to a large extent, it was built from off-the-shelf parts. Expectations were low for the vehicle, which was developed by a team at NASA's Jet Propulsion Laboratory. They would have been happy with a simple hover flight, with the vehicle flying up several feet and back to the Martian surface. Instead, Ingenuity survived long winter nights and dust storms, amazing us all by flying 72 missions over the course of nearly three years. During its lifetime, Ingenuity spent a total of 2 hours and 9 minutes soaring through the thin Martian air. It was an incredible little machine, the Wright Flyer of this century.

1. Falcon Heavy launch, dual rocket landing

By popular demand, this mission in February 2018 ranks in the top spot. The visuals were irresistible. The rocket launch itself was impressive, with the combination of 27 Merlin rocket engines generating a brightness that one almost had to look away from. Then the twin boosters separated and returned to Earth, landing like a pair of synchronized swimmers. Finally, there was the arresting view of a cherry red Tesla (and Starman) flying away from Earth in the general direction of Mars.

It was a spectacle that understandably captured the public’s attention. But the new rocket was more than a spectacle. By designing, building, and launching the Falcon Heavy, SpaceX demonstrated that a private company could independently fund and fly the largest and most powerful rocket in the world. This showed that commercial, heavy-lift rockets were possible. By providing competition to the Delta IV Heavy, the Falcon Heavy saved the US government billions. It's likely that the US government will never design and develop a rocket ever again.