Today at around 7:39 am ET, NASA's Dawn spacecraft reached Ceres, an icy dwarf planet that lies in the asteroid belt between Mars and Jupiter.
This event isn't getting the coverage Philae got when it landed on a comet this past November. It may not even be getting as much attention as a few recent space accidents, such as the explosions of a Virgin Galactic space plane and uncrewed rocket this past October.
That's a shame. Because today is the culmination of a seven-year mission to visit something remarkable — a world that's quite close to us, but is such an unknown that, until a month or so ago, the best pictures we had of it looked like a hazy blob.
We still know very little about Ceres — it could have a thin water vapor atmosphere, or a moon of its own, or even a liquid ocean under the surface. Right now, we have no idea.
That will change over the course of the next year, as Dawn orbits Ceres and collects data. Here are six reasons why you should be paying attention to this mission.
1) Ceres isn't quite a planet, but it's close
When Ceres was first discovered in 1801, astronomers thought it was a planet, and considered it to be one for decades. Eventually we found other, smaller rocks orbiting in the same belt as Ceres — all of which are now considered asteroids.
Ceres, though, has always occupied a gray area. In 2006, it was swept up in the bigger International Astronomical Union debate on the status of Pluto. One proposed planet definition would have made both Ceres and Pluto planets, but the one that was ultimately adopted downgraded them both to the new category of dwarf planet (because neither had "cleared the neighborhood around its orbit," as they were part of the crowded asteroid belt and Kuiper belt, respectively).
But the big thing to remember here is that ultimately, this is a wholly semantic, meaningless debate. Ceres is noteworthy for all sorts of reasons, regardless of what humans decide it's called.
Unlike most asteroids, it's large enough that the force of its own gravity has made it round over time. It's about a quarter as wide as the Moon, with a surface area 38 percent that of the continental US.
All the other objects this large have either already been visited by probes or are very far out, near Pluto or beyond. Ceres is an outlier. It's a big old rock nearby, and we're about to explore it for the first time.
2) We know almost nothing about Ceres
When Dawn approached Ceres and began taking photos of it this winter, scientists were surprised to see it had a cratered surface. Due to calculations based off its mass, they'd assumed it would be entirely covered in smooth ice.
This is a reminder of how little we know about Ceres — including something as simple as what it looks like. For the past two centuries, we've been aware this alien world existed, but basically just knew it as a faint smudge of light.
Most recently, Dawn spotted a pair of strange, shiny dots on Ceres' surface:
No one really knows what they are. Scientists have hypothesized that they're probably patches of subsurface ice, exposed by an asteroid impact and reflecting back sunlight — or perhaps volcanoes, erupting ice and water vapor from below the surface.
But while the dots are intriguing, what's really most interesting about them is that they're part of a bigger Ceres mystery....
3) Ceres could have a thick layer of ice — or maybe even an ocean
Either explanation for the dots would help us answer a bigger question: is Ceres entirely frozen, or does it have a liquid ocean?
Previous calculations based on its mass have suggested it may have a small rocky core surrounded by an icy mantle — one that might contain as much fresh water as is present on all of Earth. If this is true, the dots could be ice that's been exposed through the thin, dusty crust.
But some scientists believe it's possible that the ice could cover liquid water ponds, or perhaps even an ocean. Cryovolcanism occurring at the boundary between water and ice could lead to erupting vapor volcanoes — and might also explain the two jets of water vapor shooting out of Ceres' surface that scientists detected last year using a space telescope.
Now, we don't have an explanation for what would heat up Ceres' interior enough for a liquid ocean and cause this volcanism, and it's certainly the less likely scenario. But just the possibility of an ocean has some scientists excited — because, like on Jupiter's moon Europa, a liquid ocean might make it conceivable that the planet might harbor life.
4) The Dawn spacecraft itself is pretty amazing
Dawn has propelled itself to Ceres — and stopped at the asteroid Vesta along the way — by using a technology that once belonged to the realm of science fiction: ion thrusters.
These thrusters shoot out charged particles, rather than conventional chemical propellants, to push the spacecraft onward. This allows the vehicle to operate for extremely long periods of time using very little fuel.
A handful of previous missions have used ion thrusters, but Dawn has taken advantage of them to do something unprecedented: it entered Vesta's orbit, left it, and will now enter Ceres', becoming the first spacecraft to ever orbit two different interplanetary objects.
It might sound simple, but it's a big step for space exploration, and one that could be tremendously useful for future deep-space missions.
5) We won't get many more missions like this for a while
The past few decades have been filled with all sorts of fascinating missions to the planets, moons, asteroids, and comets of our solar system — uncrewed probes sent every few years, run by trained scientists, and supported by government funding.
But the sad truth is that this era is largely drawing to a close. This coming July, the New Horizons probe will visit Pluto. But after that, writes David W. Brown in an article on the dark future of American space exploration, "there is nothing budgeted in the pipeline to take its place. Yesterday invested in today. But we are not investing in tomorrow."
This is the result of cutbacks to NASA's planetary exploration budget. The OSIRIS-Rex probe will launch next year, to travel to an asteroid and bring back a sample, but it won't return until 2023. Meanwhile, a mission to Jupiter's moon Europa is in the works, but it likely won't be launched until 2025 at the earliest, and wouldn't reach Europa until the 2030s.
In other words: enjoy this mission to Ceres, and New Horizon's arrival at Pluto. After that, it's going to be a while before any NASA probe visits a new world.
6) Space isn't "somewhere else" — it's where we live
Most people think of space as some distant, irrelevant place — a setting for sci-fi movies and astronauts, but one that has nothing to do with our actual lives. Seen in this light, visiting a new dwarf planet isn't particularly meaningful, or exciting.
But this isn't the right way to see space at all. Space is where we live.
Earth is a planet in space, just as Jupiter and Mars are. There's no magical glass barrier 62 miles above our heads (the altitude commonly accepted as the boundary with space) separating us from the rest of the universe, any more than there's one at the top of Venus' atmosphere, excluding that planet's surface from space, too.
And we live much closer to what we commonly think of as "space" than you may realize. If the Earth were the size of a basketball, our entire atmosphere — the layer of gas separating us from space — would be about the thickness of a pillowcase. In the grand scheme of things, we're already living there.
This is why asteroids can come down and hit us anytime, and why space weather can disrupt our telecommunications equipment and power grid. It's why exploring our solar system matters, just as exploring Earth's surface does. And it's why all of us should be excited about seeing a new alien world — just as we'd be excited about visiting, say, a new continent on Earth for the first time.
Update: Information about the OSIRIS-REx mission was added to this article.