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Earth is the only rocky planet in the solar system with a strong magnetic field, and it's probably no coincidence that life has flourished here and not elsewhere. Magnetic fields help block the solar wind -- the stream of charged plasma constantly flying off the Sun.

And, really, thank goodness for that. If it's not blocked, the solar wind can strip a planet's atmosphere and, along with sunlight, it can destroy organic molecules, which are the kinds in our bodies. Except… here's the thing: Our magnetic field is good at blocking the solar wind, but it's not perfect.

We know that sometimes, particles sneak through and affect astronauts, wreak havoc with satellites, and even mess with power grids. But for a long time, it wasn't clear how that was happening. And then came the European Space Agency's Cluster mission.

Cluster is a set of four probes designed to measure Earth's magnetic field. Technically, it's called the Cluster II mission, but only because the first one… didn't go so well. In 1996, the rocket carrying all four satellites failed and self-destructed less than 40 seconds after launch.

But hey, four years later, everything went right. The first satellites in Cluster II were sent up in July 2000, and the other two went up in August. And the squad has been orbiting the Earth ever since.

The mission's four satellites are completely identical, with the exact same eleven instruments on each. And their purpose is pretty simple: They orbit the planet, spread out in a pyramid shape, and they measure properties of Earth's magnetic field to help us figure out what's going on up there. Having four satellites instead of one allows scientists to get a complete picture of what's happening all over the field, versus in just one spot.

And in the last twenty years, combining measurements from all four satellites has let us get a better idea than ever of how our field evolves as it gets bombarded by the solar wind. For instance, Cluster has revealed how the field changes shape depending on how much solar wind there is -- because as the magnetic field moves the charged particles, the particles warp the field right back. But it's also solved mysteries, like revealing how these sneaky bits of wind manage to sneak through.

At this point, it's found two main, related ways this happens:. One is that, sometimes, so much plasma hits the field that it makes huge waves, a lot like the kinds you see in water when there's a strong breeze. When those waves of plasma and magnetic field curl onto themselves, it makes holes in the field that solar wind can pass through.

No one was positive this was happening before because you need multiple spacecraft in different places to see the full series of events -- some to watch the solar wind, and others to look at the magnetic field. One isn't enough. So, having Cluster was huge.

But that's not the only thing going on. Cluster also found our magnetic field reconnecting. This is where lines of a magnetic field get crossed and then merge with each other, and it releases tons of energy.

In fact, this is what happens on the Sun to power explosive solar flares. And when it happens in Earth's field, that energy shoves some of the solar wind down toward the planet -- as fast as 15,000 kilometers a second, according to data from later NASA satellites. Measuring reconnection is another one of those things where it isn't just nice to have multiple satellites.

It's required: They have to surround the spot where reconnection happens to get good data. In fact, to really see this play out, you need at least four satellites. So, thanks again, Cluster.

Now, these observations alone would have been enough to get this mission a place in the history books, but Cluster hasn't just studied Earth's response to solar wind. It also lets scientists study the wind itself. For instance, in a 2012 paper, researchers announced they'd used Cluster data to discover turbulence within the solar wind.

This was the first time we really noticed that the wind had any kind of small-scale structure. But even more importantly, we also learned that turbulence can heat the wind up as it travels through the solar system. Which is the sort of thing we'll want to understand before we send astronauts on long missions outside Earth's protective magnetic bubble!

Over the last two decades, Cluster's data have led to hundreds of published papers, and the mission has given us a better, fuller understanding of the solar wind and its impact on Earth. And that's knowledge that could help us save our satellites, our power systems, and our astronauts! And the good news is, there's more to come, too.

ESA has plans for the mission through at least 2022, and other sets of probes are already up there following in its footsteps. So from establishing a legacy to revealing more about what's happening above our heads…. Happy 20th Birthday, Cluster II!

Buy yourself something nice. And if you want get yourself something nice, good news: To celebrate the Cluster mission, we made it August's Pin of the Month! Every month, we release a new space-themed pin to celebrate something in exploration.

You can get this one at DFTBA.com or in the merch shelf below — but if you want it, it's only available this August. We hope you enjoy it! {♫Outro♫}.