Rosetta and its comet are getting close to a major milestone -- here's what we know about the mission so far

There are more than 4,000 known comets in the solar system, but there’s one we understand more than the rest, thanks to the European Space Agency’s ongoing Rosetta mission.

Right now, the Rosetta mission has a satellite in orbit around Comet 67P/Churyumov-Gerasimenko as well as a lander on its surface. And this August the mission will hit a major milestone when the comet passes closest to the sun.

This will be the first time a spacecraft and probe have been this close to a comet during its closest approach to the sun. The information the instruments collect about the comet’s composition and behaviour will give scientists an unprecedented peak into how these ancient building blocks influenced the planets, including Earth, during the formation of the solar system 4 billion years ago.

Here’s a recap of the heroic mission, what it has taught us so far, and what we still hope to learn:

Here is a selfie of the Rosetta spacecraft in orbit around Comet 67P, shown in the background. Rosetta was launched in 2004 and spent 10 years in space catching up to the comet. When it arrived, scientists were shocked by the space rock's bizarre double-lobed shape.

Here you can see jets that the comet is ejecting as ice turns to a gas in the vacuum of outer space. Comet 67P is expected to get more active as it approaches the sun with increasingly more jets spewing off the surface.

This is the first picture that the camera on Rosetta took of Comet 67P after starting to orbit it on August 6, 2014. The spacecraft is about 60 miles above the comet's surface in this brilliant shot.

After achieving orbit, the Rosetta spacecraft's main goal was to snap pictures of all the places on the comet so scientists could determine where to set down the lander, called Philae, that Rosetta was carrying inside of it. Here's a map of the comet (all of the names are of Egyptian gods):

Choosing a touch down point wasn't easy because the comet's surface is riddled with steep, jagged cliffs and large boulders, as shown in the image below. Scientists needed a smooth surface.

Ultimately they decided upon 'site J,' which was later named Agilkia. The site is located on the head of the comet's smaller lobe, identified below. These were exciting times because no one had ever attempted to land a probe on a comet. This was a mission for the history books.

On November 12 last year, Rosetta released Philae. The desk-sized lander descended for 7 hours as it carefully guided itself to its designated touch down point. Here's a picture taken by Rosetta of Philae taken during its descent:

Philae responded in kind and took this shot of Rosetta from below:

The landing didn't go as planned and Philae bounced off course into a shadowy corner. Without direct sunlight, the lander only lasted about 60 hours before powering down to wait for more solar energy. Here's one of the last photos it snapped before going into hibernation underneath the cliff it landed near.

Despite Philae's silence, Rosetta continued to study the comet. Last February, the spacecraft flew closer to the comet than ever before to get a good look at the surface. Here's what some of that desolate surface looks like from just 5.4 miles up. You can see loose boulders in the upper right corner:

By studying the surface in great detail, Rosetta can help scientists determine one of the lingering mysteries about the Comet 67P: its odd shape. They want to find out whether the thin neck eroded away or if the two lobes were once two separate comets that collided at one point.

Then, after 7 months of silence, the Philae probe got enough sunlight to send a message. Its data shows Philae has been awake for a little while before working up the strength to send signals all the way back to Earth. Unfortunately, Rosetta's team doesn't know exactly where Philae ended up, though they do think it's on the comet's smaller lobe, perhaps in one of these regions.

The Philae lander has been collecting information about the surface of the comet -- such as the temperature, which was a cool 64.4 degrees Fahrenheit a month away from perihelion -- but it hasn't't had much of a chance to do it since after waking up, the ESA had to run tests to make sure all the instruments were good to go. Here's where ESA scientists think the lander ended up:

From these close-up shots by Rosetta, scientists have learned just how varied the surface of Comet 67P truly is. Here you can see a smooth featureless region called 'Ash' in the foreground right next to a more rugged, layered region called 'Seth' shown in the background to the left.

For example, the holes that dot the surface of the comet might actually be giant sinkholes so deep they could fit the entire Washington Monument inside. Scientists suspect that these sinkholes might be the reason for the jets of dust that stream off the comet.

The intriguing neck that connects the two lobes is called Hapi, shown here connected to Anuket, a rough region on the comet's small lobe.

This week, the ESA released images of Imhotep, an area on the larger lobe of the comet. It's close to the equator, and it's incredibly flat. But even more interestingly, it had a wide variety of features, from smooth to rocky terrains, round terrains, basins, and bright patches that could potentially be ice.

In June, ESA extended the mission to September 2016, at which point there won't be enough sunlight to power Rosetta's scientific gadgets and we'll stop getting images of the comet, like this one from July 14. Rosetta's having a hard time connecting to Philae in the past two weeks, so let's hope the lander starts responding to Rosetta soon to be sure the probe is able to get as much information as possible.

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