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The ESA Hipparcos team worked for 20 years on the project, then had to watch as the mission ALMOST failed! But somehow, they turned it around, and today, this little-known mission has totally transformed what we know about space.

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Go to to learn more. {♫Intro♫}. Imagine working on a huge project for 20 years, and then having to watch — almost helpless — as it appears to fail.

That’s what happened to the team working on the European Space Agency’s Hipparcos mission in the 1980s. They had poured years of time, energy, and money into their satellite only to see it suffer what seemed like a devastating blow. But somehow, they turned it around.

And today, this little-known mission has totally transformed what we know about space. The Hipparcos mission was named after Hipparchus, an ancient Greek mathematician who made the world’s first known star map. And that was the satellite’s aim, too!

Well, not to make the first map, but to make the best and most complete one! The plan was to do this by performing an astrometry and photometry survey. In other words, the satellite would measure the positions and movements of the stars, and it would measure their brightnesses and colors.

In total, the mission’s two instruments were supposed to survey five hundred thousand stars over two and half years. But at first, things didn’t go as planned. When the satellite was launched in 1989, it was supposed to travel high above the Earth and then get pushed into a nice, round orbit.

But once the satellite was up there, the booster responsible for changing its orbit didn’t fire. Which was, to put it lightly… not great. Because of this, the satellite was caught in a weird, highly elliptical orbit that passed through the Van Allen Belts.

These are big lobes of hot, electrically-charged gases caught in the Earth’s magnetic field, and they can degrade solar panels by creating a bunch of small short-circuits. Incidentally, solar panels were how Hipparcos was supposed to keep itself running. So, yeah.

Not great. Thankfully, the engineering team managed to find a way around this. Using some leftover fuel, they were able to modify the satellite’s orbit a little bit to minimize the amount of time it spent in the Van Allen Belts.

The solar panels did still degrade a little, but this new orbit helped reduce the damage. And as a nice bonus, the materials they were made of turned out to be tougher than the team thought. So in the end, Hipparcos pulled through!

And by that I mean it lasted a year longer than the original mission design and exceeded its goals. Overachiever. Ultimately, scientists used the satellite’s data to put together catalogs that included more than two point five million stars.

And besides being a major accomplishment, that helped us make some important discoveries and major predictions. For one, we solved a long-standing problem about the universe’s age. For a while, scientists have estimated the minimum age of the universe based on globular clusters — ancient groups of stars that are some of the oldest structures in existence.

When Hipparcos was launched, we thought the oldest clusters were between fourteen and sixteen billion years old. And this was a problem, because all our other evidence suggested that the universe was between eleven point six and fourteen point nine billion years old. So except for a tiny amount of overlap, it seemed like the oldest clusters were older than the universe.

And that’s... not quite right. To figure out what was going on, scientists used Hipparcos observations of two special kinds of stars: Cepheid variables and RR Lyrae stars. These stars pulse really regularly, and scientists have found that their pulse is related to how bright they are up-close.

So by comparing that brightness to how dim they appear from Earth, astronomers can pretty accurately calculate how far away they are. Thanks to Hipparcos, scientists were able to calculate that the earliest globular clusters are actually only about eleven billion years old, so it turns out, they line up with our other data after all. And since the Hipparcos mission, other missions have put the universe at thirteen point seven nine billion years old, completely in agreement with that cluster data.

Much closer to home, Hipparcos also helped predict the Shoemaker-Levy 9 impacts on Jupiter! Not just the fact that they were going to happen, but exactly when. Shoemaker-Levy 9 was a set of comets that collided with Jupiter in 1994.

Well, it was originally one comet, but it broke apart into multiple pieces. Before they hit, scientists looked at all their data on the comets’ positions and compared them to Hipparcos’ data about the stars in the background. They used those stars as a reference to figure out exactly where the comets were and how they were moving.

And that reference frame was so precise that the impact predictions were within minutes of the actual impact times! Now, compared to figuring out the age of the universe, that might not sound like much — especially since no one lives on Jupiter, so far as we know. But Jupiter isn’t the only planet that sometimes gets hit with comets and asteroids.

Earth does, too! So they better we can learn to track comets in space and predict their paths, the better off we’ll be. Besides all this, Hipparcos data has also taught us more about our Galaxy’s tilt and certain clusters of stars, and it laid the groundwork for tons of research about stars and their planets.

The stuff the mission taught us could — and does — fill many books! But in the grand scheme of things, there’s something kind of funny about it, too. After all, it’s a mission where a bunch of small, short-lived humans looked into the ancient vastness of space and decided they wanted to count the stars.

There’s something aggressively optimistic about that. And the amazing thing is that it actually worked and taught us more about our corner of the universe. Today, you might hear about other missions studying the stars, but it’s worth remembering.

Hipparcos, too. It laid the foundation for some of today’s most groundbreaking astronomy research, and we’ll likely be learning from its data for years to come. And to think — it could have failed just minutes after launch.

Thanks for watching this episode of SciShow Space! If you enjoyed it and want to learn more about the universe and everything in it, you might want to check out CuriosityStream. CuriosityStream is a subscription-based streaming service, and they offer more than 2000 documentaries about everything from science and technology to society and history.

They even have a series about space exploration! It’s called Space Probes, and it looks at missions to Saturn, Mars, and Pluto among other places. So if you liked learning about Hipparcos, you might enjoy that series as well.

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