What is star stuff?
Nearly everything you see around you began deep inside a star. Only three elements, hydrogen and tiny amounts of helium and lithium, were formed at the beginning of the universe in the Big Bang. All of the other elements that make up our world need stars in order to form. Colossal explosions of stars and violent collisions between their collapsed cores spew fresh material out into the universe. As Carl Sagan famously stated, “We are made of star stuff.”
How does star stuff become us?
Star stuff bursts forth from stellar explosions as individual atoms. Some of these elements are rare but quite familiar. The gold in that necklace? It formed when two neutron stars collided billions of years ago. But at some point, these individual atoms started to come together. Just as the letters in an alphabet are combined to form tens of thousands of words, the elements created by stars are combined into virtually limitless varieties of molecules. That salt on your table? Its sodium and chlorine atoms formed in a supernova explosion of an enormous star.
“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars.” -Carl Sagan
Over 90% of your body is made of star stuff. Yet, there’s still much we don’t know about how atoms drifting through space come together into gas, solid particles, and eventually planets and life. Comets can help us answer some of these questions, and CAESAR could bring back a piece of one for us to study.
What makes comets special?
Comets are like time capsules, preserving the molecules from the beginning of our solar system’s formation, filling in the information gap between star stuff and planets, and possibly the origins of life. Most comets used to orbit at the frigid outer reaches of the solar system where they formed from the cloud that swirled around the infant Sun.
Far from the radiation and heat of the Sun, their chemistry has changed little from the beginning of the solar system. Occasionally, one drifts in toward the Sun, pulled by gravity, and passes through the inner solar system where we can see it in our night sky. This gives us the opportunity to send spacecraft like CAESAR to study these planetary fossils up close.
Previous missions have confirmed that comets contain large amounts of ice, as well as ammonia, simple organic compounds like methane, and even more complex ones like tholins and the amino acid glycine. So comets impacting the newly formed Earth may have seeded the surface with water and key building blocks of life. Indeed, life as we know it may not have been possible without the help of comets.
Why are we going to Comet 67P?
To understand the sample we would bring back, we would need to have a lot of information about where it came from. There have been ten successful missions to seven different comets, and one important thing we’ve learned is that each comet is unique. Comet 67P has already been thoroughly studied by the Rosetta spacecraft with all its science instruments and its Philae lander. Choosing this comet as CAESAR’s target means we only need cameras for navigating, picking a sample site, and documenting the sample collection, along with the robotics for collecting the sample and the capsule in which to store the sample. All the science can be done back here on Earth. This allows us to devote precious resources to acquiring and preserving the most scientifically valuable sample possible.
Comet 67P is also conveniently located, close enough to reach with a spacecraft, but far enough away and new enough to the inner solar system that it has not been as chemically altered by close passes with the Sun as some other comets.
How can comets help us discover habitable planets?
Just a few decades ago, our solar system was the only planetary system we knew. Scientists proposed theories as to how it formed, but had no other systems on which to test these theories. That all changed in 1989 with the first detection of an exoplanet, a planet outside our solar system orbiting another star. To date, we know of 2,792 other planetary systems, and studying them has revolutionized our understanding of our own system. By greatly increasing our knowledge of how our planets formed and where the ingredients for life on Earth came from, CAESAR would help us in our search for other habitable planets and possibly extraterrestrial life.
Why bring a sample back to Earth?
Imagine holding a piece of a comet in your hand. That’s exactly what the planetary scientists working on CAESAR want to do! While it is possible to send some scientific instruments on a spacecraft like Rosetta to fly next to the comet and make measurements there, it’s just not the same as studying something in labs back here on Earth. Some instruments are just too big, too expensive, or need the sample prepared by a person in a way that is difficult to duplicate using robotics. Even when instruments can be miniaturized, they inevitably sacrifice sensitivity and quality of measurements to do so. By bringing some of the comet back to Earth, we would dramatically increase the science we can do because we could use any and all instruments that we currently have, or will develop in the near future, to understand comets as well as we possibly can.
What are the challenges of sample return?
The first challenge would be collecting the sample. Some parts of Comet 67P are too rugged to be safely and effectively sampled by CAESAR. Fortunately, scientists already know that Comet 67P also has some suitable areas, and they would have a few years of studying the surface up close with CAESAR’s many cameras to pick the best spot to get the sample they need.
Once stored safely aboard the spacecraft, the next hurdle would be getting the sample back to Earth in pristine condition. At the time of sample collection, the surface of the comet would be about -100°C ( about -150°F). If the sample were to warm up too much, chemical reactions could take place that would permanently change the sample. The sample would need to be kept well below the freezing temperature of water during the fiery descent through Earth’s atmosphere in order for it to remain pristine. Innovations from the engineers and scientists working on CAESAR can make this possible!