Rosetta Gets Up-Close With a Comet

comet nucleus
The OSIRIS narrow-angle camera aboard the Space Agency's Rosetta spacecraft captured this image of comet 67P/Churyumov-Gerasimenko on September 30, 2016, from an altitude of about 10 miles (16 kilometers) above the surface during the spacecraft's controlled descent. The image scale is about 12 inches (30 centimeters) per pixel and the image itself measures about 2,000 feet (614 meters) across. ESA

The Rosetta mission, the European Space Agency spacecraft that circled the nucleus of a comet for two years, came to an end at the end of September 2016. It did a "soft" crash-landing on the icy nucleus of Comet 67P/Churyumov-Gerasimenko, taking pictures and data all the way down. The mission's last image showed "boulders" of ice on the surface that were about the size of a coffee table. The final crash took place at 7:19 a.m. EDT on September 30, 2016, and the spacecraft stopped transmitting upon landing. It was likely destroyed or badly damaged.

Astronomers decided to end the mission because there was little likelihood that the mission, which was orbiting the nucleus, would have gotten enough solar power to continue orbiting. It was better to control the landing/crash, so the mission team programmed Rosetta for its final descent. The spacecraft became one with the comet and will continue to ride the nucleus as the comet circles the Sun.

What Did Rosetta Tell Us About Comets?

The Rosetta mission showed astronomers that comets are very complex bodies. Comet 67P, like other comets, is really a fluffy ball of ice grains and dust barely cemented together. It's got a ducky-shaped nucleus that tumbles as the comet moves through its orbit around the Sun. As it got closer to the Sun, the comet began to "sublimate" (like what happens if you leave dry ice out in the sunlight). 

It's long been known that these chunks of ice and dust are made of some of the oldest materials in the solar system. Some of the ices actually pre-date the formation of the Sun and planets. That makes them treasure troves containing precious information about conditions in the infant solar system. Since we can't travel backward in time to watch as our Sun and planets formed, studying the ices and dust and rocks embedded in comets is a big step toward "seeing" back into that tumultuous period in history.

The Rosetta spacecraft's instruments were designed to study the ices in Comet 67P and help scientists figure out how much of each kind of ice the comet contained. They also uncovered an important clue to the origin of water on Earth. For a long time, people though that much of Earth's water came from comets as they crashed into the infant planet. Comets probably DID play some role, but Rosetta determined that comets identical to Comet 67P probably did NOT contribute their water ices to create Earth's oceans. How do they know this? There's a tiny chemical difference in the water on the comet that is not seen in Earth's water. However, other comets may have contributed, so perhaps studies of others will help astronomers figure out how Earth got its water.

The mission also cataloged the different ices that make up the comet and, essentially, sniffed its atmosphere. There are exotic compounds in the nucleus, including formadehyde, acetone, and acetamide, as well as dust particles made up of carbon similar to the rocks and minerals that make up some asteroids. In addition to the usual carbon dioxide ice and gas that scientists expected, they also found the amino acid glycie, as well as the life precursor molecules methylamine and ethylamine.

The Rosetta spacecraft's specialized chemistry instruments "sniffed" the comet's atmosphere to determine what kinds of gases were emanating from the nucleus. It turns out Comet 67P was surrounded by a mist of molecular oxygen (called O2). This has never been seen in a cometary nucleus before, and was unexpected because the oxygen was largely destroyed as the Sun and planets formed. For it to be seen in a cometary nucleus means that the oxygen was incorporated into the ices when conditions were quite cool in the young solar system. The comet's existence in the outer solar system's Kuiper Belt means that the ices and hidden oxygen were preserved by the cooler temperatures "out there".

What's Next?

Although the Rosetta mission is now ended, the science it provided during its time on orbit around Comet 67P remains invaluable to comet scientists. There are years of analysis to be done using the archives of data amassed by the mission. Ideally, we could send spacecraft out to as many other comets as possible. Rosetta was years in the making, and other missions could well be designed. But, for now, the next missions to small worldlets will focus on asteroids, which are also building blocks of the solar system. Rosetta may have been the first spacecraft to do a long-term study of a comet, but in the coming years, perhaps other missions will follow its lead and land on other comets that come closer to Earth and the Sun.