September 30, 2016 – The Rosetta mission is over. The mission came to a dramatic end on Friday, September 30, with a controlled touchdown of the spacecraft on a region of comet 67P/Churyumov-Gerasimenko known for active pits that spew comet dust into space. The final descent gave Rosetta the opportunity to study the comet’s gas, dust and plasma environment close to its surface, as well as take very high-resolution images.
Rosetta is an international mission led by the European Space Agency (ESA) with instruments provided by its member states, and additional support and instruments provided by NASA. The mission launched in 2004 and arrived at comet 67P/Churyumov-Gerasimenko on August 6, 2014. The mission included many notable firsts including the first rendezvous with a comet, and the first spacecraft escorting a comet as it orbited the sun.
On November 4, 2014, Rosetta deployed a small lander named Philae, which bounced several times before settling on the surface of the comet. Philae obtained the first images taken from a comet’s surface and sent back valuable scientific data. Although the robot ran out of power after 64 hours of operations, it found a rich array of organic molecules, supporting the theory that comets kick-started life on Earth.
Rosetta continued to monitor the comet’s evolution as it made its closest approach to the sun on August 13, 2015. Rosetta studied the comet’s nucleus and its environment and made spectacular observations of gas and dust eruptions.
Continuing the study in the year since perihelion has given scientists a full picture of how the comet’s activity waxes and wanes during its 6.5-year orbit. The scientific team carried out new and potentially riskier investigations, including flights across the night-side of the comet to observe the plasma, dust and gas interactions in this region as well as to collect dust samples ejected close to the nucleus.
“The European Space Agency’s Rosetta Mission is a magnificent demonstration of what excellent mission design, execution, and international collaboration can achieve,” said Geoff Yoder, acting associate administrator for NASA’s Science Mission Directorate in Washington. “Being neighbors with a comet for more than two years has given the world invaluable insight into these beautiful nomads of deep space. We congratulate ESA on its many accomplishments during this daring mission.”
NASA’s Deep Space Network supported ESA’s Ground Station Network for spacecraft tracking and navigation throughout the mission. During the 14-hour descent, Rosetta sent uninterrupted data to ESA’s control center in Darmstadt, Germany, until the last signal was received at 5:19 a.m. MDT (13:19 CEST) on Friday.
The decision to end the mission is a result of Rosetta and the comet heading out beyond the orbit of Jupiter. Rosetta relied on solar power to operate the vehicle and its instruments, and as the comet moved away from the sun., there was less energy available to the spacecraft. Mission operators were also faced with an imminent month-long period when the sun is close to the line-of-sight between Earth and Rosetta, meaning communications with the craft would have become increasingly more difficult. Consequently, ESA decided to crash-land the spacecraft on the comet.
Science Until The Very End
Controlled hard-landing has become a common way to end the missions of planetary probes, but while most are subjected to very high-velocity impacts, Rosetta’s touchdown was made at a leisurely walking pace of 2 km/h.
The final hours enabled Rosetta to make many once-in-a-lifetime measurements, including analyzing gas and dust closer to the surface than ever before, and taking very high-resolution images of the comet nucleus. The images included views of the open pits of the Ma’at region, where the spacecraft made its controlled impact.
Pits are of particular interest because they play an important role in the comet’s activity and provide a unique window into its internal building blocks. Ma’at is home to several active pits more than 330 feet (100 meters) in diameter and 160 to 200 feet (50 to 60 meters) deep. The walls of the pits exhibit intriguing lumpy structures about 3 feet (1 meter) wide called “goose bumps.” Scientists believe those structures could be the signatures of early cometesimals that assembled to create the comet in the early phases of solar system formation.
NASA’s three science instruments were among those collecting data until the very end. The NASA instruments were: the Microwave Instrument for Rosetta Orbiter (MIRO); an ultraviolet spectrometer called Alice; and the Ion and Electron Spectrometer (IES). They were part of a suite of 11 science instruments on the orbiter.
MIRO was designed by NASA’s Jet Propulsion Laboratory (JPL) to provide data on how gas and dust leave the surface of the nucleus to form the coma and tail that give comets their intrinsic beauty. Studying the surface temperature and evolution of the coma and tail provides information on how the comet evolves as it approaches and leaves the vicinity of the sun. MIRO had the ability to study water, carbon monoxide, ammonia and methanol.
Alice, an ultraviolet spectrometer built by the Southwest Research Institute (SwRI), analyzed gases in the comet’s coma and tail; measured how fast the comet produced water, carbon monoxide and carbon dioxide (clues to the surface composition o the nucleus); and measured argon levels. These measurements will help determine the temperature of the solar system when the nucleus formed more than 4.6 billion years ago. Alice also documented a surprising lack of exposed water ice on the comet’s surface and identified an extremely volatile, unexpected gas in the comet’s atmosphere – molecular oxygen.
“Alice did its job perfectly, taking over 70,000 spectra in two years, providing a gold mine of data for comet scientists to study for years to come,” said Dr. Alan Stern, Alice principal investigator and an associate vice president of the Southwest Research Institute’s (SwRI’s) Space Science and Engineering Division in Boulder, Colorado.
The Ion and Electron Spectrometer (IES) was also built and operated by SwRI and led by principal investigator Dr. Jim Burch. IES was part of a suite of five instruments that analyzed the plasma environment of the comet, particularly the coma. The instrument measured the charged particles in the sun’s outer atmosphere, or solar wind, as they interacted with the gas flowing out from the comet.
Additionally, NASA provided part of the electronics package for the Double Focusing Mass Spectrometer, which was part of the Swiss-built Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument. ROSINA’s objective was to find out whether comets contain the basic elements of life, such as organic molecules and water. The yield was enormous:
“ROSINA has discovered over 60 molecules, 34 of which have never been found before on a comet,” said Andre Bieler from the Center for Space and Habitability (CSH) of the University of Bern.
ROSINA was also instrumental in showing that water on Earth was probably not brought here by comets, but that comets do contain oxygen and the amino acid glycine, a basic building block of life.
Rosetta’s Science Legacy
Communications with the orbiter ceased as it reached the comet’s surface, but the huge amount of data gathered during the mission will likely lead to new scientific findings for decades to come.
Overall, the results delivered by Rosetta so far demonstrate that comets are the likely leftovers of early Solar System formation, rather than fragments of collisions between larger bodies later on. Comet 67P/Churyumov-Gerasimenko gives scientists unparalleled insight into what the building blocks of the planets may have looked like 4.6 billion years ago.
“Just as the Rosetta Stone after which this mission was named was pivotal in understanding ancient language and history, the vast treasure trove of Rosetta spacecraft data is changing our view on how comets and the Solar System formed,” said project scientist Matt Taylor. “Inevitably, we now have new mysteries to solve. The comet hasn’t given up all of its secrets yet, and there are sure to be many surprises hidden in this incredible archive. So don’t go anywhere yet – we’re only just beginning.”
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta’s Philae lander was provided by a consortium led by the German Aerospace Center, Cologne; Max Planck Institute for Solar System Research, Gottingen; French National Space Agency, Paris; and the Italian Space Agency, Rome. JPL, a division of Caltech in Pasadena, manages the U.S. contribution of the Rosetta mission for NASA’s Science Mission Directorate in Washington. JPL also built the MIRO and hosts its principal investigator, Mark Hofstadter. The Southwest Research Institute (San Antonio, Texas and Boulder, Colorado), developed the Rosetta orbiter’s IES and Alice instruments and hosts their principal investigators, James Burch (IES) and Alan Stern (Alice).
Rosetta’s Final Hour