Dubbed broom stars by Ancient Chinese astronomers, comets have captivated humanity for centuries. From our perspective on Earth, the ephemeral nature of these visitors in the sky were a source of great mythology. The physical understanding of these objects began with Edmond Halley’s prediction in 1705 that a previously observed comet would return in 1758. When the comet made its scheduled rounds that year, it became perhaps the most celebrated of celestial objects and was named after Halley. Photography and spectroscopy beginning in the late 1800’s began to unearth the composition of comets. In 2014, ESA’s Rosetta mission was the first to land a probe on a comet. While comets are only short-term visitors in the night sky on Earth, they are relics from the formative era of the Solar System 4.6 billion years ago.
Rosetta was launched in 2004 and reached Comet 67P/Churyumov-Gerasimenko in 2014. This comet is thought to have originated in the Kuiper Belt beyond Uranus and was gravitationally perturbed into an orbit that reaches just beyond Jupiter every 6.45 years. Why did it take so long for Rosetta to reach its target? The trajectory utilized three flybys of Earth and one of Mars to ramp up to the required velocity to get to the comet. This saves fuel reducing weight lowering mission costs. Along the way, Rosetta also flew by the asteroids 2867 Steins (in 2008) and 21 Lutetia before going into a 31 month deep space hibernation. Below is a video of the complex trajectory of the Rosetta mission.
The presence of water is the key to life on Earth. A crucial question is how did the water get here? The early Earth was very hot, hot enough to boil water so it must have been delivered afterwards with the prime suspects being comets and/or asteroids. Rosetta, named after the stone containing hieroglyphics that deciphered Ancient Egyptian writing, was hoped to have deciphered this part of the ancient Solar System history. What it found was that the water on the comet was heavier than that on Earth.
If you are a World War II buff, you may have heard of heavy water. This type of water has an hydrogen atom with an neutron in addition to the usual proton and is thus, heavier than normal water. Heavy water can be used to produce weapon grade nuclear material and Germany had a program set up in Norway to do so. A series of commando raids in 1942-43 by the Norwegian Resistance knocked these plants, and Germany’s nuclear ambitions, out of commission.
What Rosetta found was Comet 67P had three times the amount of heavy water isotopes than found on Earth, making this type of comet unlikely to have delivered water to Earth. For now, asteroids seem to be the likely candidates but our sample size is still small and more work needs to be done to arrive at a definitive answer.
The other key to life is organic material. Could comets have delivered organic material to Earth during the early bombardment phase of its existence? The mission lander, Philae, was tasked with detecting such material near or on the comet surface. The landing did not go as planned as harpoons designed to keep Philae in place failed to deploy. The lander bounced in the light gravity environment and settled in a shadowed region on the surface. Although this caused Philae’s solar battery to shut down after 60 hours, the lander detected 16 forms of organic material. This material, delivered to Earth billions of years ago, could have served as the precursor to the complex organic chemistry that produces life on Earth.
What Philae also found is that the surface of the comet was covered with about one foot of soft, dusty material over a hard ice surface. Comets, despite the brightness of their appearance as they get close to the Sun, are actually among the darkest objects in the Solar System. In fact, comets are darker than coal. Keep in mind, coal is organic in nature, representing the end of the life cycle of plants on Earth some 300 million years ago. When we study comets, we are studying ourselves and our origins.
In the final weeks of the mission, it was announced that the Rosetta orbiter detected organic material more complex than what Philae had found. This discovery was made by spectroscopy performed on dust grains captured by Rosetta. Rosetta and Philae would also work in tandem to analyze the interior of the comet. Philae transmitted radio waves through the comet which were received by Rosetta. An analysis of the radio waves indicated Comet 67P is porous and low in density. The nature of the dust grains which are fluffy, rather than compacted, is the cause. Gravity measurements by Rosetta indicated there are no underground cavities in the comet.
And the gravity field of Comet 67P was quite complex. Gravity fields around spherical objects are fairly predictable to orbit. However, Comet 67P has what is now known as a rubber duck shape. This made Rosetta’s orbital maneuvers tricky. It was determined that the rubber duck nucleus was caused by a slow collision of two comets that eventually stuck together.
This just a summary of Rosetta’s discoveries. I would recommend visiting the Rosetta website for the whole shebang.
While it is a sad event, especially for the mission ops team, it’s not the end of the story. The data sent back by Rosetta will be analyzed for years to come and certainly more discoveries will be made. The Mercury Messenger mission ended in similar fashion in 2015 and it was announced this week its data indicated Mercury was still shrinking. In announcing its discoveries, Rosetta had an innovative social media team including a series of great educational animations for children promoting public support for the mission.
Watching the end of Rosetta reminded how different it was during the 1980’s when the mission was first conceived. Back then, you did not get live updates of planetary missions. What you got was maybe a couple minutes on the nightly news and an article in the newspaper the following day. Rosetta was hatched during a difficult time for space exploration.
The global recession of the early 1980’s was not as bad as the Great Recession, but bad enough. Unemployment spiked over 10% both in America and Europe. At one time, the Reagan administration considered axing NASA’s planetary program including Voyager, before it had reached Uranus. Fortunately, that did not happen, but there were no planetary missions launched from 1977 to 1989. We face a similar lull in outer Solar System exploration as both Cassini and Juno will end their missions in 2017 as Rosetta did today. That lull is a result of funding cutbacks after the 2008 financial crisis. While Solar System exploration will not come to a standstill, the budgetary cuts of the early 2010’s scaled back missions that would have been launched in the next few years.
If all good things come to an end, the same is true of all bad things.
I remember during the late 1980’s and early 90’s the future plans for space exploration including the Mars Rovers, the Venus Express, and Cassini which I got to see being built at JPL. This, along with the Great Observatories program, motivated me to return to school and study astronomy. Little did I know back then I would deliver the results of these missions in same day’s time via the internet to my future students.
A new generation of scientists are planning missions to go Europa, Ganymede, and Jupiter. These missions will not launch until the 2020’s and it may not be till the 2030’s when they reach their targets. I look forward to presenting those mission results to a new generation of students just as I have with Rosetta.
* Image atop post is from Rosetta final outreach animation. Credit: ESA/Rosetta