Comets are delicate, transient objects, frozen visitors from afar that make an incredible journey from their home in our Solar System’s dark and frigid outer limits, into the warm inner regions where our Sun glows with a brilliant fire and melting warmth. Comets are as beautiful as they are alien, flashing through the skies with sparkling, thrashing tails as they invade Earth’s domain relatively near our Star. In June 2014, planetary scientists announced they had discovered that Comet 67P/Churyumov-Gerasimenko is shooting out the Earthly equivalent of two glasses of water into interplanetary Space every second! This amazing observation was made by the Microwave Instrument for Rosetta Orbiter (MIRO) aboard the European Space Agency’s (ESA’s) Rosetta spacecraft on June 6, 2014. The detection of Comet 67 P’s water vapor has important implications not only for cometary science, but also for mission planning, as the Rosetta team prepares the wandering spacecraft to become the very first ever to orbit a comet in August 2014–as well as the first to deploy a lander to its icy, fragile surface, scheduled for November 11, 2014!
Icy Visitors From The Outer Limits
Comets are lovely visitors from afar that many planetary scientists think carry within their frozen hearts the most pristine remnants of primordial ingredients that went into the formation of our Solar System. This very ancient stew of icy stuff has been preserved in a kind of “deep freeze” at the fringes of our Solar System, where it is both very cold and very dark. These breathtaking, flashing objects–sometimes dismissively called “icy mud balls” or “dirty snowballs”, depending on the observer’s point of view–travel in from their distant domain beyond the outermost of the eight major planets, the beautiful blue ice-giant Neptune. Planetary scientists think that by obtaining an understanding of what composes the fragile, icy comets; they can likewise gain insight into what mysterious ingredients went into the wonderful recipe that cooked up our entire Solar System!
Comets are really icy planetesimals. This means that they are relics of the vast population of ancient building blocks that constructed the majestic quartet of giant major planets inhabiting the outer regions of our Solar System–Jupiter, Saturn, Uranus and Neptune. In contrast, rocky planetesimals, such as the asteroids that circle our Star in the Main Asteroid Belt between Mars and Jupiter, are the relic building blocks of the quartet of rocky inner planets–Mercury, Venus, Earth, and Mars. Both rocky and icy planetesimals blasted into each other, and then merged together to create ever larger and larger objects at the beginning of our Solar System’s existence approximately 4.56 billion years ago.
A comet’s nucleus is normally only about 10 miles–or less. But some comets show amazing comas, that can be more than 1 million miles wide! Some especially showy and impressive comets have been known to sport tails extending for 100 million miles!
Water, Water, Everywhere!
The MIRO instrument aboard the Rosetta spacecraft first spotted signs of water vapor being hurled out from its comet-target when it was approximately 217,000 miles away from it. At the time, Comet 67P was about 363 million miles from our Sun. After the initial June 6, 2014 discovery, water vapor was spotted again and again each time the MIRO instrument was targeted on the comet. Current observations are continuing to monitor the variability of the water vapor’s rate of production, in order to calculate the global gas production rate, as a function of its distance from our Star. The rate of gas production that MIRO detected gives planetary scientists a measure of the evolution of Comet 67P as it journeys both toward, and then away, from our Sun’s melting warmth. The rate of gas production is also critical to the Rosetta navigation team controlling the spacecraft. This is because the flow of the gas can change the trajectory of Rosetta.
Rosetta will only be four miles away from its prey, and it will lazily travel ever closer. At this point it will map 67P’s shape, surface, gravity, and rotation to hunt for the best spot to dispatch its Philae lander, while brushing over the comet at a height of merely two kilometers!