One of the most fascinating and unpredictable objects of the night sky is a comet. Comets are one of the most beautiful sights with their fuzzy coma and large, streaky tail flowing across the sky. Because of their unpredictability, each seems to have its own personality. We long to see a comet that will become the "comet of a lifetime".
For all their beauty, comets were at one time feared and considered a bad omen. During medieval times the Greek philosopher Aristotle taught that the stars were fixed and unchanging while the cycles of the Sun, Moon, and planets had order and predictability. The Earth was the center of the universe and everything beyond Earth had order and never changed. Comets were changeable and thought to be a part of Earth's atmosphere. It was believed that they were warm, earthly exhalations that rose up into the upper atmosphere and were ignited there by the fiery upper regions. The size, shape, and duration of a comet depended on the nature of the exhalation. Aristotle claimed that comets foreshadowed winds and drought. This led to a superstition that comets were a warning of impending doom such as storms, disease, war, change of rule, and the death of leaders and kings. Medieval Europe, which relied on faith and the teachings of Aristotle for knowledge, endured an extended dark age for science.
Sir Isaac Newton finally proved in 1672 that comets were part of the universe and not Earth's atmosphere. He proved that a comet's orbit could be calculated using the changing positions of the comet each night. Edmond Halley studied the orbit of comets and found one that appeared to return near Earth roughly every 76 years. He successfully predicted the return of the comet in 1758 and although he did not live to see its return, the comet was named after him in his honor. The next challenge was to determine what comets are made of and where they come from. Fred Whipple proposed the "dirty snowball" theory in 1950 and recent spacecraft flying past several comets have proven him correct. He figured that comets were nothing more than a mixture of ice and rock and that the warmth of the Sun vaporizes the ices explosively to create jet-like fountains. This stirs up the ice and dust and creates a large cloud around it called the coma, which can grow very large, even larger than Jupiter, but does not hang around long before transforming into another beautiful feature. The solar wind is constantly blowing, causing the loosely bound particles in the coma to flow back away from the comet and outward from the Sun. The result is a long, wispy tail that can stretch millions of miles into space. As huge as a brilliant comet may appear, the tail barely has enough material to roll up into a snowball. The tail always points away from the sun, and the color is due to its composition: dusty tails are yellowish-white while ion-tails appear bluish-white. Comet Hale-Bopp vividly displayed both tails.
Jan Oort accurately suggested that comets originated from a cloud of comets in an area far beyond Pluto where gravity is too weak for them to collide and form a planet. The passage of a star or unseen dark object would disrupt the cloud with some of the comets falling towards the Sun. It is possible that Earth's oceans may have received its water from impacting comets. Comets could also cause mass extinctions on Earth; the power of a comet's impact was vividly demonstrated in July 1994 when fragments of Comet Shoemaker-Levy 9 impacted Jupiter leaving massive black clouds as large as Earth.
Comets hold the key to understanding the origins of the Solar System because their vast distance from the Sun where it is colder than -400 degrees F has left them nearly unaltered. The best way to understand comets is to send a spacecraft to one. Flybys of Comet Halley in March 1986 were too quick to make an in-depth analysis, but they started to unlock some of the mysteries. Comet Halley was very dark, like charcoal, and had bright jets of ice and lots of dust, which proved the "dirty snowball" theory. Stardust flew through the tail of Comet Wild-2 on January 2, 2004 and used a sticky gel to successfully capture particles within the tail and returned them to Earth. Deep Impact fired a 700-pound copper projectile into Comet Temple 1 on July 4, 2005 and analyzed the resulting impact cloud, but the best way to explore a comet is to orbit, land and ultimately return surface samples to Earth for analysis.
Rosetta is a European Space Agency (ESA) mission that was launched on March 2, 2004 and arrives at Comet Churyumov-Gerasimenko next month where it will go into orbit becoming the first spacecraft to orbit a comet. It will observe the comet's nucleus, coma, and tail as it travels closer to the Sun and measure its increasing activity. A lander, named Philae, will be deployed in November and will attempt to land on the comet. Rosetta takes its name from the Rosetta Stone and Philae is the name of an island in the Nile where an obelisk was found that was used along with the Rosetta Stone to decipher Egyptian hieroglyphics. Rosetta carries a sophisticated suite of instruments including visible, ultraviolet, and infrared imagers, a sounding experiment, ion mass analyzer, grain impact analyzer, microwave instrument, and a spectrometer. Rosetta will orbit Comet Churyumov-Gerasimenko until December 2015 and possibly longer. The comet will reach its closest point to the Sun during August 2015, which will allow Rosetta to observe it during its most active phase.
Philae is a complex lander that will have to carefully land under very weak gravity, literally docking the comet instead of landing. It has three legs that will unfold for a gentle touchdown after a ballistic descent. The legs will dampen out any vibrations upon landing and reduces the chance of bouncing. The legs can rotate, lift, or tilt to return Philae to an upright position. Immediately after touchdown a harpoon will be fired to anchor the lander to the ground and prevent it from escaping the comet's extremely weak gravity. The primary mission will last one week, but the lander could last several months. It is solar powered and bristles with scientific equipment. It has a panoramic and microscopic imaging system, radio sounder, gas analyzers, magnetometer, plasma monitor, a drill to sample and sample retriever, and a dust monitor. These instruments will measure the elemental, molecular, mineralogical, and isotopic composition of the comet's surface and subsurface material. It will also measure the characteristics of the nucleus such as near-surface strength, density, texture, porosity, ice, thermal properties, and analyze the texture and grains of the surface material. The drill will obtain samples a few inches beneath the surface and deposit them in different ovens or deliver them for microscopic inspection.
Comet Churyumov-Gerasimenko, discovered in 1969, is a football-shaped object about 2x3 miles in size, orbits the Sun once every 6.57 years, and rotates on its axis once every 12 hours. It comes as close to the Sun as 120 million miles and is unusually active. This makes it a reliable target because it develops a good coma with jets and a distinct, if small, tail. It is known for its brief outbursts and is classified as a dusty comet, although it is estimated to produce about 40 times less dust than Comet Halley. The Hubble Space Telescope is able to resolve the nucleus and confirm its size and shape. It shines as bright as magnitude +12.
A surface sample return mission is the ultimate mission that is under consideration and will someday fly once a way is found to preserve the extremely cold and delicate comet material. Rosetta and Philae are on the verge of potentially rewriting comet science and possibly giving better insight on the history and evolution of Earth and the Solar System.