Although astronomers correctly placed comets in orbit around the Sun in the 1700s, they had yet to determine the physical nature of the comet itself. Spectrometers indicated some of the compounds surrounding comets, but could not penetrate the coma to detect the character of the nucleus. Around 1950, using all the available observations, Harvard astronomer, Fred Whipple, was the first to deduce that a comet nucleus must be a block of dirty ice. He noted that the composition of gas streaming off the nucleus indicated subliming ice, and that the dust particles must have been trapped in the ice of the nucleus. Whipple’s theoretical picture of a comet nucleus came to be called the dirty iceberg model.
Fred Whipple in 1927. Click here for original source URL
Direct confirmation of Whipple’s model came when an international fleet of five spacecraft (two Japanese, two Russian, and one European) investigated Halley’s Comet in 1986. The European probe came closest, flying only about 600 kilometers from the nucleus. That probe was named Giotto, after the Italian artist whose 1304 painting of Halley’s Comet may have been the first to represent it in a work of art.
Giotto and the other probes found the region around the nucleus to be a hazy, dusty environment. This made the pictures they returned fairly fuzzy. Giotto was actually hit by a dust grain when it was about 960 kilometers from the nucleus, knocking it partially out of commission for 32 minutes during the closest approach. However, the spacecraft recovered in time to take spectacular images of the nucleus. The probe’s close-up pictures revealed a floating iceberg-like object, about 15 kilometers long and 8 kilometers wide. For comparison, that’s about the same size and shape as Deimos, the smaller satellite of Mars.
Halley’s nucleus is as dark as black velvet. It reflects only 4% of the light that strikes it, compared with 60 to 80% for clean ice. Earth-based observers have found similar results for other comet nuclei. The black material is believed to be carbon-rich dust, which is finely distributed through the ice. When the ice sublimes, it leaves behind a concentrated layer of the dark dust. Jets of gas and dust shoot off "active" spots on the black nucleus. The gas in the head of the comet, around the nucleus, was found to be about 80% water vapor by volume.
Halley's comet was the first one to be observed by space craft, but it wasn't the last. The 2001 flyby of Deep Space 1 past comet Borelly and the 2010 EPOXI fly-by of comet Hartley 2 (which strongly resembles a peanut) are just two examples. The most recent visit to a comet was the Rosetta mission of the European Space Agency and the spectacular landing by its Philae probe in late 2014. Rosetta targeted Comet 69P/Churyamov-Gerasimenko as it headed into the inner Solar System. Rosetta went into oribt around the irregularly-shaped 4-kilometer wide comet, mapped it, and scouted out landing sites. Then the Philae lander was released for the hazardous attempt at a soft landing. Due to the comet's weak gravity the 20-kilogram lander weighed no more than sheet of paper and it drifted in at 2 mph, bounced lazily twice, and came to rest under a cliff on the comet. Although mostly successful, there were failures of the Rosetta reaction drive system and the Philae harpoon system designed to tether it to the comet surface. Philae ran out of battery power wuite quickly but the work of Rosetta continues.
Nucleus of Halley's comet. Click here for original source URL
Together, these missions are building a picture of a comet nuclei as dirty, tumbling snow balls that vary from tightly packed masses to loose ices. The ice in many comet nuclei is very weak, so that some nuclei spontaneously break into pieces. This can result from either gravitational tidal forces when a comet passes close to the Sun or a planet, or from pressure that builds up as more ice turns to gas, expanding as it does so. This is why Comet Shoemaker-Levy 9 split into many fragments before it impacted Jupiter in 1994.
Close up of Comet Hartley 2. This image was captured by NASA's EPOXI mission between Nov. 3 and 4, 2010, during the spacecraft's flyby of comet Hartley 2. It was captured using the spacecraft's Medium-Resolution Instrument. Click here for original source URL.