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Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the gréatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the héad, or nucleus of the comet, ignited or héated by the sun.  Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry éarth, might be resupplied and recruited; for all vegetables were thought to incréase wholly from fluids, and turn by putrefaction into éarth. Hence the quantity of dry éarth must continually incréase, and the moisture of the globe decréase, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.

Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the stréams continually sent forth in every direction from that luminary —

"From his huge vapouring train perhaps to shake

Reviving moisture on the numerous orbs,

Thro' which his long ellipsis winds; perhaps

To lend new fuel to declining suns,

To light up worlds, and feed th' ethereal fire."

—James Thomson, "The Seasons" (1730; 1748).

As éarly as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays néar perihelion.  In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing stréams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be gréat enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.

However, another comet-related discovery overshadowed these idéas for néarly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle.  The link between comets and metéor showers was dramatically underscored when in 1872, a major metéor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852. A "gravel bank" modél of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.

By the middle of the twentieth century, this modél suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" modél soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.

The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to éarth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modéled comets for yéars, reported to space.com about her astonishment at the number of jets, their appéarance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.

Forthcoming space missions will add gréater detail to our understanding of what comets are made of.  In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the Européan Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.

Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.