Comets—clumps of ice and rock—come near Earth much less frequently than asteroids, but they travel much faster—one hundred times the speed of sound on Earth. So even a relatively small comet would impact with very high energy, and with little warning.
As a rule, comets are much harder to find and track than asteroids. Trillions of comets are hidden in a zone far beyond Pluto's orbit called the Oort cloud. Because comets are less reflective than asteroids, they can remain invisible until they're very close to Earth. As a comet nears the Sun, solar energy begins eroding its surface, forming a bright tail that always points away from the Sun.
Astronomers don't know exactly how hazardous a comet impact would be, because comets are less dense than asteroids, and because so little is known about comet composition. However, the incredibly high speeds of most comets mean that the energy of an impact would be very high. A comet could probably cause about as much devastation as an asteroid of the same size.
Subtopic: Milky Way Galaxy
Keywords: Astrogeology, Astrophysics, Collisions (Astrophysics), Comets, Comets--Collisions with Earth, Meteorites, Natural disasters, Oort cloud