Halley's Comet returns to the inner solar system, and can be seen from the Earth, about once every:

Halley's Comet is a short-period comet that returns to the inner solar system about every 75 to 76 years. It is the only short-period comet regularly visible to the naked eye from the Earth.

Edmond Halley's prediction:

• The English astronomer Edmond Halley studied records of bright comets seen in 1531, 1607 and 1682.
• Using Isaac Newton's laws of motion and gravitation, he concluded in 1705 that these were the same comet returning at intervals of about 76 years.
• He predicted that it would return in 1758.
• Halley died in 1742, but the comet reappeared as predicted at the end of 1758, and it was named after him.
• This was the first time the return of a comet had been successfully predicted, and it was a strong confirmation of Newton's theory.

Recent and future appearances:

• It was last seen from the Earth in 1986.
• It is next expected to come close to the Sun in mid-2061.
• Its period varies slightly, between about 74 and 79 years, because of the gravitational pull of the planets.

Historical records:

Its appearances have been recorded for more than 2,000 years, including by Chinese astronomers as early as 240 BCE.

Physical features:

• Its nucleus is about 15 km long and 8 km wide, shaped like a peanut.
• It is one of the darkest objects in the solar system, reflecting only about 4 percent of the light that falls on it.
• Its orbit is highly elongated, taking it from inside the orbit of Venus to beyond the orbit of Neptune.

Missions:

• In 1986, several spacecraft, together called the "Halley Armada", studied it.
• The European Space Agency's Giotto probe passed within about 600 km of its nucleus and took the first close-up images of a comet nucleus.

Why not the other options?

• ✗10 to 11 years is close to the sunspot cycle.
• ✗100 years and 250 years do not match the comet's period.