The planet-building process left leftovers: rocky and metallic asteroids, most between Mars and Jupiter (about $2.1$ to $3.3$ AU), and icy comets, which spend most of their time far out, in the Kuiper belt beyond Neptune or the much more distant Oort cloud.
💡 Why it works
A comet's nucleus is a lump of ice and dust a few kilometers across. Far from the Sun it is frozen and dark. On a long, stretched orbit it falls in toward the Sun, where sunlight warms it: ices turn to gas, releasing dust, and a huge glowing cloud, the coma, surrounds it. Two forces then shape the tails. The solar wind, a stream of charged particles flowing out from the Sun, sweeps the gas straight away from the Sun in a bluish ion tail. Sunlight itself gently pushes the dust away, into a broader, curved dust tail. Both pushes point away from the Sun, whichever way the comet is going.
🔑 Key idea
Comets are icy leftovers on long, stretched orbits. Near the Sun they grow comas and tails that always point away from the Sun. When Earth passes through dust a comet has left along its orbit, the grains burn up in our air as a meteor shower, on the same dates each year.
Halley's Comet ($a = 17.8$ AU, $e = 0.967$). The tail grows near the Sun and always points away from it, even when the comet is heading outward.
Meteors and meteor showers
A meteoroid is a small bit of rock or dust in space; when one plunges into the atmosphere at tens of kilometers per second and glows, it is a meteor; a piece that survives to the ground is a meteorite. Comets shed dust all along their orbits. Earth crosses some of these trails at the same point in its orbit each year, so showers return on schedule: the Perseids every August from Comet Swift–Tuttle, the Orionids every October from Halley's Comet.
Worked example
✏️ Worked example: A comet's year
A comet comes within $0.6$ AU of the Sun and goes out to $35$ AU. How long is its orbit?
Kepler's third law: $P = a^{3/2} = 17.8^{1.5} = 75$ years.
These are about the numbers for Halley's Comet, which last passed in 1986 and returns in 2061.
Answer: About $75$ years.
⚠️ Common mistake
Drawings of comets streaking along with tails behind them are misleading. The tail points away from the Sun, not backward along the path. On the way out, after rounding the Sun, the tail actually goes ahead of the comet. Check: what pushes the tail is the solar wind and sunlight, which both flow outward from the Sun.
Practice
Pass with 3 right answers, the last 2 in a row. Each right answer earns 1 XP.