- How much material is there altogether?
- Not much. The whole main belt together comes to about 3 per cent of the mass of the Moon, which is why no planet formed there — and why the crowded asteroid field of films has no counterpart in reality, since the average gap between belt objects runs to millions of kilometres.
- What is a near-Earth asteroid?
- One whose orbit brings it within 1.3 astronomical units of the Sun, close enough to come near the Earth's own path. Those are the ones surveyed hardest, and the reason continuous asteroid surveys are funded at all.
- Have any been visited?
- Several. Spacecraft have orbited Eros, Vesta, Ceres and Bennu, brought samples of Itokawa, Ryugu and Bennu back to Earth, and in 2022 deliberately hit the small moon Dimorphos to measure how much its orbit changed.
- Who names them?
- The discoverer proposes and the IAU decides, but only after the orbit is secure enough for the object to be given a permanent number. Until then it carries a provisional designation recording the year and the half-month it was first seen. A body that turns out not to have come from here is renumbered again, with an I for interstellar — there are three so far, and the first of them was catalogued as an asteroid.
- Will an asteroid hit the Earth?
- Small ones do constantly and burn up harmlessly. For anything large enough to cause regional damage, surveys have catalogued the great majority of the kilometre-scale near-Earth population and none of them is on a collision course this century. The risk that remains sits with smaller, darker and less complete populations, which is why the surveys continue.
- What is the difference between an asteroid, a meteor and a meteorite?
- They are three stages of one thing. An asteroid or a smaller meteoroid is the body while it is in space. The meteor is the streak of light when it burns through the atmosphere. A meteorite is what reaches the ground. The great majority never survive the third stage.
- How big is the largest asteroid?
- Ceres, at about 940 km across, which is large enough to be round under its own gravity and is therefore also classified as a dwarf planet. It holds about a third of the mass of the entire belt on its own. Vesta and Pallas follow at around 500 km, and after that the sizes fall away quickly — most catalogued asteroids are a few kilometres or less.
- Could an asteroid be deflected?
- It has been done once, as a test. In 2022 a spacecraft was flown into the small moon of a near-Earth asteroid and measurably shortened its orbital period, which showed that a kinetic impact can change an orbit. Doing it for real would need years of warning, which is the reason the tracking effort exists.