Why missions visit asteroids and comets

Asteroids and comets are leftovers from the formation of the solar system, time capsules that have changed little in more than four billion years.^[1] The Trojan asteroids that share Jupiter's orbit, for example, are thought to be remnants of the material that formed the outer planets.^[2] Visiting them shows what the planets were built from, and it also tells planetary defense what an asteroid is made of and how it would respond to a push.

Flybys: the first look

A flyby spacecraft passes its target and observes it on the way, without being captured into orbit; flybys made the first reconnaissance of the solar system.^[3] One spacecraft can visit several targets this way. NASA's Lucy is planned to fly past three asteroids in the main belt and eight Trojans.^[2] New Horizons flew past Pluto in 2015 and then Arrokoth in 2019, the most distant object ever explored up close.^[4]

Orbiters: the long study

An orbiter must carry enough propulsion to slow down and be captured by its target, and it follows the first reconnaissance with in-depth study.^[3] NASA's NEAR Shoemaker became the first spacecraft to orbit an asteroid when it reached Eros in 2000.^[5] Psyche is planned to spend about two years in orbit around the metal-rich asteroid of the same name, mapping its surface and measuring its composition.^[6]

Landing and touch-and-go

A lander reaches the surface and returns data from there.^[3] NEAR Shoemaker touched down on Eros in 2001 even though it was never designed to land.^[5] Japan's Hayabusa touched down twice on Itokawa, and NASA's OSIRIS-REx collected rock and dust from Bennu on 20 October 2020.^[7][8]

Sample return: the laboratory on Earth

Hayabusa delivered the first asteroid sample in 2010, and its grains matched the most common kind of meteorite found on Earth.^[7] Hayabusa2 brought back 5.4 g from Ryugu in 2020, and OSIRIS-REx returned 121.6 g from Bennu, the largest asteroid sample collected in space.^[9][10] NASA's Stardust collected particles from comet Wild 2 for return to Earth in 2006.^[11] The Bennu sample contains 14 of the 20 amino acids that life on Earth uses to make proteins.^[12] This is called sample return.

Impactors: hitting the target

Some missions strike their target on purpose. In 2005, NASA's Deep Impact released an impactor into comet Tempel 1, blasting out material from beneath the surface so its interior could be studied.^[11] In 2022, DART struck the asteroid moonlet Dimorphos and shortened its orbit, the first deliberate change to the motion of a celestial object.^[13] It showed that a kinetic impactor can change an asteroid's path, given warning time of several years at a minimum.^[14]

What this means for defense

Deflecting an asteroid depends on knowing its mass, density, porosity, interior structure and how it responds to outside forces.^[1] Each kind of mission adds to that knowledge. The missions in this section are listed with what each is for, starting with DART and Hera.