Why asteroids?

Asteroids are an integral part of the solar system but probably don’t get as much attention as missions to the planets. The asteroid belt lies between the orbits of Mars and Jupiter (Figure A.1).

Asteroids (and related bodies such as dwarf planets) are extremely important because they represent nearly pristine objects originating from the formation of the solar system. It’s thought that the asteroid belt includes some icy objects composed of carbon-rich material from the outer solar system, while other components originate from the rocky inner part of the solar system.

Position of the asteroid belt relative to the inner planets and Jupiter.
Figure A.1: Position of the asteroid belt relative to the inner planets and Jupiter.

The belt exists because Jupiter produces such large gravitational perturbations that objects within the belt have continuing collisions that inhibit the formation of large objects (Figure A.2). While slow collisions lead to the building of larger asteroids, high speed collisions tend to produce fracturing of the incident objects. Within the asteroid belt there are millions of objects with a size less than 1 km and just a handful of objects with a diameter greater than 300 km.

Some of the asteroids have been kicked out of the belt. Of these ejected asteroids, some have fallen towards the inner solar system. Those that have been nudged into orbits in Earth’s neighborhood are called near-earth asteroids. Some are kicked outwards, and settle into an orbit where the gravity forces of the sun are balanced by the gravity forces from Jupiter. The asteroids that are in Jupiter’s orbit and are forward of the planet are called “Greeks” while the asteroids trailing Jupiter are called “Trojans.”

Artist impression of the breakup of a large asteroid and the creation of a family of asteroids.
Figure A.2: Artist impression of the breakup of a large asteroid and the creation of a family of asteroids.

Asteroids and their families

Because of the continuing collisions, asteroids come in many shapes and sizes, as shown in Figure A.3. These objects, because of their many different characteristics, can provide us with clues on the role of gravity in the formation of the main constituents of the solar system as well as insight into their composition. Asteroids can also offer potential clues to the properties of the interiors of the planets since some of the asteroids may originate from small planets that have been broken up by previous collisions.

The largest body in the asteroid belt is the dwarf planet Ceres, which is about 950 km in diameter. Ceres, like Pluto, is considered a dwarf planet because although it has a spherical shape produced by its own gravity, it has yet to clear its orbit of other objects. It is also thought to have a core. The largest asteroid (and not a dwarf planet) in the asteroid belt is Vesta, which has a diameter of 525 km. As shown in Figure A.3, it has the appearance of a flattened sphere. Unlike most asteroids, Vesta shows signs of having had a liquid core at one time, with lava flows marking its surface. The large craters on Vesta are evidence of the violent impacts that occur in the asteroid belt — and astronomers have identified several thousand asteroids that are believed to have originated from Vesta. These asteroids are called V-type or “vestoids,” and some have even impacted the Earth.

Comparison of the shapes of Ceres with asteroids and the Martian moon Phobos.
Figure A.3: Comparison of the shapes of Ceres with asteroids and the Martian moon Phobos. https://astronomy.com/magazine/ask-astro/2017/08/the-diameter-of-spherical-bodies

Smaller asteroids like Itokawa, which is not part of the asteroid belt, can be very much more irregular in shape, as you can see in Figure A.3. For comparison, Figure A.3 also shows the Martian moon Phobos. It, too, is non-spherical and there is controversy over whether it is a captured asteroid or whether it was produced by the impact of a Vesta-sized asteroid with Mars.

Regardless of which theory is correct in the case of Phobos, asteroids played an important role in the formation of the solar system and continue to provide clues on such processes. They also have the potential for providing important resources for human exploration beyond low Earth orbit, which was the aim of the now defunct company Planetary Resources, which championed the concept of asteroid mining.

Missions to asteroids (and comets)

Some of the key missions that observed small solar system objects are listed in Table A.1; the images of the relevant spacecraft are shown in Figure A.4.

Flyby missions

Asteroids have made for excellent secondary science objectives for spacecraft that are “flying by” on their journeys to other targets in the solar system. Galileo, on its journey to Jupiter, provided the first close-up glimpse of asteroids — Gaspra and Ida in the asteroid belt. The asteroid Braille, also in the asteroid belt, was observed in a flyby mission by Deep Space 1, which was on its way to a technology demonstration of the use of plasma thrusters in deep space. The Stardust mission did a flyby of asteroid Annefrank as a secondary mission objective — its main objective was to fly into the tail of comet Wild 2 and bring back samples to Earth. These samples showed that comets contain several complex organic compounds that are fundamental building blocks of life on Earth.

In 2014, the New Horizons spacecraft, on its way to visit Pluto (one of the furthest known dwarf planets), flew by Arrokoth, a small object in an orbit beyond Neptune. Arrokoth is composed of two smaller chunks, called planetesimals, that are joined together. This makes it an interesting example of what happens when objects impact each other in a slow collision — exactly the type of collision that does not occur in the asteroid belt, as mentioned above.

DateAsteroid/CometAv Length (km)Mission
1999Braille1.6Deep Space 1
2004Wild 24Stardust
2012Touttis2.5Chang’e 2
2014Arrokoth20New Horizons
Table A.1: Flyby missions of asteroids/comets
Images of the spacecraft in Table A.1.
Figure A.4: Images of the spacecraft in Table A.1.

Sample return missions

The fact that so many asteroids are relatively close to Earth and have low gravity has led to missions developed to orbit, land, and even return samples back to Earth. These missions are listed in Table A.2; images of the spacecraft are shown in Figure A.5.

Results from the Deep Impact mission to Tempel 1 showed that comets contain more rocky material than previously thought, which supports the theory that material from the inner solar system has been transported outwards and material from the outer solar system has been transported inwards.

The lander Philae, part of the Rosetta mission to comet Churyumov–Gerasimenko, unfortunately had a hard bounce upon landing and so the data return was limited. Philae’s instruments nevertheless showed that the comet’s surface is quite pungent, with ammonia, hydrogen cyanide, and hydrogen sulfide being present. Rosetta also found the presence of simple amino acids, which is the basis on life on Earth.

The samples returned during the Hayabusa mission to asteroid Itokawa suggested that Itokawa was probably part of a larger asteroid in its past. The samples from Itokawa also contained material like that of meteorites that have impacted Earth.

DateAsteroid/CometAv Length (km)Mission
1998Eros17NEAR Shoemaker
2005Tempel 1Deep Impact
2019Ryugu0.8Hayabusa 2
Table A.2: Orbiting, landing, and/or sample return missions to asteroids or comets
Past missions to asteroids and comets.
Figure A.5: Past missions to asteroids and comets.

ROADS on Asteroids -related missions

The Dawn mission surveyed the largest asteroid in the belt, Vesta, and showed that Vesta is rich in carbon material as well as material that needed water in order to form. Dawn then travelled (using plasma propulsion) to the inner most dwarf planet, Ceres, and found that it once had oceans. On the one hand, the presence of ancient oceans on Ceres suggests that the ingredients of life may have been present on the dwarf planet — but on the other hand, Ceres’ relatively low gravity means the presence of such oceans would like have been short lived, and if so life would not have been able to take hold.

Currently, there are also two ongoing sample return missions to asteroids: the Hayabusa 2 mission to the asteroid Ryugu by JAXA (the Japanese space agency) and the OSIRIS REx mission to the asteroid Bennu by NASA. Hayabusa 2 is unique because it used a hypervelocity impactor to expose and collect underlying material to help better determine the age and origin of the asteroid Ryugu. Bennu is a carbonaceous asteroid, and the OSIRIS REx mission seeks to determine the potential abundance of water and organic compounds.

These sample return missions require a substantial amount of energy to make the necessary transfers in an out of orbit on its journeys both to and from the target objects. Using standard chemical rockets would be too expensive, and so these systems use plasma propulsion — where strong electric fields are applied to strip the electrons off atoms and the charged particles are accelerated to speeds nearly 10 times faster than can be attained within chemical rockets. Because this requires electrical power, Hayabusa 2 and OSIRIS REx carry large solar arrays. Yet even with this large array, the total mass is nevertheless much less than carrying chemical rocket systems.

The year 2021 will be an exciting year for asteroid research as there are two missions to be launched. The Double Asteroid Redirection Test (DART), scheduled to launch for asteroid Didymos in July 2021, seeks to redirect Didymos’s “moonlet,” a small secondary object in orbit around the asteroid. The Lucy mission, scheduled to launch in October 2021, will investigate the Trojan asteroids that trail Jupiter’s orbit.

Recommended reading on missions to asteroids







Recommended reading on asteroids and other missions into deep space