Asteroid Exploration

Asteroids originated in the early inner system jostle for planetary control and the migrated their orbits out towards Jupiter’s influence while most comets come from families of remote objects in the Oort Cloud and Kuiper Belt.


There was a discovery in October 2017 of an extra-terrestrial asteroid named ‘Oumuamua, which whipped into our Solar System at 87 km/second from above the system plane, and rushed back out before anyone had much chance to study it. In 3 months it came to the inner system, flew inside the orbit of Mercury, had its trajectory altered and picked up speed and spewed dust and ice as it left.


This object is not well understood, but its hyperbolic trajectory and speed indicated it originated outside the Solar System, and would only visit the one time.  It is thought to be an oblong, 50×500 metre, part asteroid, part comet tumbling body.  It certainly confirmed that travel between star systems occurs.

This object has likely been travelling for hundreds of millions of years or more from its origin.  From its home system, it may have visited other star systems, as it has our own, passing quickly around central star and again heading out into interstellar space.  An epic interstellar voyage speaks to the enduring patience of an old rock.

Astronomers noted that it appeared to have a high density (indicative of a rocky and metallic composition) and that it was spinning rapidly and picking up speed as it left the solar system.  Its arrival trajectory indicate that it did not come from Earth’s nearest star system, and that it may in fact have been travelling for a very long time.

Asteroids (planetismals)

Historically, the terms asteroidminor planet, and planetoid have been more or less synonymous. An asteroid is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet. Some moons do exhibit signs of being captured asteroids.  Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects.

Our system currently has five identified dwarf planets, but may there are likely many hundreds or more dwarf planets still to be identified out to the edge of the Oort Cloud, the boundary of our Solar System.

Trojans are astronomical objects that share the same orbit as a larger planet, such as Jupiter, Mars and Neptune at 60° ahead or behind the main body. Centaur asteroids are those that roam between the gas giants planets; Jupiter, Saturn, Neptune, and Uranus.  Kuiper Belt is the region beyond the gas giant planets.

The designation asteroid is kind of a bucket for the large number of small and not well understood astronomical objects in our solar system.  As remnants of our varied solar system, they are believed to be quite diverse. More subtle definitions will come with our expanding knowledge of these objects.   A good source for astronomical, solar system knowledge is Worldbuilding by the Numbers.  This provides a nice introduction to asteroid.

Hollywood has given many of us the unfortunate idea that asteroid belts (often called “asteroid fields” in movies like Star Wars™), are densely packed regions into which it is suicide to venture.  This is probably true of ring systems such as Saturn’s (except that the particles are much smaller on average), but it is far from the case for asteroid belts (at least in the Solar System).

In the Solar System’s asteroid belt, fully half of the mass (about 4% of the mass of the Moon, in total) is contained in just four bodies (Ceres, Vesta, Pallas, and Hygiea), and fully a third is contained in Ceres, alone.  The rest of the mass is divided between about 200 bodies of about 100km in size, 1-to-2 million bodies around 1km or larger in diameter, and many millions of smaller bodies, down to the size of dust particles, created by the rare collisions between asteroids.

There are literally thousands and tens-of-thousands of kilometers between bodies in the asteroid belt.  Though collisions do happen, they are comparatively rare.  “Collisions between main-belt bodies with a mean radius of 10 km are expected to occur about once every 10 million years.”

Asteroids’ first discovery

The first asteroid, Ceres was discovered on January 1, 1801, by the astronomer Giuseppe Piazzi at Palermo, Italy, correctly believing it to lie in the orbital region between Mars and Jupiter where Kepler registered a gap in his harmonic scheme.

By the end of the 19th century, 464 objects had been found, and that number grew to 108,066 by the end of the 20th century. As of 2018, the orbits of 757,626 minor planets were archived at the Minor Planet Center, 516,386 of which had received permanent numbers.

Part of the challenge of acknowledging asteroids is knowing their orbit, which takes adequate observation.  Asteroids can disappear before this information is captured, making it difficult to know whether the body was new, or had possibly been observed before.  This is why knowledge of an asteroid’s orbit is needed before it is named.

Asteroid taxonomy

In 1975, an asteroid taxonomic system based on coloralbedo, and spectral shape was developed.  Asteroids fall into three broad spectral/compositional categories, commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.  As scientists expand their direct understanding of asteroid composition, these categories will continue to break in many sub-categories.

C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroidsS-type asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. Approximately 17% of asteroids are of this type.  M-type or X-type asteroids are asteroids of partially known composition, although some are thought to have high metal content.

Metallic M-type asteroids, are made of nickel–iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts, and are thought to be the source of iron meteorites. M-type asteroids are the third most common asteroid type.

Asteroid density

Another feature that distinguishes asteroids is their density, ranging in structure from solid monolithic to semi-fused monolithic fragments with differing depths of settled regolith top layer to loose rubble pile.

Recent thinking is that many asteroids are to varying degrees ruble piles, collections of fragments that gently come together.  Touchdowns on these distant objects have yet to find much dust, but rather look like landing on old rock slide.

An asteroid’s low mass, compared to planets  means very little gravity holds it together.  A high impact, or a gravitational flyby between two asteroids may cause an explosive dustball of fragments.  This cloud would settle back into a newly mixed, loosely density surface. The tiniest fragments are most likely to escape, and with time, dissipate.

Solar events striking asteroids can also energize surface material and rotation, further blasting the body with radiation, further challenging the smallest grains more so than larger fragments.

Asteroid Moons

One way to determine the density of an asteroid is to find an orbiting moon.  Variations in body’s orbit are influenced by a moon, and thereby give astronomers an indication of density.

An example is 216 Kleopatra (217 km × 94 km), resembling the shape of a ham-bone.  In September 2008, Franck Marchis and his collaborators discovered two moons orbiting Kleopatra, later named Alexhelios (outer – 9 km – 2.5 day orbit) and Cleoselene (inner – 7 km – 1.25 day orbit).


These moons indicate Kleopatra to be a rubble pile, a loose amalgam of metal, rock, and 30–50% empty space by volume, likely due to a disruptive impact prior to the impact that created its moons.  It is assumed that the moons and the main asteroid have nearly identical compositions, although the moons’ density may be higher.  The larger, fractured Kleopatra would represent the majority of re-clustered asteroid fragments ejected by the large collision event that created its two moons.

Asteroid Rotation: Fast vs Slow 

Another method to be able to test for monolith vs rubble pile is to check the rotational period.  Bodies below a period of 2.2 hours – also known as the “cohesion-less spin-barrier” – can not be merely held together by self-gravity, but must be formed of a contiguous solid, as they would fly apart otherwise.

As of 2019, a group of approximately 800 bodies – most of them are stony near-Earth asteroids with small diameters of barely 1 kilometer– have an estimated period of less than 2.2 hours. (See fast rotators). The encounter with Bennu shows how an asteroid’s rotation can be sped up by solar forces causing them eject material.


Robotic visits in the Asteroid Belt

Human missions to small bodies in the solar system have been some of the most exciting astro-scientific moments.  My touchpoints are a short review of asteroid bodies Earth’s spacecraft have visited to 2019. Each visit in has its own scientific discovery and achievement, feeding a growing, dynamic field.


The largest asteroid body is Ceres, which is said to hold 30% of the total asteroid belt mass.  1 Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter, slightly closer to Mars’ orbit. With a diameter of 945 km. Ceres is the largest of the minor planets, and the only dwarf planet inside Neptune’s orbit. It is the 33rd-largest known body in the Solar System and is estimated to comprise approximately one-third of the mass of the entire asteroid belt.

It was visited by the Dawn mission in 2015.

1 Ceres


4 Vesta is the second-most-massive and second-largest body in the asteroid belt, after the dwarf planet Ceres at approximately 550 km in diameter.  It contributes an estimated 9% of the mass of the asteroid belt.

It was also visited by the Dawn mission in 2011, before it went on to Ceres.

Visited larger asteroids


21 Lutetia is a large asteroid in the asteroid belt of an unusual spectral type. It measures about 100 kilometers in diameter (120 km along its major axis). It has an irregular shape and is heavily cratered, with the largest impact crater reaching 45 km in diameter.

ESA’s Rosetta mission has returned the first close-up images of the asteroid in 2010.  It has a minor planet designation, a relatively complex composition, and up to 3 kilometres depth of regolith covering the surface.

21 Lutetia

Comet exploration

It is appropriate to review visits to comets in this collection of robotic exploration of asteroids.  Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound, out-gassed atmosphere surrounding their central nucleus. Yet certainly there are bodies that have expended their gases, and some asteroids are now also known to outgas.

Known comets originate in the Oort Cloud and Kuiper Belt where the vast majority of objects have never been in the inner solar system and hold more ice and frozen gases. Asteroids are believed to have originated in the early inner system jostle for planetary control and the migrated their orbits out towards Jupiter’s influence and are more rocky.

Robotic space missions now include a Kuiper Belt object, Ultima Thule.  This object blurs the line between asteroid rock and stone with stellar particles causing it to out-gas like a comet.  Comet missions are an important and fascinating facet of small body exploration.

Hailey’s Comet

Halley’s Comet was visited by the Giotto spacecraft.  In March 1986, the spacecraft succeeded in approaching Halley’s nucleus at a distance of 596 kilometers. Images showed Halley’s nucleus to be a dark peanut-shaped body, 15 km long, 7 km to 10 km wide.

Only 10% of the surface was active, with at least three outgassing jets seen on the sunlit side, material analyzed to be 80% water, 10% carbon monoxide, 2.5% a mix of methane and ammonia, as well as trace amounts of other hydrocarbons, iron, and sodium.  Halley’s nucleus was dark, which suggested a thick covering of dust



comet Grigg–Skjellerup

The Giotto spacecraft later had a rendez-vous with comet Grigg–Skjellerup, coming within 200 km of the 2.6 km wide body.  Unfortunately the space craft had its camera destroyed during its encounter with Hailey’s comet, so no photos of the object were taken. Its orbit is being influenced by Jupiter iteratively pushing its perihelion orbital point from 0.77 AU in 1725 to 1.12 AU in 1999.

comet Harley 2

Hartley 2 was the target of a flyby of the Deep Impact spacecraft, as part of the EPOXI mission in November 2010, which was able to approach within 700 kilometers.  It is believed to be 1.2 to 1.6 km wide.


Comet Borrelly

This object is a periodic comet (solar orbits of 200 years or less), which was visited by the spacecraft Deep Space 1 in 2001. The comet’s nucleus is particularly notable for being shaped like a bowling pin. Its dimensions are 8×4×4 km


Comet Churyumov-Gerasimenko

67P/Churyumov-Gerasimenko was the primary target for the Rosetta mission which arrived in 2014 for a two year study. The mission included a semi-successful landing the Phillea lander on the comet.

The comet consists of two lobes connected by a narrower neck, with the larger lobe measuring about 4.1 × 3.3 × 1.8 km and the smaller one about 2.6 × 2.3 × 1.8 km, and loses 10 – 25 cm of material per year, most when closer to the sun.


Tempel 1

Deep Impact was a NASA space probe was designed to study the interior composition of the comet Tempel 1, by releasing an impactor into the comet. The shape of this object is somewhat pyramidal, with a mean radius of 2.8 km. On July 4, 2005, the Impactor successfully collided with the comet’s nucleus.

The impact excavated debris from the interior of the nucleus, forming an impact crater. Photographs taken by the spacecraft showed the comet to be more dusty and less icy than had been expected. The impact generated an unexpectedly large and bright dust cloud, obscuring the view of the impact crater.

A follow-up mission was the Stardust NExT in 2011 was able to review the site, as well as further map and study the body..

converted PNM file


Wild 2

The presence of iron in samples from Comet 81P/Wild (5.5 × 4.0 × 3.3 km) by the Stardust mission and returned to Earth in 2006 have been interpreted as evidence for space weathering, giving the comet its rust-red hue.


Smaller Asteroid visits

The following is a scale composite of some of the objects visited by space craft.  It gives an indication of the range of objects.

Visited smaller asteroids


253 Mathilde is an asteroid in the intermediate asteroid belt, approximately 50 kilometers in diameter. The asteroid has a number of extremely large craters, with the individual craters being named for coal fields and basins around the world.  Measurements by the spacecraft NEAR Shoemaker suggest that the asteroid is very loosely packed rubble pile.

253 Mathilde


243 Ida is a 60 km by 25 km asteroid in the Koronis family of the asteroid belt and has a 1.5 km wide natural satellite, named Dactyl. The Koronis family is a large family of stony asteroids, which are thought to have been formed in a catastrophic collision 2 billion years ago. The Koronis family travels in a cluster along the same orbit and has 5949 known members. It was visited by the Galileo spacecraft in 1993.

243 Ida – Dactyl


433 Eros is a stony and peanut-shaped asteroid 34 km x 11 km in the Amor family.  The Amor asteroids is a group of 7217 near-Earth asteroids which do not cross the orbit of Earth, but most do cross the orbit of Mars.

The NEAR Shoemaker probe visited Eros twice, first with a 1998 flyby, and then by orbiting it in 2000 when it extensively photographed its surface. On February 12, 2001, at the end of its mission, it landed on the asteroid’s surface using its maneuvering jets.  It showed the surface to be covered in a regolith layer, although it is not clear how deep it may be.

433 Eros


NASA’s Deep Space 1 experimental spacecraft successfully flew closely above the surface of asteroid 9969 Braille (2.1 × 1 × 1 km.) in July 1999. It was the first interplanetary spacecraft to use an ion engine and tested a number of navigational, communications and science data technologies.

The following year, Engineers develop a new way to operate the Deep Space 1 spacecraft after the potentially mission-ending failure of its star tracker. Software is radioed to the probe using the camera on board to serve as a replacement navigational tool. The operation marks one of the most successful robotic space rescues in the history of space exploration.

The probe came within 26 km of Braille, but the images and spectra were taken from an approximate distance of 14 000 km, due to problems with the tracking system.

9969 Braille


951 Gaspra is an stony asteroid that orbits very close to the inner edge of the asteroid belt, It was the first asteroid ever to be closely approached when it was visited by the Galileo spacecraft, which flew by on its way to Jupiter October 1991, where it made the first, and so far only, direct observation of a comet colliding with a planet’s atmosphere (Shoemaker-Levy 9).

Gaspra is approximately 18 km x 10 km. Grooves about 100–300 m wide, up to 2.5 km long, and tens of meters deep are seen on Gaspra’s surface, which may be related to Gaspra’s formation in an asteroid collision that shattered the underlying rock.

It was suggested in 2007 that the fresh, steep craters on Gaspra were formed by the Baptistina family-forming event that happened near it.  The Baptistina family is an asteroid family of more than 2500 members that was probably produced by the breakup of an asteroid 200 km across 80 million years ago following an impact with a smaller body.

951 Gaspra


2867 Šteins, is an irregular, diamond-shaped asteroid from the inner regions of the asteroid belt, approximately 5 kilometers (3.1 miles) in diameter. It was visited in 2008 by the Rosetta probe.

2867 Šteins


25143 Itokawa is a stony sub-kilometer contact binary asteroid, classified as near-Earth object of the Apollo group and potentially hazardous asteroid, that measures approximately 350 meters in diameter.

It was the first asteroid to be the target of a sample return mission, the Japanese space probe Hayabusa, and currently is the smallest asteroid photographed and visited by a spacecraft. The sample capsule was returned to Earth in 2010. Its small asteroid lander MINERVA failed at deployment.

25143 Itokawa


In February 2018, a second spacecraft sent by the Japanese space agency JAXA, Hayabusa2  neared Ryubu (C-type) asteroid.  This mission is more of a robotic team.  It has 4 surface robots to deploy, as well as 3 sample collections planned.  So far this mission has been very robust and gives us a glimspe of the dynamic swarm that could be used on future missions to asteroid.

162173 Ryugu



This object, provisional designation 1942 EM, is a stony Florian asteroid and suspected contact binary from the inner asteroid belt, 6.6 × 5.0 × 3.4 km. In November 2002, the Stardust space probe flew past Annefrank at a distance of 3079 a target to practice the flyby technique that the space craft would later use on the comet Wild 2.

5535 Annefrank


NASA’s OSIRIS-REx spacecraft arrived at the 500 metre asteroid Bennu.  Researchers hope to bounce its surface and return a sample to Earth for detailed analysis in 2023.


The team found that the asteroid has an accelerating spin, enough to have periods where it jettisons material off its surface.


Ultima Thule

On January 1, 2019 NASA spaceship New Horizons blasted by the Kuiper Belt body Ultima Thule, (2014 MU69), a 33 kilometre bi-lobed object. The first news of the fly-by shows that it is a contact-binary object.

This is not the first time explorers have found two bodies that have come together slowly enough to adhere, rather than break each other. An interesting feature in this mission was that the object was discovered after takeoff of New Horizon, and the redirected to it.


Future Missions

The flyby and direct missions continue to promise exciting science, as it begins a more in-depth study of the asteroids and planetary fragments that populate the outer edges of our Solar System, on the far side of the gas giants, Jupiter, Saturn, Neptune and Uranus.  The Kuiper Belt, and the Oort Cloud out beyond it likely hold many millions of objects and surprises.

In 2021, the feat of navigation that is the Lucy mission will launch. To steer Lucy towards its targets doesn’t simply involve programming a map into a spacecraft and giving it gas money – it will fly by six asteroid targets, each in different orbits, over the course of 12 years. In 2022, another mission takeoff will be the Psyche probe to that 200  large metal, core-remnant asteroid. “It has been proposed that the rocket may be shared with a separate mission named Athena, that would perform a single flyby of asteroid 2 Pallas, the third largest asteroid in the Solar System.”

We are learning more about asteroid bodies with these close-encounter missions, in addition to advanced astronomical systems. These science missions will help expand our knowledge of the origins of our star system, as well as the composition and variety of multitudes of solar objects. Projects such as NASA’s Center for Near-Earth Object Studies (CNEOS) and mission plans such as Planetary Resources’ Arkyd programme extend our familiarity with asteroids.

Asteroids and Humans

The expanding science of asteroids comes at a time when humans are actively working to visit and set themselves up off Earth.  The science to accomplish this is well underway on places like the International Space Station, and other missions such as the Chinese lander mission to the far side of the moon.  In the news are many teams working towards going to Mars, and ‘settle’ there.

Away from the Earth and Moon system, there are few planets to host humans. Asteroids fill in the detail of our system as our enhanced abilities reveal the details.  Recent refinement of space exploration capabilities have allowed scientists to study many small and dim targets including exo-planets in other star systems. They continually map the millions of unidentified asteroids in our solar system. Asteroids are a vast complex field in space sciences. Advanced study of small bodies in the solar system will guide our understanding of their mysteries, their dangers and possibilities.

Direct exploration of most of our eight large planet systems will likely remain the domain of robotic missions.  In addition to the rigors of space, these large system bodies and their moons host a number of extremely exotic environments and varying gravity wells.  The complexity of preparing for and achieving escape velocity from distant bodies is hardly only possible on the smaller moons. Otherwise it’s better to do the science in situ and transmit results.

Asteroids, by contrast, are almost free of atmosphere and gravity, and offer extractable resource opportunity.  Increasingly able-bodied space craft have started to bring back pieces of asteroid for closer examination. In addition to core elements, many are also distinct, carrying minerals never seen. Their secrets will inform humanity’s future away from Earth. Asteroids will certainly play a role as humans and robots settle in space.


Aroh Wendelin