The small bodies of the Solar System – asteroids and comets – have often received short shrift in the past, but as their importance to our understanding of how the Solar System formed and evolved has increased, more and more missions are being sent to explore these diminutive objects.

Being able to better explain how the planets, and particularly Earth, formed and evolved is not only of interest to planetary scientists, but also astrobiologists seeking to understand how planets develop the necessary conditions for life.

In 2016 the European Space Agency’s Rosetta mission completed its two-year exploration of comet 67P/Churyumov–Gerasimenko, while NASA’s Dawn spacecraft has visited the asteroid Vesta and is now in orbit around the dwarf planet Ceres. In August 2018, NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) spacecraft will rendezvous with the near-Earth asteroid 101955 Bennu on a mission to retrieve a sample of the asteroid and return it to Earth. Further afield, the Psyche mission will visit the asteroid 16 Psyche in 2026, while the Lucy mission will venture into new territory by exploring Jupiter’s trojan asteroids a year later.

“Some of these small bodies tell us about the formation of the planets during the early part of the Solar System,” says NASA’s new Chief Scientist, Jim Green, who spoke about current and forthcoming missions at a NASA Planetary Sciences Division seminar earlier this year. “These missions are all about learning about the formation of the Solar System and how the planets were built up from these small bodies.”

Rocky planets are understood to form through a process known as accretion, where dust in a disc around a young star agglomerates together to form increasingly larger objects, which collide and merge, eventually growing into protoplanets and then fully-fledged planets. Asteroids and comets are the leftovers of this process, and therefore they retain valuable information about this period of planet building. They also played a role after the planets had formed, crashing into them and depositing their caches of volatiles and organic molecules.

“Small bodies were impacting the Earth and providing a variety of important compounds, perhaps even amino acids, certainly water,” suggests Green.

The origin of Earth’s water

Whether it was asteroids or comets that delivered Earth’s water remains to be discovered. Water is usually made from two atoms of hydrogen and one of oxygen, but sometimes water molecules can contain heavier isotopes of hydrogen called deuterium, creating what we call ‘heavy water’.

A distinct fraction of Earth’s water consists of heavy water (defined by the deuterium to hydrogen ratio; in Earth’s water the ratio is 0.000156:1) and scientists have attempted to figure out which asteroids and comets share this ratio. The results have been mixed. Some asteroids do, others do not, whereas most comets studied so far do not, with one exception in the shape of comet 103P/Hartley 2. Among the comets found to have a different ratio is 67P/Churyumov–Gerasimenko, which Rosetta discovered has a deuterium to hydrogen ratio three times greater than Earth’s water.

Comets contain more than just water, however. They carry organic molecules that could have been the building blocks for life on Earth. NASA is currently considering a new mission called CAESAR for its New Frontiers program. CAESAR’s name is an acronym, standing for Comet Astrobiology Exploration SAmple Return. It is currently at the proposal stage, with a decision on whether it will fly or not expected in spring 2019. If it is given the go-ahead it will purposefully follow in Rosetta’s footsteps by visiting 67P/Churyumov–Gerasimenko and retrieving a sample for scientists on Earth to study in the laboratory.

“CAESAR is following up on the fact that we know an enormous amount about the comet [thanks to Rosetta]” says Green. “The plan is to bring back dust, and as Philae [the little lander deployed by Rosetta] has shown us, there is plenty of dust on the comet.”

Analysis of this dust could inform researchers about what raw materials comets could have delivered to our planet, not only when Earth was forming but also in the 500 million years or so afterwards, when the majority of impacts took place.

Of course, NASA already has a sample-return mission on the menu, in the form of OSIRIS-REx. Its aim is to collect at least 100 grams, and hopefully up to one kilogram, of material from the surface of the asteroid Bennu.

“Bennu is a carbonaceous chondrite, as dark as charcoal, and is of the type of bodies that we believe impacted Earth when our planet started cooling down and which brought a variety of material to begin the process of replenishing the organics that perhaps Earth lost in the formation process,” says Green. “This will give us some idea of what type of material was out there at the very beginning of the Solar System, potential seeds that may come to planets and grow into environments for life to exist.”

Exploring protoplanets

Meanwhile, a third possibility regarding Earth’s water has begun growing in popularity, which is the idea that not all of the water was lost in our planet’s formation process, allowing the water in Earth’s oceans to have originated from the interior of the planet, from where it was vented out onto the surface and into the atmosphere. To better understand if this hypothesis is viable, scientists would like to study some protoplanets to see how much water they contain. Fortunately, some of them are still around. NASA’s Dawn mission has visited two of them, namely Ceres and Vesta.

“What’s really exciting about Vesta, which is the second largest object in the Asteroid Belt, is that Dawn showed that it went through a process that we call differentiation,” says Green.

Differentiation occurs in larger bodies that are warm enough inside for denser rocks to sink to the core, while lighter rocks find themselves filling the outer layers, creating the familiar crust–mantle–core structure common to all rocky planets. Alas, this means that some of the most interesting components of Vesta are locked away, hidden deep within its iron-nickel core.

“However, there’s an exposed core out there in the Asteroid Belt, called Psyche,” says Green. “Psyche is a metal world for which most of the exterior material has been blasted away through impacts, [exposing the denser core].”

To take advantage of this, NASA is launching the Psyche mission in 2022 to study this exposed core, learn more about how the process of differentiation creates planetary cores and discover what volatiles, if any, such cores contain that could have found their way into the Earth when such protoplanets were colliding and merging in the final stages of building the planets.

Wandering worlds

Another quandary from the era of planet formation is migration. Scientists strongly suspect that large swathes of the Solar System – particularly the outer parts beyond Mars – saw a lot of movement during the Solar System’s formative years. This migration probably involved Jupiter moving inwards and Saturn, Uranus and Neptune moving out, in the process jostling the orbits of the small bodies. Some of these small bodies were sent inwards, where they may have been captured by Jupiter’s gravity and are now located at Jupiter’s Lagrange points.

Lagrange points are regions of space around a planet where gravitational forces balance out, allowing small objects at those points to hold their position. Each planet has five Lagrangian points, and the two of relevance here, L4 and L5, are found 60 degrees ahead and 60 degrees behind Jupiter in its orbit around the Sun. At both points there are heavily populated clusters of small bodies called the trojans. No spacecraft has ever explored Jupiter’s trojans before, so a new NASA mission called Lucy will be the first to do so.

“The Lucy mission is all about going to these small bodies that have been trapped in Lagrangian points and which can perhaps tell us a lot about how the Solar System rearranged itself,” says Green.

Lucy, which is launching in 2021, will fly to both the L4 and L5 points to study up to six small bodies that inhabit those positions. Lucy will reach the L4 point in August 2027 and explore four objects in the space of just over a year, before heading towards L5, where it will arrive in March 2033 to investigate a binary trojan (featuring two distinct and similarly-sized bodies) called Patroclus.

These six missions – Dawn, Rosetta, OSIRIS-REx, Lucy, Psyche and CAESAR – promise to transform what we know about the small denizens of the Solar System and how they fit into the story of the planets and how one of those planets – Earth – became habitable.

“This set of missions go after some of our top questions that we want to know the answers to,” concludes Green. “We’re making tremendous progress in understanding the role of small bodies in the origin and evolution of our Solar System.”