We’re interested in the smallest bodies in the Solar System, but why?  

ISAS is focussed on exploring the smallest bodies in our Solar System.

“It’s like crime scene,” explains Hal Levison from the NASA Lucy mission to the Trojan asteroids that escort Jupiter around the Sun. “Sometimes the blood splatters on the wall will tell you more about the murder than the bodies on the floor.”

A major focus for current space programs are the smallest bodies in our Solar System. At JAXA, our Martian Moons eXploration mission to Phobos and Deimos is aiming for a launch in 2024. Hayabusa2 is enroute to asteroid Ryugu and we are planning the Solar Power Sail (SPS) mission to Jupiter’s Trojan asteroids, while DESTINY+ will visit the asteroid Phaethon; the source of the annual Gemini meteorite shower.

Across the Pacific, NASA launched OSIRIS-REx to head towards asteroid Bennu last September. At the start of this year, the US space agency further announced two new small body missions for their Discovery Program; Lucy and a mission to a unique metal asteroid, Psyche.

And of course, the ESA’s historic mission to Comet 67P came to an end last September as the spacecraft went into a controlled impact with the comet’s surface.

Speaking on a panel at the Japan Geoscience Union Meeting in May for the six current missions, Levison’s vivid analogy sums up why there is such intense interest in our Solar System’s smallest bodies. These rocky leftovers from the planet formation process are evidence for how the Solar System formed. From understanding how planet orbits may have changed in the past, to how water and other elements were mixed through the Solar System and the delivery of ingredients for life on Earth, these rocky splatters are the key.

Lindy Elkins-Tanton from the Psyche mission agrees with Levison’s statement that going to the small bodies is the key to understanding how the planets formed.

“For years, people thought the Solar System was all about the big planets,” she explains. “But there are so many more small bodies than large bodies. Primitive bodies [unchanged by events such as geological processes or strong radiation] are like the eggs that went into cakes. You can’t go to the cakes (the big planets) and still find the eggs, you have to go to the eggs to take measurements.”

But are six independent missions really necessary? The importance of multiple data sets is that the comparison reveals far more information than the sum of the individual missions.

“We have over 100,000 meteorites in the world collection, but we don’t know where they come from!” said Scott Messenger from the OSIRIS-REx mission, explaining that any geologist would ask for the origin of a rock in order to understand its formation. “We don’t have any in-situ [collected from their original position] samples. One is not enough!”

Our first Hayabusa mission returned rock samples from the asteroid, Itokawa, and the NASA Apollo program brought home lunar rocks. However, neither of these samples are primitive, having experienced extensive alternation since their formation in the early Solar System. They therefore cannot tell us about conditions at the time our planet was forming. Since a single sample risks being an unusual or atypical example, having multiple missions reveals far more about the building materials around the young Sun.

Speaking for the DESTINY+ mission, Tomato Arai adds that the flyby of Phaethon will provide complementary information on how fresh primitive material is formed and travels to the planets.

While both heading to the Jovian Trojans, the different mission plans for Lucy and SPS will produce an essential combination for understanding this distance population. Lucy is set to do a series of fly-bys, exploring the group to understand how diverse and well mixed the different asteroid compositions are. SPS will focus on a single asteroid in much greater detail to map its evolutionary history.

For understanding the Trojans, SPS is “the nail in the coffin,” says Levison.

The combination of inner and outer Solar System destinations is also extremely valuable. Yoko Kebukawa from the SPS mission points out that we class small bodies as asteroids or comets, but are these really distinct groups or is there a continuous distribution of objects between the icy comets and rocky asteroids?

Messenger adds to this by asking how material such as water and other elements becomes mixed between the Sun and outer Solar System? To understand the distribution and each planet’s supply of minerals and organics, we need to sample a wide range of objects.

So have any lessons been learnt from Rosetta that will help these future encounters will the Solar System’s smallest of worlds?

For Messenger, it was the surprising structure of Comet 67P that is worth remembering. The duck-like shape of the comet suggested that it had once been two different rocks that had collided and stuck together. This collision must have been sufficiently gentle not to shatter the two halves, countering many models that propose such impacts should be violent. Not only can similar mobile shapes uncover how larger bodies and planets grow, but missions must be prepared for arriving at a location different from any they have seen before.