Water on Itokawa and Processing It

The JAXA Hayabusa sample-return mission brought back material from the S-type asteroid Itokawa. The samples were collected from the smooth patch in the middle of the two lobes. Scientists from ASU found direct evidence of high concentrations of water within the samples. Credit: JAXA.

Water has been directly observed from two samples returned from the S-type asteroid Itokawa. It was hypothesized that S-type asteroids contained low concentrations of water bearing minerals due to light curve data and studying similar minerals on Earth. However, the two Itokawa samples contained high concentrations of water at 698 and 988 parts per million weight, respectively. This holds great promise for asteroid processing targets if similar S-type asteroids have similar water concentrations.

A team of scientists from the School of Earth and Space Exploration at Arizona State University were the first to directly study material returned from Itokawa for water. Samples from Itokawa are the first and only samples collected and returned from an asteroid so far. The samples returned are mostly olivine and pyroxene, which can contain water bearing (hydrous) material within the crystal structure of the minerals.

The two samples from Itokawa (left most two diamonds) have similar water concentrations to those from terrestrial sources. Finding high concentrations of water in S-type asteroids would provide many more targets for asteroid processing missions. Credit: Jin et al., 2018, Figure 5.

25143 Itokawa is an Apollo group asteroid that orbits the Sun between 0.95 and 1.7 AU every 18 months. In addition to being classified as a Near Earth Object (NEO), Itokawa is also a Potentially Hazardous Asteroid (PHA) because its orbit crosses Earth’s orbit. The peanut shaped Itokawa measures about 535 x 294 x 209 meters with a mass around 35 billion kilograms (about 48 Empire State Buildings or 23 million Honda Accords).

Itokawa is an S-type asteroid that is composed mostly of silicates and other stony objects. S-type asteroids are the second most common asteroid type in our Solar System after carbonaceous C-type asteroids. They are dominant within the main asteroid belt until about 3 AU.

Silicate minerals can store water through higher temperatures compared to water ice bearing C-type asteroids. This means that between a comparable S-type and a C-type NEO, the S-type asteroid would retain its water much longer than the C-type asteroid due to the water ice on the C-type asteroid sublimating away at lower temperatures.

The Honey Bee system in its fully expanded form, shown after encapsulating an asteroid in its containment bag (yellow bag at bottom). Credit: TransAstra Corporation.

The key challenge with processing hydrated material from asteroids is the heat required to bake the water out of the material. Water ice melts above 0 C, but extracting water from silicates requires temperatures over 600 C. Some proposed processing systems may be able to accommodate this challenge, including the optical mining system developed by TransAstra Corporation. This system captures a small asteroid within a bag, and then bombards the surface of the asteroid with concentrated solar energy funneled from large solar reflectors. Temperatures within this system can achieve those required to process a S-type asteroid.

The discovery of high concentrations of hydrous minerals within an S-type asteroid hold promise for expanding available targets for asteroid processing systems. The key factor is determining if Itokawa is unique in having high concentrations of water, or if other S-type asteroids also contain high levels of water. Future sample return missions will be able to answer these questions.