By Larry O’Hanlon

Scientists have come out with an estimate of how much water might be available in near-Earth asteroids.

A new study in the Journal of Geophysical Research: Planets, a publication of the American Geophysical Union, suggests there are between 26 and 80 hydrated near-Earth asteroids larger than a kilometer in diameter. Of those, 8 to 26 of the asteroids are easier to get to than the surface of the Moon. The new study also estimates there are between 350 and 1,050 smaller hydrated objects easier to reach than the Moon.

The study’s authors estimate there are between 400 and 1200 billion kilograms (440 to 1.3 billion U.S. tons) of water that could be extracted from the minerals in these asteroids. In liquid terms, that’s between 400 billion and 1,200 billion liters (100 billion and 400 billion U.S. gallons) of water. That’s enough to fill between 160,000 and 480,000 Olympic-sized swimming pools.

The near-Earth asteroid Bennu is 500 meters (1,600 feet) wide and contains hydrated minerals, according to scientists working on the OSIRIS-REx spacecraft. It could one day be mined for water by future explorers. Credit: NASA/Goddard/University of Arizona

Asteroids are rocks in orbit the Sun, generally between Mars and Jupiter, while comets are made up of rock and ice and originate in the outer solar system. Both asteroids and comets are made up of material from the early formation of the solar system. Approximately 19,000 near-Earth asteroids of various sizes have been discovered.

Water in asteroids can provide hints about the nature of the early solar system, including clues about where Earth’s water and the Moon’s polar ice came from. It could also supply water and fuel to future interplanetary space missions, according to the authors of the new study.

“We know that there are minerals with water in them on asteroids. We know that from meteorites that have fallen to the ground.” said Andrew Rivkin of Johns Hopkins University Applied Physics Research Laboratory in Laurel, Maryland. Rivkin is the lead author on the new paper with F. E. DeMeo, of MIT. “It’s also possible that Earth’s water came largely from impacts.”

Meteors are bits of asteroids and comets that enter and burn up in the Earth’s atmosphere and cause meteor showers. If these bits are large enough to make it to the ground, those surviving chunks of space rocks are called meteorites.

For water, follow the iron

Piecing together estimates of how many near-Earth asteroids are rich in hydrated minerals – minerals containing water or hydroxide — is tricky, Rivkin said.

Using telescopes to detect the spectral signals of hydrated minerals in the light from asteroids should be straightforward. Water and hydroxides absorb very specific wavelengths of infrared light, which leaves dips in a hydrated asteroid’s spectrum – what are called absorption bands. Unfortunately, in this case, those absorption bands are the same parts of the spectrum that are either filtered out by most ground-based telescopes or are flooded with signals from water in Earth’s atmosphere.

“Ideally we’d do it in space, but don’t have that capability right now,” Rivkin said. “So instead, we look for absorption due to iron in hydrated minerals.”

Previous research found asteroids that contained the signal of a particular kind of oxidized iron also contained hydrated minerals, so that oxidized iron can serve as a proxy for these hydrated minerals. “Where ever we see the water band, we also see this specific oxidized iron band,” Rivkin said.

The study’s authors combined these iron signals with information about known trajectories of small bodies near Earth to come up with their estimate of the amount of water in near-Earth asteroids.

To get a better estimate would probably require a space telescope, like the James Webb Space Telescope, which is scheduled to launch in 2021, said Rivkin.

Water is expected to be a hot commodity in space, as it is essential for human survival and can be used to propel spacecraft to other parts of the solar system, or to make propellant to refuel Earth-orbiting satellites.

“It’s been argued that it makes sense to mine water,” Rivkin said.

— Larry O’Hanlon is a freelance science writer, editor and online producer. He manages the AGU Blogosphere.