In 2006, astronomers with the NASA-backed Catalina Sky Survey in Arizona discovered a peculiar body floating amid the sea of thousands of human-made satellites orbiting our planet. After taking a closer look, they determined that the object wasn't just another piece of space junk. Rather, it was a natural satellite that had been temporarily yanked into a tagalong orbit with the Earth, similar to the moon.
This "minimoon," designated 2006 RH120, was just a few meters in diameter. But unlike the actual moon, this cosmic body was a transient Earth companion, traveling around the planet for only a year before being ejected from our planet's orbit. More than a decade later, scientists with the Catalina Sky Survey spotted another minimoon (2020 CD3) this one about the size of a small car roaming through Earth's orbit, before it was flung out of the Earth-moon system's influence in March 2020.
Because of their proximity to Earth, these minimoons have warranted close scientific scrutiny. But more recently, some experts have eyed minimoons and other near-Earth asteroids for a different reason: They have the potential to act as stepping stones in our exploration of the cosmos.
"We have yet to become an interplanetary species," Richard Binzel, a professor of planetary sciences at the Massachusetts Institute of Technology, told Live Science. Minimoons could become milestones "to achieve as you're learning how humans can operate in interplanetary space, and ultimately reach Mars."
In September 2016, NASA launched the uncrewed OSIRIS-REx spacecraft on a mission to collect a sample from the potentially hazardous asteroid Bennu, which has a 1-in-2,700 chance of slamming into Earth in 2182. Seven years later, OSIRIS REx returned to Earth with a tiny chunk of the 4.5 billion-year-old asteroid.
The success of the OSIRIS-ReX mission has inspired scientists planning the next phases of near-Earth exploration. One idea is to use close asteroids as stepping stones for missions to Mars, Binzel said.
Retrieving Bennu was a step in the right direction, he said, but there might be a better target when it comes to testing our technology to expand further into the cosmos. At its closest, Bennu is around 186,000 miles (300,000 km) away from Earth and only crosses the planet's orbit around the sun every few years. As a result, the mission took seven years and cost an estimated $1.16 billion.
Minimoons, on the other hand, are some of the easiest asteroids to reach from Earth, Binzel said.
"To go anywhere in space, you have to change your velocity," Binzel said. Minimoons are small bodies with very little gravity, and have a low required change in velocity, or delta-V, which means that it doesn't take much propulsion to transport a spacecraft from low Earth orbit to a rendezvous with the asteroid.
Given these properties, minimoon missions would require less fuel than journeys to many other cosmic bodies. "It only takes a puff of fuel to leave the Mini-Moon and head back towards Earth," Binzel told Live Science in an email.
Journeys to minimoons would take about 100 days to get there and back, research suggests. (Our permanent moon is about a three-days' journey away, but it took NASA's Saturn V rocket 203,400 gallons (770,000 liters) of kerosene fuel and 318,000 gallons (1.2 million liters) of liquid oxygen just to get off the ground.
While minimoon missions are promising, the flaw with this strategy goes back to their ephemeral nature, which could make it difficult to plan and execute a mission before the natural satellite is ejected from its short-term journey around Earth.
"They are in tagalong orbits with the Earth, so they're like a pet," Binzel said. "Temporary pets that you keep for a while and then they wander off."
By conducting missions to minimoons and other near-Earth asteroids in their vicinity, NASA and other space agencies can test their technologies' effectiveness in deep space, including life support systems, engines and propulsion systems, Paul Abell, chief scientist for small body exploration at NASA, told Live Science.
"Going to Mars is a big, big step," he said. "There's a lot of things that have to happen, so why don't we look at some of these near-Earth asteroids that are in between the Earth-moon system and Mars."
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These minimoon journeys could also help scientists who are dedicated to a different pursuit that could be just as crucial for getting to Mars: mining for water.
Water is vital not only for hydration but also for the creation of additional rocket fuel, which is typically liquid hydrogen. This will be required to get as far as Mars, which is, on average, more than 140 million miles (225 million km) from Earth.
Currently, spacecraft have to carry all of the water and fuel they will need from Earth. The massive weight added by the liquid drives the "tyranny of the rocket equation," which states that as payload mass increases, so must the amount of propellant required to break free from Earth's gravitational pull.
Essentially, if NASA increases the payload mass of a spacecraft even slightly, they have to add much more fuel to get it off the ground and into orbit and the fuel itself adds even more weight to lift, creating a vicious cycle. The key to breaking this cycle is finding a way to refuel in space, Abell said.
"When you go on vacation, when you fly or drive anywhere, you're not taking all your oxygen, all your food, everything with you for the entire round trip," Abell said. "Well, it's the same type of thing. We want to get away from having to take everything with us from Earth, all the way out and then come back, because that's super expensive."
The good news? Near-Earth asteroids may be ideal candidates for space gas stations. A growing body of research shows that many near-Earth asteroids are rich in minerals and water that's locked inside the rock. If this water can be accessed, it could be split into hydrogen and oxygen, both key elements for creating rocket fuel.
"If you can access that water and leverage it, all of a sudden you have water to drink, you have oxygen to breathe and, more importantly, you have rocket fuel," Abell said.
Currently, most of NASA's efforts are focused on harvesting water from the moon, but many commercial companies including Karman+, TransAstra and AstroForge have their sights set on asteroids for water and metal mining.
These operations haven't gotten off the ground yet, largely due to the cost and technology required to get to these floating rocks, experts say. But minimoon missions could help streamline operations by providing companies a training ground to test "feasibility of asteroid mining technologies for future commercial applications," according to a 2018 study.
However, minimoons themselves may not be the best option for fueling up spacecrafts because they are small, with surfaces dried out from "sitting in the sun, cooking for a long time," said Binzel.
Robert Jedicke, an astronomer at the University of Hawaii and lead author of the 2018 study, suspects that many minimoons aren't water-bearing, because they may have either broken off from the moon or were pulled in from the edge of main asteroid belt, both of which suggest a low potential for water.However, other scientists, including Abell, think it could be possible. Binzel, for his part, is more optimistic, saying there's lots of uncertainty in the modeling. "You don't know til you look!" he suggests.
Even if minimoons lack water, they could enable companies "to test their ability to maneuver spacecraft near an asteroid," said Jedicke, who is currently working with TransAstra to develop techniques for mining water from asteroids.
Related: 'Stepping stone to Mars': Minimoons may help us become an interplanetary species, says MIT astrophysicist Richard Binzel
Minimoons' small size and rapid motion make them incredibly difficult to detect with existing ground-based telescopes. However, a new telescope could soon change that. High in the Chilean Andes, construction is almost complete on the Vera C. Rubin Observatory, which will hold the world's largest digital camera.
Starting in 2025, the camera, known as the Legacy Survey of Space and Time, will snap 700 pictures each night for 10 years to catalog the solar system at a high-precision 6-terapixel level. This thorough exploration of the universe will help scientists understand mysterious substances such as dark matter and dark energy. And with a tailored approach, it could also help astronomers detect a minimoon as often as every three months, according to a 2020 simulation.
And in 2027, another NASA instrument, dubbed the NEO Surveyor, will detect asteroids from space. The surveyor will complete a full scan of the sky every two weeks to characterize potentially hazardous asteroids and comets near Earth's orbit. While the primary focus of this infrared space telescope is to keep humanity safe from "planet-killer asteroids," it has the potential to uncover tiny minimoons in the process.
It's too soon to tell whether minimoon missions will play a key role in spacecraft technology or mining operations, experts told Live Science. But no matter what, studying these temporary Earth companions and other near-Earth asteroids could provide crucial clues to the mysteries of our solar system, Binzel said. Many scientists think near-Earth asteroids, like Bennu, probably brought the seeds of life to Earth early in our planet's history.
For tracing the chemical origins of the solar system and finding the ingredients that made life on Earth, minimoons are a great place to go, he said.
"But the reason we haven't gone to them before is there aren't very many of them," Binzel said. "We're just now discovering them. But they will come to the forefront because we have new telescopes coming online."
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