Last week, NASA announced the discovery of 2016 HO3, a little asteroid that dances around Earth at a distance of about 9 million miles. While the space rock technically orbits the Sun and not our planet, Earth’s gravity does influence it, keeping it close by.
It turns out, 2016 HO3 isn’t Earth’s only celestial buddy. Over the last two decades, astronomers have cataloged a constellation of interesting space rocks near Earth including quasi-satellites, Trojans and mini-moons. Here are a few of the more interesting finds:
As far as we know, the moon is the only significant natural satellite in orbit around the Earth—though over the last two centuries there have been several claims otherwise. One such mini-moon is the three-mile wide asteroid 3753 Cruithne. Discovered in 1983, Cruithne is a quasi-satellite similar to 2016 HO3, orbiting the sun on its own. Its wobbly horseshoe-shaped orbit, which took over a decade to map out, makes it appear as if Cruithne is making a “messy ring around Earth’s orbit,” Duncan Forgan, research fellow at St. Andrews University, writes for The Conversation.
Because it has a similar orbital period as Earth—it takes one year to make it around the sun—Cruithne often appears as if it is sticking close to the planet. Researchers have found a few other of these "resonant asteroids," which give the impression that they are orbiting Earth.
According to Deborah Byrd at EarthSky, computer models show Cruithne will likely spend about 5,000 years in its wonky orbit. Then it may move into true orbit around the Earth for about 3,000 years, becoming a true second moon before eventually moving on.
Forgan says it might be worth visiting Cruithne in the near future. “Cruithne could be a practice site for landing humans on asteroids,” he writes, “and perhaps even mining them for the rare-earth metals our new technologies desperately crave.”
2010 TK7 is a so-called Trojan asteroid, which tags along in front or behind a planet along the same orbit. In 1906, astronomers identified the first Trojan in the solar system, an asteroid dubbed Achilles following the same orbit around the sun as Jupiter. Since then, researchers have found thousands of Trojans, most around Jupiter, but also preceding or tailing Mars, Neptune, Venus and Uranus.
The physics is a bit complex, but when a planet orbits the sun, interactions in gravity create five points of stability called Lagrangian points, explains Phil Plait for Slate. The two most stable points lie 60 degrees ahead of and behind an orbiting body. Any asteroid, minor planet or moon at that point enters the same orbital path around the sun as the nearby planet. The object can also orbit the Lagrangian point while also orbiting the sun.
Plait explains: "[T]hink of it like a small dip in a flat table. Put a marble there and it'll stay put, even if you push it a little."
It wasn’t until 2011 that NASA's Wide-field Infrared Survey Explorer identified Earth’s first Trojan, 2010 TK7—a 1,000-foot-wide asteroid orbiting about 50 million miles ahead of the planet. It’s not really a moon, but acts more like a little brother showing us the way around the sun. Scientists expect that we may have more Trojans somewhere in our path.
In late 2006, astronomers at the Catalina Sky Survey in Arizona discovered a mysterious white object orbiting Earth. At first they assumed it was a spent rocket booster or other space debris, but further investigation showed it was a small asteroid. Dubbed 2006 RH120, it was the first recorded Temporarily Captured Object, or “mini moon” orbiting Earth. By the middle of 2007 it was gone.
Pulled in by the gravity of the Earth and the Moon, researchers now believe that temporary mini moons are common, though we rarely see them, and researchers suggest that finding and sampling or visiting these asteroids might be simpler and more cost effective than sending astronauts to deep space asteroids.
“We think they can probably achieve a lot of NASA’s goals for the human spaceflight program in terms of visiting other worlds,” Bill Bottke of the Southwest Research Institute in Boulder tells Leonard David at Space.com. “But we think we can do it at lower cost, possibly within the current NASA budget, and for a lot less mission risk.”