A recently discovered asteroid plays leapfrog with Earth as they both orbit around the Sun.

The orbit of 2016 HO3
The strange orbit of Apollo asteroid 2016 HO3. (Click to see bigger image.)
NASA / JPL-Caltech

Our fair planet has a tiny companion, an asteroid that shares our annual journey around the Sun in a complex dance.

Astronomers recently announced the discovery of 2016 HO3, an asteroid between 40 and 100 meters in size that behaves as Earth's quasi-satellite. Discovered as a faint blip on April 27, 2016, by the Pan-STARRS 1 survey based on Haleakala, Hawai'i, 2016 HO3 occupies an orbit very much like Earth's. Calculations suggest that, though it evaded detection until this year, 2016 HO3 has hung out in Earth's vicinity for a century or so, and will continue to play orbital leapfrog with our world for centuries to come.

2016 HO3's Weird Orbit

“Since 2016 HO3 loops around our planet, but never ventures very far away as we go around the Sun, we refer to it as a quasi-satellite of Earth,” says Paul Chodas (NASA Jet Propulsion Laboratory). “In effect, this small asteroid is caught in a little dance with Earth.”

2016 HO3's orbit takes it alternately sunward and ahead of Earth for six months at a time, then it's slightly farther from the Sun and falls behind us. Its orbit is slightly tilted relative to the ecliptic plane, resulting in a corkscrew twist in the orbit over several decades. Too distant to be considered a true second moon, 2016 HO3's journey takes it as close as 38 times the Earth-Moon distance (9.1 million miles or 0.1 astronomical unit) from our planet, and as far as 100 times the Earth-Moon distance (24 million miles or 0.25 astronomical unit).

Chodas explains that this body doesn't qualify as a "minimoon" of Earth because our planet's gravity is simply too weak to truly capture it. "One easy way to distinguish a captured object from a quasi-satellite is the period of the loop about our planet," he explains. "Captured objects are true satellites of Earth and will loop around with periods as large as a few months, much less than 1 year. Quasi-satellites loop around Earth with periods of almost exactly 1 year."

As an Apollo asteroid, 2016 HO3 joins the small but growing list of objects tracked in a solar orbit near Earth. Asteroid 2003 YN107 was discovered by the LINEAR sky survey about a decade ago, when it followed a similar track, but the rock has since departed our neighborhood. Asteroid 2006 RH120 — the only true minimoon yet discovered — made several distant looping passes of Earth from April 2006 to September 2007 before ejection. It turns out that our Moon does a pretty good job at celestial goaltending, assuring such secondary hopefuls never hang around for long.

Simulations carried out by Mikael Granvik (University of Helsinki, Finland) in 2012 suggest that most "Temporarily Captured Objects" (TCOs) only complete three orbits of the Earth-Moon system before ejection, and only 1% ever impact Earth. (Read Sky & Telescope's September 2015 issue for more info.)

Claims for a tiny, second moon around Earth, dubbed "Lilith," go all the way back to alleged sightings in the 19th century. "Petit's Moon" created a temporary sensation in 1846, until it too proved to be spurious. In modern times, spent boosters from the China's Chang'e 2 and NASA's Apollo 12 lunar missions were temporarily identified as "asteroids" 2010 QW1 and J002E3, respectively. Other objects, such as 3753 Cruithne occupy strange horseshoe-shaped orbits around Earth. Venus also has its own quasi-satellite, designated 2002 VE68.

Hunting Temporary Minimoons

A new generation of all sky surveys could swell the ranks of known TCOs. The enormously successful PanSTARRS project, which currently operates with just one of four proposed telescopes, might one day get its full complement for such a dedicated search. A 2014 study suggests that the Subaru telescope could stand a 90% chance of nabbing a potential TCO after only 5 nights of dedicated scanning of the sky. Then there's the Large Synoptic Sky Survey, (LSST) set to see first light in 2022 — they're working on the primary mirrors now (watch the video below).

The discovery of 2016 HO3 spawns far more questions than it answers: Where did it come from? Did the rock spall off the Moon during an impact, or is it merely an asteroid that wandered too close to Earth? Could 2016 HO3 make the candidate list of possible targets for a future crewed mission to an asteroid? Future discoveries will help put 2016 HO3 into context and help reveal its origin.




Image of John-Murrell


June 25, 2016 at 3:23 am

So we now have firm evidence that the Earth has not cleared it's orbit. As a result Earth is no longer a Planet according to the IAU defintion. Welcome to Dwarf Planet Earth !

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Peter Wilson

June 27, 2016 at 2:51 pm

There might be a little dust in the corner this week, but that doesn't mean I didn't sweep last week.

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Image of Bryan


June 29, 2016 at 12:00 pm

"Clearing the orbit" doesn't mean that there is nothing else there, but that the main body is gravitationally dominant over anything else that is there.

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June 28, 2016 at 1:00 pm

Since 2016 HO3 might be with Earth for a while, I wonder if one day it might be "captured " for detailed

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June 28, 2016 at 6:46 pm

What a great target for a mission, that would orbit this mini-moon like Dawn at Ceres. Could the funds be scrambled together? Surely there are rockets available, since the Defense Department is always lobbing satellites into orbit. Wouldn’t have to be a grand-slam type of exploration, unless a landing option was chosen. Just cameras and some spectrometers might be enough.

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Charles Peterson

June 29, 2016 at 3:24 pm

In 1974 inspired by a proposed sample-return mission to Mars by the MIT Center for Space Research, I designed for Phobos a spacecraft orbit which is essentially the same as that of recently-discovered asteroid 2016 HO3 about the Earth. Using JPL's HORIZON web-interface facility, I have determined that asteroid 2016 HO3 is indeed captured in a 1-to-1 resonance orbit about the Earth, and that its orbit has a libration period of about 39 years, a radial libration excursion of about 0.00125 AU, and a tangential libration excursion of about 0.070 AU. (The total combined tangential excursion from both libration and heliocentric eccentricity is about 0.29 AU leading/trailing the Earth.) Its tangential-to-radial excursion ratio is thus about 56. My 2-D analysis of this orbit derives simple formulas that predict a 34-year libration period and a tangential-to-radial ratio of 54. The dynamics and stability of a co-planar version of this orbit is analytically treated and simply explained in part 2 of my 1976 doctoral thesis. If I see a subsequent comment on this article requesting it, I will happily compose an additional comment that includes an internet URL link to that thesis (if Sky & Telescope permits).

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December 20, 2017 at 2:22 pm

Aloha Charles. I recently spoke with an astronomer Savid Tholen of IfA, Hawai'i, who desribed the 'orbit' of HO3 as passing cyclically thru the same section of sky annually, because of it geo-lunar synchronous co-orbit. Q: what is HO3's path through the sky, and what backdrop of stars does it appear to pass in front of each year? To put it another way, what are it's constellations? I understand since it passes inside and outside of our earth-moon system, as well as north and south of the ecliptic plane, that it will appear to cruise thru a great number of constellations outside of the traditional ecliptic. What is the apparent annual path of HO3 thru our sky?

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