Asteroid

2006 RH120

The first mini-moon ever found. A few meters across, 2006 RH120 was captured by Earth in mid-2006, looped around us for about a year and left in mid-2007.

At a glance

Next close approach
Aug 17, 2028 (UTC) · 11 lunar distances
Impact risk
On NASA's impact-risk list · 1 in 120 chance of impact, 2044–2124
Kind
Temporarily captured orbiter[1]Bound to Earth and inside about three Hill radii for about a year, completing roughly four revolutions
Size
A few meters across[1]Absolute magnitude 29.5
Captured
Mid-2006 to mid-2007[2]A series of distant loops around Earth
Where it is now
Orbiting the Sun, just outside Earth's orbit[3]Semi-major axis 1.03 au, period 384 days; next pass August 2028 at 0.027 au
Close approaches and the impact-risk list

From discovery to the flyby

  1. September 2006
    Discovered[1]
    Catalina Sky Survey, while already in orbit around Earth
  2. September 11, 2006
    Close pass[3]
    0.0056 au from Earth, about 840,000 km
  3. June 14, 2007
    Last close pass[3]
    0.0019 au from Earth, about 280,000 km, closer than the Moon
  4. February 18, 2008
    Designated 2006 RH120[5]
    Minor Planet Center circular; orbit solution by P. Chodas (JPL) allowing for sunlight pressure
  5. August 2028
    Next pass[3]
    0.027 au from Earth, about ten lunar distances

2006 RH120 is the first minimoon ever found: the first natural object other than the Moon confirmed to be in orbit around Earth.[1] It was captured in mid-2006, made a series of distant loops around our planet and departed in mid-2007.[2] The guide Minimoons explains what makes an object one.

A year in orbit

The Catalina Sky Survey found it in September 2006 as a faint moving point of light.[1] It is a few meters across and stayed bound to Earth for about a year, making roughly four revolutions.[1] That makes it a temporarily captured orbiter, the kind that completes at least one loop; a flyby is bound but gone before finishing one.[1] NASA's Center for Near-Earth Object Studies describes it as somewhat larger than the second minimoon, 2020 CD3, and like it a temporary mini-moon.[2]

JPL's catalogue records its passes during the stay. It came within 0.0056 au of Earth on 11 September 2006, about 840,000 km, then swung past the Moon and Earth repeatedly until its last close pass on 14 June 2007 at 0.0019 au, about 280,000 km, inside the Moon's distance.[3] Its own orbit around the Sun lies just outside Earth's, with a semi-major axis of 1.03 au and a period of 384 days, so it drifts slowly past us and comes back every few decades.[3]

How it got in, and out

A study by Brian Anderson of the University of Southern California and Martin Lo of JPL traced the dynamics. The asteroid was in a 27:29 resonance with Earth, approached along a pathway linked to the Lagrange point L1 between Earth and the Moon, and escaped about a year later along a pathway from the L2 point on the far side, into a 21:20 resonance with Earth.[4] They compare this to the way the Galileo and Cassini spacecraft used resonant passes to save fuel, and to the temporary captures of comets by Jupiter.[4]

The Minor Planet Center gave it the designation 2006 RH120 on 18 February 2008, after it had gone, with an orbit computed by Paul Chodas of JPL that accounts for the push of sunlight, a force that matters for an object this small.[5] It is listed with an absolute magnitude of 29.5.[5]

Where it is now

It is back in orbit around the Sun. JPL's next listed pass is in August 2028 at 0.027 au, about ten times the Moon's distance, far too distant and faint for anyone but a survey telescope.[3] Minimoons are too small to be a danger; the guide explains why.

Sources

  1. [1]Frontiers in Astronomy and Space Sciences, Earth's Minimoons: Opportunities for Science and Technology — doi.org, retrieved November 4, 2026
  2. [2]NASA/JPL Center for Near-Earth Object Studies, Tiny Object Discovered in Distant Orbit Around the Earth — cneos.jpl.nasa.gov, retrieved November 4, 2026
  3. [3]NASA/JPL Small-Body Database, 2006 RH120 — ssd.jpl.nasa.gov, retrieved November 4, 2026
  4. [4]NASA Technical Reports Server, Dynamics of Asteroid 2006 RH120: Pre-Capture and Escape Phases — ntrs.nasa.gov, retrieved November 4, 2026
  5. [5]IAU Minor Planet Center, MPEC 2008-D12: 2006 RH120 — minorplanetcenter.net, retrieved November 4, 2026

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