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2006 RH120 ( = 6R10DB9) (A second moon for the Earth?)

Discovered by the Catalina Sky Survey on 14 September 2006 as a magnitude +19 object moving at 9"/min it was added to the NEO Confirmation Page (NEOCP) later that same day, so that confirming observations could be made, just like any other relatively fast moving NEO candidate picked up by the NEO surveys.

2006 RH120 ( = 6R10DB9) on 20 March 2007
6R10DB9  2007 Mar 20 Mag +19.3, moving at 24"/min
At discovery it was just over twice as far away as the Moon (2.2 Lunar Distances, or LD), receding from Earth and fading. As further observations were made the calculations showed that it had approached the Earth in an orbit very similar to the Earth (a=1.04 AU, i=1°, e=0.034 and P=1.06 years) but would approach close enough to become caught in the Earths gravity. With an absolute magnitude of about +30, its likely size would be at most only 3 - 6 meters and it was assumed that it was likely to be a piece of man-made space junk returning to the vicinity of the Earth. The day after discovery it was removed from the NEOCP with the comment 'was not a minor planet', normally used when artificial satellites have been picked up inadvertently during NEO searches. However, it was obvious that it was in a very unusual orbit and was followed for two weeks until 28 September 2006, by which time it had faded to 20th magnitude.

Perigee had occurred a few days before discovery, at 2.18 LD, the next perigee was due on 4th Jan 2007 at only 1.4 LD, with mid December 2006 being favourable to recover the object again.

The Spacewatch II 1.8-m and the Mt. Lemmon 1.5-m reflectors in Arizona observed it during the period 11 - 17 December 2006 when it was between 2 and 3 LD, shining at magnitudes between 19 and 20. These observations allowed the orbit to be improved enough to be reasonably certain that there had been close approaches to Earth as far back as October 1958. By perigee in early January 2007 it was fading fast as it approached conjunction with the Sun and was not observed.

By the next perigee on 25 March 2007 the orbit of 2006 RH120 had evolved so that it would approach closer to the Earth than it had since discovery and would skim 31,000 Km inside the Moons orbit, or 0.91 LD.

It was picked up initially from the Southern African Large Telescope (SALT) in Sutherland, South Africa using the 10.5-m reflector on 11 March 2007 at about 2.2 LD. Further observations from that site and from Mt. Lemmon were made in the following week. The observations from SALT indicated that the rotation period was only 2.75 minutes and with a large 1.2 mag. variation in brightness, suggesting the object is very elongated. On 20 March 2007 it was observed from Great Shefford (see image above) by which time it was just 1.2 LD away and still approaching. Attempts by Carl Hergenrother to obtain visible spectroscopic observations using the 6.5-m MMT on 28 March 2007 were unfortunately beaten by the weather and attempts to observe it from La Palma were also unsuccessful.

Using the positions obtained from 14 September 2006 until 20 March 2007 Bill Gray has determined that in all likelihood the object is natural and not a piece of manmade space junk. On his 6R10DB9 page he comments that calculations to determine the area to mass ratio for solar radiation pressure calculations lead to an uncertainty of only about 5% in its value and adds "the area/mass ratio is way too low to be a rocket booster".

The next perigee was early on 14 June 2007 and that pass saw the orbit further perturbed, with the minimum distance from Earth reduced to just 276,845 Km or about 0.7 LD. Approaching Earth from the day lit side of the sky it moved swiftly out of the solar glare immediately following perigee at high northern declinations, initially moving at 50-60"/minute southwards, having been brightening by about 1 magnitude per day in the days before perigee. When at its closest and for the following week it passed through dense areas of the Milky Way, shining at 18-19th magnitude and maintained that brightness until the end of the month. As it moved swiftly south it faded and by mid July 2007 was magnitude +21 and at -50° declination.

Update 16 June 2007:

Further positions were obtained from Great Shefford on 15 June 2007 when the object was passing through rich star fields in Draco, near the border with Cepheus. It was at a distance of 0.8 LD, moving at 47"/min and ranging in brightness between magnitude +19 and +20.

Using all available positions 14 Sep 2006 - 22 June 2007 to determine an orbit with FindOrb causes slight systematic residuals of up to 5" in the March and 7" in the June 2007 positions and probably indicates that the model used to determine Solar Radiation Pressure may be too simple to adequately describe its motion over a long period of time. With its likely elongated shape and also with it potentially showing different amounts of its surface to the Sun as it orbits around Sun and Earth the effects of Solar Radiation Pressure will not be constant and would lead to systematic trends in the residuals.

Considering this however, the residuals in the pseudo-MPEC here show a good fit to the observations over the 10 month span.

2006 RH120 (= 6R10DB9)

15 June 2007 22:45 - 23:04 UT
Animation composed of four stacks, each of 20 x 4 second exposures.
Motion 47"/min, field of view 7' x 16'
North up, binned 2x2, scale 2.1"/pixel
0.40m f/6 Schmidt-Cassegrain + CCD

After the June perigee 2006 RH120 is perturbed out of geocentric orbit, its orbit by 1 September 2007 being very similar to Earth's, with a=1.018 AU, i=0.5°, e=0.014 and P=1.03 years.

Previous close approaches in 1958, 1969, 1979 and 1992 all appear to have been unfavourable with the object passing between Sun and Earth, therefore at small solar elongations, large phase angles and with the object extremely faint. A necessarily uncertain prediction for the next close approach is around the year 2028.

After 17 months of being referred to by its temporary observer assigned designation 6R10DB9, it was officially designated as minor planet 2006 RH120 on 18 February 2008 in MPEC 2008-D12.

Below is a link to pseudo MPEC pages generated using Bill Gray's FindOrb software including a daily ephemeris covering the period from discovery to the body's exit from the Earth vicinity:

Ephemeris Sep 2006 - Aug 2007 (using observations Sep 2006 to Mar 2007)
Ephemeris June 2007 - July 2007 (using observations from Sep 2006 to 22 Jun 2007, J95-centric 'dark hours only' covering the final perigee 12 Jun 2007 to 04 Jul 2007)

The table below contains groups of geocentric orbital elements at approximately 10 day intervals covering each of the four perigees, showing the large changes to the orbit over the 10 month period. Outside of these dates the motion of the object is dominated by the Sun's gravitation. All calculations were performed with FindOrb using elements derived from positions 14 Sep 2006 to 22 June 2007.

Epoch Perigee Peri.° Node° Incl.° e a/km P/d



(Referred to Earth equator)

20060902 20060912.3 180.3 159.1 41.4 0.475 1589136 230.7
20060912 20060911.2 175.7 159.5 41.3 0.567 1936139 310.3
20060919 20060909.7 167.2 159.7 41.8 0.559 1857264 291.5

20061218 20070101.7 125.2 105.4 90.9 0.519 1132535 138.8
20061221 20070102.2 126.7 105.9 91.6 0.486 1070442 127.6
20061231 20070103.7 137.4 105.4 91.7 0.410 905792 99.3
20070110 20070103.8 138.4 105.2 91.2 0.512 1087484 130.6
20070119 20070103.4 132.0 104.2 89.9 0.517 1040366 122.2

20070311 20070325.6 94.5 102.5 87.9 0.637 938816 104.8
20070321 20070325.4 90.8 102.8 85.7 0.646 1008880 116.7
20070331 20070325.6 94.3 102.9 85.3 0.664 1060707 125.8
20070409 20070325.1 93.0 102.6 84.3 0.624 970339 110.1

20070531 20070614.4 78.1 124.9 89.2 0.702 867738 93.1
20070609 20070614.2 73.1 124.2 89.6 0.712 959197 108.2
20070619 20070614.3 74.8 124.3 89.7 0.720 997522 114.8
20070628 20070614.6 77.3 124.5 90.5 0.748 1119652 136.4

Details of the four perigees are summarised here, showing the decrease in perigee distance with each pass of the Earth, given in kilometres and in Lunar Distances (384400 Km). 2006 RH120 leaves the Earth-Moon system following the closest approach in June 2007.

Perigee & Epoch q/km q/LD
20060911.32 839340 2.18
20070103.75 533460 1.39
20070325.52 353475 0.92
20070614.23 276845 0.72

Other pages on 2006 RH120 ( = 6R10DB9):

Bill Gray's pseudo MPEC page including analysis and orbit details.

Dr. Lance Benner (JPL): RADAR Planning page including a description of the object and the detailed planning for Goldstone RADAR observations scheduled for June 2007.

Sky & Telescope news: Earth's "Other Moon" by Roger W. Sinnott from 17 Apr 2007.

[Information on this page has been assembled in part from details published in Bill Gray's 6R10DB9 pseudo MPEC page, the MPC's Distant Artificial Satellite Circulars (as at 16 June 2007 6R10DB9 appearing in #'s 68-73, 75-77, 92-93, 95, 97 & 106-109) and Dr. Lance Benner's RADAR planning page here.]

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