|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
+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
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
2006 RH120 (= 6R10DB9)
15 June 2007 22:45 - 23:04 UT
Animation composed of four stacks, each of 20 x 4 second
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
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.
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.
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
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