[meteorite-list] Initial Results from the Close Approach of Asteroid 2014 JO25

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 9 May 2017 16:24:02 -0700 (PDT)
Message-ID: <201705092324.v49NO2na004643_at_zagami.jpl.nasa.gov>

https://cneos.jpl.nasa.gov/news/news196.html

Initial Results from the Close Approach of Asteroid 2014 JO25
Center for NEO Studies (CNEOS)
May 5, 2017

A relatively large asteroid called 2014 JO25 approached within 4.6 lunar
distances (within 1.1 million miles or 1.8 million kilometers) of the
Earth on April 19, 2017. This was the closest approach by an asteroid
at least 600 meters in size since 4179 Toutatis, a 3 mile (5 kilometer)
sized asteroid, approached within four lunar distances in September 2004.
The close approach provided an outstanding opportunity to study the physical
properties of the asteroid, and the images obtained by ground-based radars
are comparable in resolution to those that could be obtained by a spacecraft
flyby.

2014 JO25 was discovered by Al Grauer of the Catalina Sky Survey (CSS)
near Tucson, Arizona in May 2014. The Catalina Sky Survey is a project
of NASA's Near-Earth Object [NEO] Observations Program in collaboration
with the University of Arizona.

Figure 1: Part of the Catalina Sky Survey, this 1.52-meter Cassegrain
telescope was used to discover 2014 JO25 in May 2014. The observatory
is located just north of Tucson, Arizona in the Santa Catalina Mountains.
Figure 1: Part of the Catalina Sky Survey, this 1.52-meter Cassegrain
telescope was used to discover 2014 JO25 in May 2014. The observatory
is located just north of Tucson, Arizona in the Santa Catalina Mountains.

Shortly after its discovery Jet Propulsion Laboratory (JPL) astronomer
Joe Masiero, a member of the NEOWISE science team, used observations made
by the NEOWISE spacecraft in 2014 to estimate 2014 JO25's size as roughly
650 meters (2000 feet), and its optical albedo as 0.25. Albedo is the
proportion of incident sunlight that a body reflects back into space.
For comparison, the Moon has an albedo of 0.12, meaning that it reflects
only 12% of the sunlight that reaches it. Based on initial estimates 2014
JO25's surface would be twice as reflective as the Moon's, fairly
bright for an asteroid.

Until the recent close pass, the asteroid's spectral class, rotation
period, and pole direction were unknown. This close approach provided
an opportunity for very detailed radar and optical observations, which
allowed astronomers to better determine the characteristics of this unique
object. But precision astrometry - measurements of the asteroid's
position in space relative to stars in the background sky - was needed
first to determine a more precise orbit, crucial for the radar observations.

So in September 2016 Joe Masiero made a special effort to obtain more
astrometric observations of 2014 JO25, which was distant at the time,
and therefore very faint. He had to use the very large Gemini South 8.2-meter
telescope on Cerro Pachon, Chile to make these measurements. These observations
significantly reduced the orbital uncertainties for the asteroid. Using
the more accurate orbit, Peter Veres of the Center for NEO Studies (CNEOS)
at JPL looked through archival Pan-STARRS images taken in 2011, before
the object was known to exist. These astrometric measurements were crucial
for reducing the pointing uncertainties for this close pass in April 2017
and enabled the successful radar observations.
Your browser does not support the video tag. You can the video instead.
Figure 2: This animation shows the orbit of 2014 JO25 about the Sun. The
orbit is inclined ~25 degrees with respect to the ecliptic; perihelion
at 0.24 AU and aphelion at 3.9 AU [for reference Jupiter orbits the Sun
at 5.2 AU]. The orbit of 2014 JO25 seems to resemble that of an Encke-like
comet. For a high resolution version, download the video for external
display. (NASA/JPL)

Radar observations were performed at the National Science Foundation's
Arecibo Observatory equipped with the NASA planetary radar system by a
team led by Patrick Taylor of Arecibo Observatory between April 15-21,
and at NASA's Goldstone Solar System Radar by a team led by Lance Benner
of JPL from April 16-21. These dates cover the actual closest approach
time at 08:24 EDT on April 19, and due to the proximity of the asteroid,
the observations produced hundreds of radar images with resolutions of
7.5 meters/pixel from both observatories and a smaller number of images
at 3.75 meter/pixel resolution at Goldstone.

Figure 3: This sequence of images was obtained by NASA's 70-meter antenna
at Goldstone near Barstow, California, on 18 April 2017 - the day before
2014 JO25's closest approach. The double-lobed asteroid safely passed
by the Earth at a distance of 1.8 million kilometers (or ~4.6 times the
average distance from Earth to the Moon (NASA/JPL). Figure 3: This sequence
of images was obtained by NASA's 70-meter antenna at Goldstone near
Barstow, California, on 18 April 2017 - the day before 2014 JO25's
closest approach. The double-lobed asteroid safely passed by the Earth
at a distance of 1.8 million kilometers (or ~4.6 times the average distance
from Earth to the Moon (NASA/JPL).

The radar images reveal that 2014 JO25 has an irregular and deeply bifurcated
shape with two major components that are connected by a relatively narrow
neck. The longest axis of the asteroid is about 1 km in extent and the
short axis is roughly 600 meters. The two components (or lobes) are each
several hundred meters across, but one is roughly 60% larger than the
other and the smaller lobe appears more oblong and less rounded. The two
lobes are in contact with their short axes pointed approximately toward
each other. Significant portions of each lobe appear rounded but there
are also small areas where the surfaces appear angular.

The neck between the lobes is more than 200 meters deep in some places
and its depth varies with location.

The general appearance of the asteroid, depending on viewing angle, is
vaguely reminiscent of a lopsided peanut, or a rubber ducky. Understanding
the true detail of the three-dimensional shape will require extensive
reconstruction after the radar data have been fully processed.
Figure 4: This animation of 2014 JO25 was compiled from the observations
made by the 300-meter Arecibo Observatory near closest approach on 19
April 2017. The resolution is 7.5 meters /pixel. There are small bright
features that may be boulders on the surface as well as raised topography
that is casting shadows (Arecibo Observatory/NSF/NASA). Figure 4: This
animation of 2014 JO25 was compiled from the observations made by the
300-meter Arecibo Observatory near closest approach on 19 April 2017.
The resolution is 7.5 meters /pixel. There are small bright features
that may be boulders on the surface as well as raised topography that
is casting shadows (Arecibo Observatory/NSF/NASA).

The overall shape of the asteroid also resembles the now famous "rubber
ducky shaped" nucleus of Comet 67P/Churyumov-Gerasimenko that was recently
explored by the European Space Agency's Rosetta spacecraft, but the
comet's nucleus is more than four times larger than this asteroid.

Prior to the close approach, all that was known about the physical properties
of 2014 JO25 were an estimate of the size, ~650 meters, and its reflectivity,
~25%, based on "effective diameter" infrared measurements from NASA's
NEOWISE spacecraft, i.e. what the size would be if the object were roughly
spherical. The results from the radar images are consistent with the size
and reflectivity estimates from the NEOWISE data given the irregular shape
of the asteroid.

The most detailed radar images of 2014 JO25 reveal evidence for smaller
scale features such as flat regions up to ~200 m long, ridges, concavities,
possible impact craters several tens of meters in diameter, hills, and
collections of bright spots that may indicate large boulders.

Based on changes in the appearance of the asteroid from the beginning
to the end of the radar observing campaign, the line of sight to the asteroid
was relatively close to its equator from April 15-17, at least several
tens of degrees away from the equator on April 18, and then again near
its equator again on April 20 and 21. The asteroid moved about 150 degrees
across the sky during the observations, so substantial excursions in the
latitude visible each day could be expected. The smaller lobe nearly disappeared
during a narrow interval on April 20, indicating that the larger lobe
was eclipsing the smaller lobe and that the radar line-of-sight was looking
down the long axis that joins the two lobes.

The radar teams used sequences of images from Arecibo and Goldstone to
estimate the asteroid's rotation period by tracking specific individual
features visible during each observing session across observations obtained
on multiple days. This yielded a rotation period of about 4.5 hours.

Figure 5: The Arecibo radio telescope spans just over 300-meters across.

The radar observations were also used to measure the asteroid"s distance
and line-of-sight velocity (range and range-rate) on several occasions,
information that has been used to improve calculation of the asteroid's
past and future motion around the Sun. Although it was already known that
the 2017 pass would be the closest by the asteroid for more than 400 years,
the detailed radar measurements now obtained will allow the motion to
be computed reliably for thousands of years, an unusually long interval
compared to the predictions for the more than 16000 near-Earth asteroids
discovered to date.

The radar measurements can also be used to gauge the roughness of the
asteroid's surface on size scales of tens of centimeters. Measurements
from Arecibo and Goldstone both show that the surface roughness of 2014
JO25 is similar to those estimated by radar for asteroids 433 Eros, 25143
Itokawa, and 4179 Toutatis, all of which have now been visited by spacecraft
so that there are "ground-truth" images for comparison. The surface
roughness is also comparable to the average seen for more than 200 other
near-Earth asteroids previously studied with radar that have not been
explored by spacecraft.

In principle, the radar reflectivity can be used to constrain the composition
of the asteroid. This requires a detailed 3D shape model that is not yet
available, but using estimates of the dimensions visible in the images,
the radar albedo is about 0.2, which is consistent with a rocky composition
but not similar to a metallic composition.

The NASA Infrared Telescope Facility (IRTF) was also used by Josh Emery,
Lauren McGraw and Mike Lucas (University of Tennessee) and Cristina Thomas
(Planetary Science Institute) to observe 2014 JO25. The 3-meter telescope
is located atop Mauna Kea, Hawaii and can take spectra of celestial bodies
to better determine their bulk composition.
Figure 6: The SpeX instrument (a spectrograph) on the IRTF observed 2014
JO25 on 21 April 2017. This dataset shows the spectrum of the asteroid
from 0.7 to 2.5 microns. This spectrum contains two large diagnostic absorption
features at 1 and 2 microns and is consistent with a spectral classification
of S-type asteroids (J. Emery/UT-Knoxville et al.) Figure 6: The SpeX
instrument (a spectrograph) on the IRTF observed 2014 JO25 on 21 April
2017. This dataset shows the spectrum of the asteroid from 0.7 to 2.5
microns. This spectrum contains two large diagnostic absorption features
at 1 and 2 microns and is consistent with a spectral classification of
S-type asteroids (J. Emery/UT-Knoxville et al.)

SpeX is a medium-resolution spectrograph built at the Institute for Astronomy
(IfA) for the IRTF. On the evening of April 21, 2017, the SpeX instrument
was used to distinguish the asteroid's spectral class (or, taxonomy).
Based on the absorption features at 1 and 2 microns, it is consistent
with an S-type asteroid. S-type asteroids are silicaceous; that is -
more of a stony composition and moderately bright. This is the same spectral
class as Toutatis, and Itokawa, the asteroid visited by the Japanese Hayabusa
1 mission.

Among the hundreds of near-Earth asteroids studied with radar to date,
about 50 have double-lobed or "contact binary" shapes. For near-Earth
asteroids larger than about 150 meters in size, about 1/6 of them have
this type of shape, so it is clearly quite common.

The 4.5-hour rotation period is quite rapid for a contact binary shape,
and given the dimensions of the asteroid, 2014 JO25 is rotating almost
fast enough to cause separation into two objects.

So how did 2014 JO25 acquire this shape? Scientists don't know for sure,
but there are a number of plausible scenarios. One mechanism is that the
asteroid formed during a slow collision between two separate objects.
Or, the asteroid could have formed by slowly spinning up and now starting
to come apart. For example, there is strong evidence that many near-Earth
asteroids are weakly bound collections of rocks and dust that are held
together primarily by their very feeble gravity. If so, then the asteroid's
shape could distort if its rotation accelerates, which can happen due
to a subtle effect related to how irregularly-shaped asteroids absorb
visible sunlight and then re-radiate it as infrared light. The difference
in the direction of the emitted infrared relative to the absorbed sunlight
produces a gentle torque that can gradually change the spin. This acceleration
has actually been observed for several asteroids, and is called the
Yarkovsky-Keefe-Radzievskii-Paddack, or "YORP effect", after four scientists
who researched the dynamical components of this concept. If the rotation
spins up enough for a loosely bound object, then the shape can change and
double-lobed objects may form.

Another possibility is that the asteroid could have more closely approached
one of the planets - Mercury or Earth (its orbit can take it close to
either of these) - and that planetary tides could have started to pull
it apart. However, the existence of dozens of contact binaries suggests
that the mechanism responsible for their formation acts across a wide
swath of the inner solar system, so that argues against the theory it
is caused by very close planetary passes, which are relatively rare. Yet
another possibility is that a pre-existing, larger asteroid was shattered
by a collision with another object, and some of the remaining debris re-accumulated
by its weak mutual gravitational attraction into a few individual rubble-piles
which settled onto each other. Whatever its origin, 2015 JO25's shape
is another clue to the fascinating histories of the small bodies of our
Solar System.
Received on Tue 09 May 2017 07:24:02 PM PDT