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

[Ron Baalke]

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.