Total solar eclipses present rare opportunities to study not just the solar corona, but also to study daytime
celestial objects using ground-based and airborne observatories that would otherwise be dominated by
intense sky brightness from the uneclipsed Sun. For example, Mercury is one of the least-studied planets,
both remotely and in situ, and its regolith is still poorly characterized. The MESSENGER orbiter measured
the surface composition of only the top few microns; the composition at larger depths, as well as the
regolith density and compaction, are still not known. These properties are important for understanding
Mercury's formation and subsequent processing of its surface by impacts and solar interaction. Vulcanoids
are another mystery. These hypothesized asteroids within the orbit of Mercury, left over from the early solar
system, have never been observed, though limits have been placed on their size/brightness distribution.
Establishing their existence, or lack thereof, would greatly impact our understanding of solar system
formation and rocky body evolution.
The proximity of these targets to the Sun makes them particularly difficult to study, both from ground and
space. Ground-based observations at twilight exploit the Sun's occultation by the horizon to minimize the
solar background, but such measurements are compromised by the large airmass at low elevation angles
causing poor seeing and high atmospheric absorption, which is especially problematic at thermal infrared
wavelengths (few microns). Weather can also cause complications when observing from the ground.
Airborne observations during the eclipse provide unique advantages that greatly reduce the above
concerns. By flying in the stratosphere, they avoid all weather and are guaranteed clear skies. At altitudes of
50 kft or higher, flying above >85% of atmospheric airmass and >99.5% of atmospheric water vapor, the
seeing quality is enormously improved and near-IR observations become possible due to significantly
reduced water absorption. By "chasing" the Moon's shadow's path across the Earth, an airborne
observatory can also increase the total eclipsed observing time by 50% or more, depending on airspeed.
We present the first results from airborne observations of the 2017 Great American Total Solar Eclipse using
two of NASA's WB-57F research aircraft, each equipped with two 8.7–8.9" telescopes feeding high-
sensitivity visible (green-line) and near-IR (3–5 µm) cameras operating at high cadence (30 Hz) with ~3″/pixel
platescale and 1.2–1.6° fields of view. The aircraft flew along the eclipse path, separated by ~90 km, to
observe a summed ~120 min of partial and total eclipse (including ~8 min of solar observations in totality)
in both visible and NIR, enabling an attempt at the first thermal imaging of Mercury and a new search for
Vulcanoids with improved sensitivity limits.
The Mission Two DyNAMITE-equipped WB-57 aircraft flew down the centerline of the eclipse, at
50,000 ft. altitude. The WB-57 has two seats, occupied by a pilot and an operator. The aircraft were spaced
precisely so that Plane 2 entered the leading edge of the lunar shadow 10–20 s before Plane 1 exited the
trailing edge. The overlap provides cross-calibration between the observation sets. Mercury was observed
by each plane for ~30 min during partial eclipse prior to and following totality. Solar observations were
prioritized for the ~4 min of totality. Calibration observations provided flat-field and dark measurements, as
well as absolute flux references and characterization of pointing stability The instruments stored science-
quality data on-board, but downlinked quick-look video via satellite, as a live digital stream.
Mercury's Nightside Temperatures
The diurnal variation of Mercury's surface temperature is a diagnostic of the regolith properties – composition, albedo,
porosity. Dayside temperatures peak at 800 °F (427 °C), while nightside temps bottom out at –280 °F (–173 °C). The
speed at which this change occurs depends strongly on these regolith properties. Measuring the temperature of the
nighttime surface as a function of local time (longitude) therefore probes the regolith cooling rate and, with modeling,
provides insight into the regolith characteristics to depths of a few cm. Only a handful of IR thermal data of Mercury are
available, and critically, no thermal IR image of Mercury yet exists.
The eclipse provided a near-ideal opportunity for such a measurement using the DyNAMITE IR (3–5 µm) camera. The
blackbody emission from the surface peaks exactly within this wavelength range. Mercury was at 10.4° solar elongation,
sufficiently far to be away from solar coronal emission, and with a nearly pure nightside geometry, at only 6% phase. The
dayside crescent was on the dawn side, adjacent to the coldest surface conditions, thus providing the best possible
measurement of the "warm" dusk side and its cooling into night.
Below, left Schematic of Mercury diurnal cycle showing temperature variation as a function of local time. Mercury is at 6%
phase with dawn terminator visible, providing a near-complete view of the nightside from just after sunset.
Below, right Estimated SNR in 3–5 µm across Mercury's nightside for a 10 min integration. At 10.8″ angular diameter,
Mercury is 4 pixels across, providing the first opportunity for resolved thermal imaging. The bright dayside emission on
the dawn side contaminates only the coldest nightside pixel, allowing the cleanest measurement of the cooling profile
from dusk through midnight. We obtained ~100 min of total observing time during partial eclipse between two aircraft.
A search for Vulcanoids
If they exist, Vulcanoids are hypothesized to be in dynamically stable orbits at 15–45 RS
(4–12° elongation). This proximity to the Sun makes observations difficult. Only a handful
of deep searches have been conducted, the most recent of which found no objects down
to a limit of Vmag=11, corresponding to a size of ~5.7 km assuming a 5% R-band albedo.
Only one prior near-IR search has been conducted (to a limit of Lmag=5), but was
hampered by bad weather.
The eclipse provided an opportunity to improve these limits through ~120 total minutes
(between two aircraft) of solar and Mercury observations during partial and total eclipse,
with lower sky brightness than prior ground/airborne searches. We estimate detection
limits of better than Vmag=11.9 and Lmag=5.7.
Below Vulcanoid search results from Steffl et al. (2013), showing processing of data after
removal of solar F-corona (left), sky subtraction to remove solar features (middle), and
fixed-source removal of background stars to reveal potential targets (inset, artist
depiction). Our processing will follow similar procedures.
The DyNAMITE Instrument The Day/Night Airborne Motion Imager for Terrestrial
Environments (DyNAMITE) on the Airborne Imaging and Recording System (AIRS) platform was built by
Southern Research to image rocket launches and other airborne objects. It is a stabilized, agile, steerable
platform with two telescopic channels: visible and infrared (IR). DyNAMITE is mounted in a modified nose of
the WB-57 aircraft. The IR channel is specifically designed for remote thermal imaging. The cameras have
in-flight-controllable zoom lenses, and acquire images at up to 30 frames per second. The visible-light
channel was upgraded with lossless HD recorders and a narrow-band green filter to maximize science.
2017 August 21 Observations
Right, top Observations of Sirius provide absolute brightness calibration for both visible and IR, and estimates
of pointing stability and SNR improvement for long stacked integrations. Despite stabilization, there is significant
jitter of 5–10 pixels (15–30″) over tens of seconds; co-alignment reduces this to <1 pixel (<3″). Stacking co-
aligned images reveals dim sources not visible in individual frames; a Vmag=+7 star (1% of sky brightness) is
observable with SNR=15 in just 20 sec. This technique is scalable and will allow us to stack tens of minutes of
observations to maximize SNR for Mercury analysis and the Vulcanoid search."
Bottom, left Sample IR image of Mercury just prior to totality (30 ms exposure time). No calibrations have yet
been applied; fixed-pattern noise is evident. Mercury is intensely bright and clearly visible, but unfortunately
appears unresolved; this is may be partly due to the ~3–5″ diffraction limit of the ~8.95" lens, but may also be
due to scattering within the various optics, and/or non-optimal focus. Deconvolution of the PSF may be possible
to reveal the underlying structure (currently under evaluation). This highlights the need for real-time quantitative
focus feedback and improved optics, including a larger aperture, for future missions.
Bottom, right Example observations of the solar corona during totality. The visible image (top) is a 1-min calibrated and
co-aligned stack with some processing to reveal coronal details; this is optimized for the Sun, but will be processed with
longer integration times for the Vulcanoid search. Mercury frames can be similarly processed. The IR image is a single
frame without calibration, but can be calibrated and processed in a similar manner to the visible (in progress).
Planetary Science from NASA’s WB-57 Canberra High Altitude Research Aircraft
During the Great American Eclipse of 2017
A. Caspi, C. Tsang, C. DeForest, D.D. Durda, A.J. Stefﬂ (SwRI), D.B. Seaton (CU/CIRES, NOAA/NCEI)
P. Bryans, S. Tomczyk, J. Burkepile, P. Judge (NCAR/HAO), E.E. DeLuca, L. Golub (SAO)
P.T. Gallagher (Trinity College Dublin), A. Zhukov, M. West (Royal Observatory Belgium)
J. Lewis, J. Wiseman, J. Collier, T.A. Casey, D. Darrow (SR)
C.J. Mallini, T. Propp, J. Gascar, C. Klemm, M. Yates, D. Johnson, D. Del Rosso (NASA JSC)
T. Parent (DynCorp), J. Warner, M. Jacyna (ViaSat)
Eclipse ﬂight path
Dim star (Vmag=+7) revealed