The goal of the CyMISS (Tropical Cy clone intensity M easurements from the ISS ) project has been to acquire image sequences of intense tropical cyclones (TCs), such as hurricanes and typhoons, from the ISS to support the development of an improved remote sensing method to determine more accurately the strength of these destructive storms using stereoscopy. Supported by CASIS (Center for the Advancement of Science in Space) which manages the ISS US National Laboratory for NASA, this project has amassed a large collection of storm images during the last five years, many of which have been shared on this site and elsewhere (see the CyMISS page). As our team has been wrapping up our current CASIS-supported activities on the ISS, I have found some more images to share, not of the numerous storms we have observed, but of the nighttime sky as seen from the ISS.

In the last phase of the CASIS-supported work on CyMISS, our team has been exploring improvements which can be made to our observing techniques. One of our investigations involves observing storms at night using moonlight as the source of illumination. On February 20, 2019, the Expedition 58 crew of ISS acquired a series of 240 image pairs of Tropical Cyclone Oma using a Nikon D5 camera with the exposure times alternating between 1/30th and 1/8th of a second. The image sequence spanned from 16:58:46 to 17:02:45 GMT when the Moon, which provided the only source of illumination for the storm, was about 25 hours past full. The estimated lunar V magnitude of -12.3 resulted in an illumination level of about 1.7X10-6 that of the Sun, all else being equal. A sample image with an exposure time of 1/8th of a second is shown above. At this time, Tropical Cyclone Oma was located at about 22.1° S, 160.6° E in the South Pacific off the east coast of Australia. A context image of Oma created by mosaicking this sequence of images is shown below (for more details including 3D images of this storm, see “CyMISS Image of the Month: A Moonlit 3D View of Tropical Cyclone Oma from the ISS”).

The purpose of these nighttime observations from the ISS was to capture simultaneous views of the moonlit clouds below the horizon with the stars visible above the horizon to be used to refine the pointing knowledge of the camera. Such knowledge, along with details of the position of the ISS at the time, would allow the images to be geolocated more accurately as well as determine cloud top altitudes using stereography. The images with an exposure time of 1/30th of a second are meant to provide ideal views of the moonlit scene below the horizon while the 1/8th of a second exposures provided better views of the stars above the horizon (while the longer exposure times provide brighter views of the Earth, the 7.2 kilometer per second orbital motion of the ISS causes about a kilometer of image smear reducing the resolution of the images). Unlike our daytime photography sessions which typically used the Nikon D5 camera set to an exposure time of 1/2000th of a second at an ISO of 400 with the lens set to f/22, the nighttime images required the lens be opened up full to f/2 with the ISO cranked up to 51,200 to make up for the significantly lower scene illumination under moonlight as well as reveal dimmer stars above the horizon. While the longer exposure time and faster f-stop setting increased the amount of light reaching the camera’s focal plane array in the nighttime images by a factor of about 30,000X, the much higher ISO setting for the nighttime photography amplified the signal from the focal plane array by another factor of about 128X resulting in noticeably more image noise compared to daytime.

During the photography session of Tropical Cyclone Oma on February 20, 2019, the ISS was looking approximately to the southeast over the expanse of the South Pacific. The view above the horizon, shown in the images above, stretches about 45° from just above the ecliptic plane in the constellation of Capricorn on the left towards the southern constellation of Grus on the right. The stars are clearly visible across this swath of sky as is the airglow layer above Earth’s horizon at an altitude of about 90 kilometers. This airglow layer caused by sodium deposited by meteors which had burned up in Earth’s upper atmosphere and energized by solar radiation to produce its distinctive yellow color. On the left side of the image, the glow of zodiacal light (i.e. sunlight scattering off interplanetary dust concentrated along the ecliptic plane) is also evident above the bright horizon where light from the Sun (which was about 8° below the horizon of the ISS at this time) is scattering through the atmosphere. In all the decades I have observed the sky, this was the first time I have ever actually seen this phenomenon.

An enlargement of the star image centered on the stars along the “neck” of Grus (“The Crane”) on the right side of scene well away from the glare of zodiacal light is shown above. The contrast has been stretched in both panels and the color table inverted in the lower panel to help improve the visibility of the stars in the image (which appear as black dots). Selected stars in this view are numbered with their HR catalog designations from the Harvard Revised Photometry Catalog and are listed in the table below along with their common name (where available) and their brightness in terms of V magnitude taken from Yale’s Bright Star Catalog.

Selected Star Brightnesses

HR No. Common Name V Magnitude 8353 γ Gruis 3.01 8366 5.46 8367 BZ Gruis 6.18 8379 5.50 8411 λ Gruis 4.46 8477 6.23 8486 μ1 Gruis 4.79 8488 μ2 Gruis 5.10 8521 π1 Gruis 6.62 8524 π2 Gruis 5.62 8556 δ1 Gruis 3.97 8560 δ2 Gruis 4.11 8636 β Gruis 2.10 8639 6.07 8657 5.51

In order to determine the limiting magnitude for these images, two very dim stars visible just below BZ Gruis (HR 8367), shown in the closeup below, were examined in addition to those listed in the table. Using the online SIMBAD astronomical database to examine stars near BZ Gruis, these stars have been identified as HD 208434 and HD 208487 with V magnitudes of 7.12 and 7.47, respectively. Comparing the appearance of these and other stars in our images with their brightness shows that we can confidently identify stars down to a V magnitude of about 7.5 in individual, lightly processed images with an exposure time of 1/8th of a second. Such star-filled views should provide adequate information to improve the pointing knowledge of ISS imagery significantly.

The CyMISS team at Visidyne would like to thank the crew of the ISS as well as the staff at NASA’s Marshall Space Flight Center and Johnson Space Center for their efforts. The original images are courtesy of the Earth Science and Remote Sensing Unit at NASA Johnson Space Center. The work presented here is supported in part under CASIS Grant UA-2019-013.

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See earlier articles on the CyMISS program here.