arXiv subjects

Anthony Mallama

Publications and source records attributed to Anthony Mallama.

At least 37 records · Page 2Linked to original sources

arXiv2023

Starlink Generation 2 Mini Satellites: Photometric Characterization

Starlink Generation 2 Mini satellites are fainter than Gen 1 spacecraft despite their larger size. The mean of apparent magnitudes for satellites in brightness mitigation mode is 7.06 +/- 0.10. When these magnitudes are adjusted to a uniform distance of 1,000 km that mean is 7.87 +/- 0.09. The brightness mitigation mode reduces distance-adjusted satellite luminosity by a factor of 12 relative to spacecraft that are not mitigated.

astro-ph.IM↗

arXiv2023

BlueWalker 3 Satellite Brightness Characterized and Modeled

The BlueWalker 3 (BW3) satellite was folded into a compact object when launched on 2022 September 11. The spacecraft's apparent visual magnitude initially ranged from about 4 to 8. Observations on November 11 revealed that the brightness increased by 4 magnitudes which indicated that the spacecraft had deployed into a large flat-panel shape. The satellite then faded by several magnitudes in December before returning to its full luminosity; this was followed by additional faint periods in 2023 February and March. We discuss the probable cause of the dimming phenomena and identify a geometrical circumstance where the satellite is abnormally bright. The luminosity of BW3 can be represented with a brightness model which is based on the satellite shape and orientation as well as a reflection function having Lambertian and pseudo-specular components. Apparent magnitudes are most frequently between 2.0 and 3.0. When BW3 is near zenith the magnitude is about 1.4.

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arXiv2023

Roll Angle Adjustment Dims Starlink Satellites

The brightness of Starlink satellites during orbit parking and orbit raising decreased significantly in 2020 when the operator modified their orientation. The mean apparent magnitude before the change was 3.90 +/- 0.18, while afterward it was 5.69 +/- 0.06. When magnitudes are adjusted to a standard distance of 1,000 km the means are 4.86 +/- 0.16 and 7.31 +/- 0.05. The difference at the standard distance indicates that spacecraft with adjusted roll angles are 90% fainter on average than the earlier ones.

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arXiv2022

The BlueWalker 3 Satellite Has Faded

Observations of BlueWalker 3 (BW3) beginning on December 8 of this year indicate that its apparent brightness had decreased. We postulate that the orbital beta angle and resultant solar power considerations required an adjustment to the satellite attitude around that time. So, the nominally zenith facing side of the flat-panel shaped spacecraft, which supports the solar array, was tilted toward the Sun. Consequently, the nadir side, which is seen by observers on the ground, was mostly dark. Thus, BW3 has generally appeared faint and on some occasions was not seen at all. The amount of fading was up to 4 magnitudes. Numerical modeling indicates that the amount of tilt was in the range 13° to 16°. This situation indicates the improvement in the appearance of BW3 from the ground that can be achieved with small tilts of the spacecraft. Satellite operators and astronomers can jointly address the adverse impact of bright satellites on celestial observations based on this finding.

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arXiv2022

Visual Magnitude of the BlueWalker 3 Satellite

Observations have been carried out in order to assess the optical characteristics of the BlueWalker 3 spacecraft which is the prototype for a new satellite constellation. The illumination phase function has been determined and evaluated. The average visual magnitude when seen overhead at the beginning or ending of astronomical twilight is found to be +1.4.

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arXiv2022

Visual Brightness Characteristics of Starlink Generation 1 Satellites

A large dataset of visual magnitudes for all three designs of Starlink satellites is analyzed. Brightness phase functions are derived for the Original, VisorSat and Post-VisorSat models. Similarities and differences between the functions for these spacecraft are noted. A metric called the characteristic magnitude is defined as the average brightness of a satellite when seen overhead at the end of astronomical twilight. When the phase functions are evaluated according to this metric, the characteristic magnitudes are: Original, 4.7; VisorSat, 6.2; and Post-VisorSat, 5.5.

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arXiv2022

The Sky Distribution and Magnitudes of Starlink Satellites by the Year 2027

Visual magnitudes and sky coordinates are projected for the full constellation of Starlink satellites. The results are presented in the form of sky maps and numerical tables. Observer latitudes from the equator to 60 degrees are considered. The solar elevations include -12 deg (the end of nautical twilight), -18 deg (the end of astronomical twilight) and -30 deg.

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arXiv2022

Lunar Eclipse Phenomena: Modeled and Explained

A model based on celestial geometry and atmospheric physics predicts the dimming and the color of lunar eclipses. Corresponding visual magnitudes and color indices for eclipses from year 2000 through 2050 are listed. The enlargement of the Earth's umbral shadow reported by observers for over 300 years is explained. The geometrical aspects of the model are the sizes and separations of the Sun, Moon and Earth. Atmospheric effects include refraction, absorption and focusing of sunlight.

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arXiv2022

OneWeb Satellite Brightness -- Characterized From 80,000 Visible Light Magnitudes

The mean apparent magnitude and the mean of magnitudes adjusted to a standard distance are reported. The illumination phase function for OneWeb satellites is determined and it differs strongly from that of VisorSat spacecraft. Brightness flares are characterized and the mean rate of magnitude variation during a pass is determined. Tools for planning observations that minimize interference from bright satellites are illustrated and discussed.

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arXiv2021

The Brightness of Starlink and OneWeb Satellites During Ingress and Egress from Terrestrial Eclipses

A model that combines celestial geometry and atmospheric physics is used to calculate the dimming of artificial satellites as they enter and exit the Earth's shadow. Refraction of sunlight by the terrestrial atmosphere can illuminate a satellite while it is inside the eclipse region determined from geometry alone. Meanwhile, refraction combines with atmospheric absorption to dim the satellites for tens of km outside of that region. Spacecraft brightness is reduced more in blue light than in red because absorption of sunlight is stronger at shorter wavelengths. Observations from the MMT-9 robotic observatory are consistent with the model predictions. Tables of satellite brightness as functions of their location in the eclipse region are provided.

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arXiv2021

Starlink Satellite Brightness -- Characterized From 100,000 Visible Light Magnitudes

Magnitudes for the VisorSat and Original-design types were analyzed separately and by time. Mean values are compared with those from other large-scale photometric studies, and some signficant differences are noted. The illumination phase functions for Starlink satellites indicate strong forward scattering of sunlight. They are also time-dependent on a scale of months and years. These phase functions improve the predictability of satellite magnitudes. A Starlink Brightness Function tailored to the satellite shape also improves magnitude predictions. Brightness flares lasting a few seconds are characterized and the mean rate of magnitude variation during a pass is determined. Observation planning tools, including graphs and statistics of predicted magnitudes, are discussed and illustrated.

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arXiv2021

A Bidirectional Reflectance Distribution Function for VisorSat Calibrated with 10,628 Magnitudes from the MMT-9 Database

A BRDF for the VisorSat model of Starlink satellites is described. The parameter coefficients were determined by least squares fitting to more than 10,000 magnitudes recorded by the MMT-9 robotic observatory. The BRDF is defined in a satellite-centered coordinate system (SCCS) which corresponds to the physical shape of the spacecraft and to the direction of the Sun. The three parameters of the model in the SCCS are the elevations of the Sun and of MMT-9 along with the azimuth of MMT-9 relative to that of the Sun. The mean VisorSat magnitude at a standardized distance of 1,000 km is 6.84 and the RMS of the distribution around that mean is 1.05. After the magnitudes are adjusted with the BRDF, the RMS reduces to 0.51. The set of MMT-9 observations transformed to the SCCS is available from the author.

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arXiv2021

The Brightness of VisorSat-Design Starlink Satellites

The mean of 430 visual magnitudes of VisorSats adjusted to a distance of 550-km (the operational altitude) is 5.92 +/-0.04. This is the characteristic brightness of these satellites when observed at zenith. VisorSats average 1.29 magnitudes fainter than the original-design Starlink satellites and, thus, they are 31% as bright.

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arXiv2020

The Brightness of OneWeb Satellites

The mean visual magnitude of OneWeb satellites at the standard satellite distance of 1,000 km is 7.18 +/-0.03 . When this value is adjusted to the nominal 1,200 km altitude of a OneWeb satellite in orbit it corresponds to magnitude 7.58 which is an indication of the mean brightness at zenith. The OneWeb satellites are fainter than the original Starlink satellites at a common distance. Preliminary data on the new and dimmer VisorSat design for Starlink suggests that they are still brighter than OneWeb at the satellites' respective operational altitudes.

astro-ph.IM↗

arXiv2020

Starlink Satellite Brightness Before VisorSat

The mean of 830 visual magnitudes adjusted to a distance of 550 km (the operational altitude) is 4.63 +/-0.02. The data on DarkSat, the low-albedo satellite, indicate that it is fainter than the others by 1.6 magnitudes or 78%. However, there is considerable uncertainty in this value due to the small number of observations. Some satellites were observed to flare by 10,000 times their normal brightness. These statistics can serve as a baseline for assessing the reduced brightness of the VisorSat design for future Starlink satellites.

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arXiv2020

A Flat-Panel Brightness Model for the Starlink Satellites and Measurement of their Absolute Visual Magnitude

The Starlink satellites are shaped like flat panels. The flat sides face zenith and nadir during normal operations. Their brightness is determined by the product of the solar illumination on the downward facing side of the panel multiplied by the area of that side projected toward the observer on Earth. This geometry leads to a unique brightness function that is not shared by other satellites. For example, the observed brightness is very sensitive to the solar elevation angle. There are circumstances where sunlight only illuminates the upward facing side of the satellite rendering it invisible to Earth-based observers. A brightness model depending on the solar aspect and the observer aspect of the flat panel, in addition to the satellite distance, is described. Absolute brightness is the only free parameter of the model, and it is taken to be that at a distance of 1,000 km when the solar and observer factors are unity. This model has been successfully fitted to a set of observed magnitudes. The absolute visual magnitude of a Starlink satellite as determined from this fitting is 4.1 +/- 0.1. The model could be used to determine the absolute magnitude of the Starlink satellite known as Dark Sat which has a special low-albedo coating.

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arXiv2018

Computing Apparent Planetary Magnitudes for The Astronomical Almanac

Improved equations for computing planetary magnitudes are reported. These formulas model V-band observations acquired from the time of the earliest filter photometry in the 1950s up to the present era. The new equations incorporate several terms that have not previously been used for generating physical ephemerides. These include the rotation and revolution angles of Mars, the sub-solar and sub-Earth latitudes of Uranus, and the secular time dependence of Neptune. Formulas for use in The Astronomical Almanac cover the planetary phase angles visible from Earth. Supplementary equations cover those phase angles beyond the geocentric limits. Geocentric magnitudes were computed over a span of at least 50 years and the results were statistically analyzed. The mean, variation and extreme magnitudes for each planet are reported. Other bands besides V on the Johnson-Cousins and Sloan photometric systems are briefly discussed. The planetary magnitude data products available from the U.S. Naval Observatory are also listed. An appendix describes source code and test data sets that are available on-line for computing planetary magnitudes according to the equations and circumstances given in this paper. The files are posted as supplementary material for this paper. They are also available at SourceForge under project https://sourceforge.net/projects/planetary-magnitudes/ under the 'Files' tab in the folder 'Ap_Mag_Current_Version'.

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arXiv2018

A Catalog of Sloan Magnitudes for the Brightest Stars - Version 2

A new version of the Catalog containing Sloan magnitudes for the brightest stars is presented. The accuracy of the data indicates that the Catalog is a reliable source of comparison star magnitudes for astronomical photometry. Version 2 complements the APASS database of fainter stars.

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