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   DRAFT, UNDER REVIEW   
   Draft version March 7, 2023   
   Typeset using LATEX default style in AASTeX631   
   PHYSICAL CONSTRAINTS ON UNIDENTIFIED AERIAL PHENOMENA   
   Abraham (Avi) Loeb1 and Sean M. Kirkpatrick2   
   1Head of the Galileo Project, Astronomy Department, Harvard University   
   60 Garden Street, Cambridge, MA 02138, USA   
   2Director of All-domain Anomaly Resolution Office   
   1010 Defense Pentagon   
   Washington DC 20301, USA   
   ABSTRACT   
   We derive physical constraints on interpretations of “highly maneuverable”   
   Unidentified Aerial Phenomena   
   (UAP) based on standard physics and known forms of matter and radiation. In   
   particular,   
   we show that the friction of UAP with the surrounding air or water is expected   
   to generate a bright   
   optical fireball, ionization shell and tail - implying radio signatures. The   
   fireball luminosity scales with   
   inferred distance to the 5th power. Radar cross-section scales similarly to   
   meteor head echoes as the   
   square of the effective radius of the sphere surrounding the object, while the   
   radar cross-section of the   
   resulting ionization tail scales linearly with the radius of the ionization   
   cylinder. The lack of all these   
   signatures could imply inaccurate distance measurements (and hence derived   
   velocity) for single site   
   sensors without a range gate capability.   
   Keywords: Interstellar objects – Meteors – meteoroids – Meteorites –   
   Bolides – asteroids: general –   
   asteroids: individual (A/2017 U1) – Minor planets – ‘Oumuamua   
   1. INTRODUCTION   
   In 2005, the US Congress tasked NASA to find 90% of all Near Earth Objects   
   (NEOs) that are larger than 140   
   meters (Loff 2014). The Congressional task resulted in the construction of the   
   Pan-STARRS telescopes. On October   
   19, 2017, the Pan-STARRS sky survey flagged an unusual NEO, the interstellar   
   object ‘Oumuamua (see, Loeb (2022a)   
   and references therein). Unlike Solar system asteroids or comets, ‘Oumuamua   
   appeared to have an extreme flat shape   
   and was pushed away from the Sun without showing a cometary tail of gas and   
   dust, raising the possibility that it was   
   thin and artificial in origin. Three years later, Pan-STARRS discovered a   
   definitely artificial object, namely NASA’s   
   rocket booster 2020 SO, which exhibited similar behavior with an extreme   
   shape, a push by the Solar radiation pressure   
   and no cometary tail because its thin walls were made of stainless steel   
   (Talbert 2020).   
   On March 9, 2017, six months before ‘Oumuamua’s closest approach to Earth,   
   a meter-size interstellar meteor   
   (IM2) collided with Earth (Siraj & Loeb 2022a). Surprisingly, IM2 had an   
   identical speed relative to the Sun at large   
   distances and an identical heliocentric semimajor axis as ‘Oumuamua had. But   
   the inclination of IM2’s orbital plane   
   around the Sun was completely different from ‘Oumuamua’s, implying that   
   the two objects are unrelated.   
   Nevertheless, the coincidences between some orbital parameters of ‘Oumuamua   
   and IM2 inspires us to consider the   
   possibility that an artificial interstellar object could potentially be a   
   parent craft that releases many small probes   
   during its close passage to Earth, an operational construct not too dissimilar   
   from NASA missions.   
   These “dandelion seeds” could be separated from the parent craft by the   
   tidal gravitational force of the Sun or   
   by a maneuvering capability. A small ejection speed far away could lead to a   
   large deviation from the trajectory of   
   the parent craft near the Sun. The changes would manifest both in arrival time   
   and distance of closest approach to   
   Earth. With proper design, these tiny probes would reach the Earth or other   
   Solar system planets for exploration,   
   as the parent craft passes by within a fraction of the Earth-Sun separation -   
   just like ‘Oumuamua did. Astronomers   
   would not be able to notice the spray of mini-probes because they do not   
   reflect enough sunlight for existing survey   
   telescopes to notice them if they are on the 10 cm scale of CubeSats or   
   smaller. At a distance d from the Sun and the   
   telescope, objects that are a meter in diameter and reflect a fraction a ≈   
   10% of sunlight impinging on their surface   
   would yield a flux of optical light of ∼ 0.2(d/1 AU)−2 nJy, well below the   
   detection threshold of even the James Webb   
   DRAFT, UNDER REVIEW   
   2   
   Space Telescope. In contrast, the radar signatures of a meter class object   
   would be detectable with our deep space   
   radars and space fence, much like IM2 was, out to beyond geosynchronous orbit   
   at an altitude above 36,000 km. Such   
   objects could also become optically detectable as they get close to Earth,   
   especially if they create a fireball as a result   
   of their friction with air.   
   Equipped with a large surface-to-mass ratio of a parachute, technological   
   “dandelion seeds” could slow down in the   
   Earth’s atmosphere to avoid burnup and then pursue their objectives wherever   
   they land. Current radar coverage   
   of the majority of first-world countries gives detectability of this   
   High-Area-To-Mass (HAMR) objects down to a few   
   centimeters depending on material, making detectability possible (Frueh et al.   
   2017).   
   Within a close range to a star, extraterrestrial technological probes could   
   use starlight to charge their batteries   
   and liquid water as their fuel. This would explain why they would target the   
   habitable region around stars, where   
   liquid water may exist on the surface of rocky planets with an atmosphere,   
   like the Earth. Habitable planets would   
   be particularly appealing to trans-medium probes, capable of moving between   
   space, air and water. From a large   
   distance, Venus, Earth or Mars would be equally attractive for probes. But   
   upon closer inspection, Earth would show   
   spectral signatures of liquid water (through reflection of blue light) and   
   vegetation (through its red edge) that might   
   attract selective attention (Seager et al. 2005).   
   What would be the overarching purpose of the journey? In analogy with actual   
   dandelion seeds, the probes could   
   propagate the blueprint of their senders. As with biological seeds, the raw   
   materials on the planet’s surface could also   
   be used by them as nutrients for self-replication or simply scientific   
   exploration. It is important to note, that given   
   the time scales associated with the propulsion scheme discussed here, it is   
   unreasonable to assert that the intention of   
      
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