Office of the Secretary of Defense: 02_Physical_Constraints_Paper

Constraints on UAP 3
light years that span the scale of the Milky Way galaxy. Such autonomous systems could be designed to survive even if
the senders are not able to communicate with them, and deposit probes upon arrival to the target planetary systems.
It is likely that any functional devices embedded in the Earth’s atmosphere are not carrying biological entities because
these would not survive the long journey through interstellar space and its harsh conditions, including bombardment
by energetic cosmic-rays, X-rays and gamma-rays (Hoang et al. 2017, 2018; Hoang & Loeb 2020). Interstellar gas
and dust particles deposit a kinetic energy per unit mass that exceeds the output of chemical explosives at the speed
of tens of km/s. However, technological devices with AI can be shielded to withstand the hazards of space, repair
themselves mechanically, or even reproduce given the resources of a habitable planet like Earth. With Machine Learning
capabilities, they can adapt to new circumstances and pursue the goals of their senders without any need for external
guidance.
As argued by John von Neumann in 1939, the number of such devices could increase exponentially with time if they
self-replicate (Freitas 1980), a quality enabled by 3D printing and AI technologies. Physical artifacts might also carry
messages, as envisioned by Ronald Bracewell in 1960Z (Bracewell 1960; Freitas & Valdes 1985) and currently used by
NASA.
3. PROPULSION METHODS
In principle, the fastest devices could be launched by lightsails, pushed by powerful light beams up to the speed
of light (Guillochon & Loeb 2015a). Natural processes, such as stellar explosions (Loeb 2023; Lingam & Loeb 2020)
or gravitational slingshot near black hole pairs (Guillochon & Loeb 2015b; Loeb & Guillochon 2016), could launch
objects to similar speeds. However, it would be difficult for relativistic payloads to slow down below the escape speed
of Earth, 10−4.5c, without having around the same facilities that generated their high initial speeds.
A better-suited propulsion technique that was used in all interplanetary space missions from Earth is chemical
rockets. Since rockets carry their fuel, they can navigate to a desired planet and slow down near it. Alternatively, it
may be possible to use one of the above methods to travel to and through a planetary system, deploying interplanetary
probes using conventional chemical propulsion.
For a rocket of total mass, m, and exhaust speed of the ablated gas relative to the rocket, vexh, momentum
conservation implies: m ˙v = − ˙mvexh, where an overdot, ˙( ), denotes a partial time derivative. The Tsiolkovsky solution
to the rocket equation (Tsiolkovsky 2000), (minitial/mfinal) = exp{(vfinal − vinitial)/vexh}, implies that for reasonable
fuel-to-payload mass ratio, the final speed vfinal will only be an order of magnitude larger than the exhaust speed.
For typical chemical propellants with vexh of order a few km s−1, this tyranny of the rocket equation explains why all
human- made spacecraft reached a speed limit of tens of km s−1 or ≈ 10−4c. Interestingly, this speed is comparable
to the escape speed from the Earth’s orbit around the Sun, vesc ≈ 42km s−1, making it possible for humanity to
launch interstellar probes which take advantage of the motion of the Earth around the Sun at vinitial ≈ 30 km s−1. In
contrast, chemical propulsion may not be sufficient for probes to escape from the habitable zone around dwarf stars,
like the nearest star, Proxima Centuari (Loeb 2018; Lingam & Loeb 2018). In summary, chemical propulsion allows
escape from the habitable zone of Sun-like stars and enables slowing down near a destination.
Devices which need to refuel would favor a habitable planet where liquid water or combustible organic fuel are
available. The exhaust velocity of hydrogen/oxygen for rocket fuel is about 4.5 km s−1 (at a mass ratio of 16.4
with steam as exhaust) and so pure liquid hydrogen/oxygen is insufficient to slow down the characteristic free-fall
speed of over 40 km s−1 from interstellar space to the habitable zone around the Sun unless the fuel mass is many
orders of magnitude larger than the payload mass. Combustion of fuels with potentially higher heat capacity (such
as hydrocarbons) would result in other chemical byproducts at the craft’s exhaust, which would have distinct spectral
signatures. This implies that chemical propulsion, while sufficient to escape the habitable zone using Earth’s motion
around the Sun, is insufficient to slow down from interstellar space to the planet’s surface in the habitable zone without
other assistance. Consequently, the mothership/probe scenario is more energetically viable. In addition, using water as
the basis of the fuel would also require cold temperatures. Between hot exhaust (steam or other chemical byproducts)
and cold storage (20K for hydrogen), this gives rise to additional signatures for characterization.
Planets can be identified from a distance as they transit their star or through direct imaging (Winn 2023). Once an
Earth-like planet is targeted, an interstellar device can plunge into its atmosphere. In principle, a multitude of tiny
devices can be released from a mothership that passes near Earth. At vf inal ≈ 10−4c, a probe would cross twice the
distance of the Sun from the Milky-Way center within a time of ≈ 0.5 Gyr. The fraction of all Sun-like stars that host
Earth-like planets in their habitable zone is in the range ∼ 3–100% (Zink & Hansen 2019; Hsu et al. 2020; Bryson02Page determined to be Unclassified
Reviewed by Chief of Staff, AARO
IAW FY24 NDAA, Section 1841(a)(1)(C)
Date: 2/5/2025