Interstellar travel using current chemical rocket technology would require millions of years. However, advancements in genetics may one day extend human lifespans sufficiently to enable astronauts to survive such journeys. And even if they live long enough to reach for other stars, interstellar space remains a profoundly unsafe venture, according to an American astrophysicist.

Harvard Professor Avi Loeb, author of the bestsellers “Extraterrestrial” and "Interstellar", fears that the space is filled with centimeter-sized objects traveling at speeds ten times that of bullets fired from conventional firearms – and they represent a real threat. He wondered in an article in the Medium how often would a spacecraft encounter these high-speed "interstellar bullets"?

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The frequency of collisions depends on the craft's size. To provide Earth-like gravity (9.8 m/s² or 1g), a spacecraft would need to spin. For example, a craft with a radius of 200 meters - a structure roughly the size of a large football field - would achieve artificial gravity with a rotation period of 28.4 seconds.

“Based on the current census of interstellar meters, there are 10 to the power of 23 meter-sized objects in interstellar space per star. Assuming an equal amount of mass in centimeter-sized objects, there should be a billion times more of those smaller impactors. Given that the typical separation between stars in the vicinity of the Sun is 6 light years, the typical separation between interstellar objects of centimeter-size is roughly the diameter of the Earth, 12 thousand kilometers,” the professor wrote.

The kinetic energy of a centimeter-sized object traveling at 40 km/s is equivalent to the explosive force of one kilogram of TNT - comparable to a high-speed head-on car collision.

Such impacts could breach the spacecraft’s protective layers, posing a severe risk to its occupants, he emphasized.

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The scientist suggests two defensive strategies to enable spacecraft pass through. One is the active radar detection. Deploying a radar system for early detection and deflection of incoming objects could mitigate the risks. This approach mirrors Earth-based planetary defense systems like NASA’s Planetary Defense Coordination Office, which tracks Near-Earth Objects (NEOs) and develops deflection methods. The successful 2022 DART mission demonstrated the potential of redirecting threats through kinetic impact.

However, detecting a centimeter-sized object from a kilometer away would require a radar system similar to Cold War-era ballistic missile defense systems. Such a radar, operating at shorter wavelengths than the object's size, could identify interstellar bullets in time for evasive action. The emitted radar signal might even be detectable up to 300 light-years away by telescopes like the Square Kilometer Array (SKA), enabling searches for interstellar defense systems in extraterrestrial civilizations.

The other strategy envisages the laser ablation method: A high-powered laser could vaporize incoming objects before impact. However, both radar and laser systems depend on early detection, and their success isn’t guaranteed.

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When defenses fail, the spacecraft could lose surface layers. These metallic fragments might resemble interstellar objects like space debris such as NASA’s 2020 SO, observed by Hawaii’s PanSTARRS observatory.

The case for AI astronauts

In Loeb’s opinion, larger spacecraft are at greater risk due to their wider surface areas. In contrast, smaller crafts, such as CubeSats (10 cm in size), face negligible collision risks. This disparity makes artificial intelligence (AI) systems more viable for interstellar travel than human astronauts. AI systems, inter alia, require no artificial gravity; are compact, resilient, and adaptable to prolonged journeys; and can endure the existential risks posed by interstellar debris.

Charles Darwin’s principle of natural selection underscores the importance of favoring the "fittest" for survival. In the context of interstellar exploration, this implies that AI "astronauts" are better suited than humans. Consequently, the professor underlined, any probes we encounter from extraterrestrial civilizations are likely to be small, AI-driven devices rather than biological entities.

As humanity contemplates its future in the cosmos, embracing the technological offspring of our ingenuity may ensure our legacy beyond Earth, he added.

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and chair of the advisory board for the Breakthrough Starshot Project. He also led the astronomy department at Harvard University in 2011–2020.

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