# The Existential Risk from Fast Bullets During Interstellar Travel

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Interstellar travel would take millions of years for the chemical rockets we developed so far. It is possible that better understanding of genetics, as hinted by the latest findings in the laboratory of the Harvard Professor David Sinclair, will extend the lifespan of humans and allow future astronauts to live for the duration of interstellar trips.

However, even with these optimistic hopes, interstellar space would constitute a hostile environment for humans since it is full of centimeter-sized objects moving ten times faster than the bullets shot by chemical propellants from common terrestrial guns. How frequently will these interstellar bullets hit a traveling spacecraft?

To gauge the risk, we must specify the size of the craft. In order to imitate the comfortable gravitational acceleration to which humans are accustomed on the surface of Earth, 9.8 meters per second squared (1g), the craft will have to spin and have a large size. To be specific, let us consider a craft which is a few times larger than the space station or a football field, having a radius of two hundred meters — a hundred times larger than the human body. The centrifugal acceleration of a spinning craft with this size would produce an artificial gravity of 1g if it rotates with a period of 28.4 seconds, nearly half a minute.

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. Given the similarity in speed between interstellar objects and a chemical rocket, of order 20 kilometers per second, this implies that a spacecraft measuring 200 meters in radius will collide with a centimeter-sized interstellar object every 10 thousand years. During that time, the craft would traverse a distance of 0.6 light years, merely a tenth of the separation between stars.

Irrespective of the craft’s speed, this implies ten impacts by centimeter-sized bullets for every trip of a 200-meter-radius craft between neighboring stars. How dangerous is that?

The kinetic energy carried by a centimeter-sized bullet at a relative speed of forty kilometers per second is equivalent to a kilogram of TNT. This is equivalent to the total kinetic energy of two cars in a head-on collision above the speed limit on a highway. Clearly, such an impact would drill a hole in any enclosure surrounding an interstellar craft and pose an existential risk to astronauts along its path. Can we mitigate this risk?

One approach would be to detect incoming objects with an active radar system and deflect the near-craft objects before arrival. This strategy imitates the planetary defense methods envisioned for Near-Earth Objects (NEOs). Apparently, the human brain is a better tool for survival over billions of years than the body size of dinosaurs. As far as we know, the dinosaurs did not develop telescopes that would warn them of asteroid impacts. As a result, they were wiped out by an impact 66 million years ago. Moreover, we should not search for dinosaur footprints on the Moon because they never had a space program. On the other hand, NASA established the Planetary Defense Coordination Office to track NEOs that arrive closer than ten times the radius of the Earth and develop methods to deflect them. The Double Asteroid Redirection Test (DART) craft successfully collided in September 2022 with Dimorphos, a minor-planet moon of the asteroid Didymos, and kicked it.

The deflection of a centimeter-sized object in interstellar space would be possible as long as the object is identified at a distance much larger than the craft’s diameter. Detecting a centimeter-sized object from a kilometer distance would require a very powerful radar system. The challenge is equivalent to detecting a ballistic missile out to a distance of a few hundred kilometers, requiring a system similar to the Ballistic Missile Early Warning Systems constructed after World War II. To spatially resolve incoming interstellar bullets, the radio wavelength must be smaller than their size.

The radio signal from the required powerful radar could be detectable out to a distance of 300 light years from Earth by the forthcoming Square Kilometer Array (SKA), as I calculated with Matias Zaldarriaga in a 2006 paper. Here, the radio signal will not be sourced within a planetary system around a star but by a giant craft moving through interstellar space. An SKA search for related radar signals could place constraints on Craft Defense systems, akin to our own Planetary Defense initiative.

An alternative protection method would be to ablate interstellar bullets with a powerful laser beam. But success with either method requires early detection and is therefore not guaranteed. When protection is ineffective, the interstellar craft may lose a surface layer. The resulting debris of a thin metallic layer the size of a football field could be pushed near the Sun by sunlight and behave similarly to the first reported interstellar object, 1I/`Oumuamua, or NASA’s space debris 2020 SO, both discovered as NEOs by the PanSTARRS observatory in Hawaii.

Of course, the existential risk from centimeter-sized impacts scales in proportion to the surface area of the craft and would be completely negligible for small CubeSats, ten centimeter in size, which carry only electronic equipment.

For this reason, astronauts with artificial intelligence (AI) offer better prospects for interstellar travel than human astronauts with natural intelligence. In addition to their compact size, AI systems do not require artificial gravity and they could be designed to have the patience and material resilience to withstand the existential risk from interstellar bullets.

Following Charles Darwin’s insight in his book “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1859)”, we should favor travelers which are the fittest to survive in interstellar space. As proud parents of our technological kids, we should send AI astronauts rather than risk our biological kids in interstellar space.

This conclusion implies that our future encounters with extraterrestrial probes are likely to involve small devices with technological rather than biological descendants of the senders.