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by Avi Loeb
July 14, 2025
from
AviLoeb Website
Image credit: Mark Garlick,
Science Photo
Library
The discovery of
interstellar objects over the past
decade raises an important question that could shape the future of
humanity:
how to distinguish
extraterrestrial spacecraft
from interstellar asteroids...?
Both types of objects reflect sunlight.
However, no telescope on Earth can resolve a
hundred-meter object - the scale of our largest rocket -
Starship, from a distance of about a billion kilometers -
the distance where
3I/ATLAS was discovered on July 1, 2025.
Unfortunately, we cannot rely on sky watchers to
alert us to the possibility that a spacecraft just entered the solar
system.
Even after the first reported interstellar
object, 1I/`Oumuamua, showed the anomalies (On the Possibility of an
Artificial Origin for 'Oumuamua)
of a flat shape and a non-gravitational acceleration without a
cometary tail that distinguished it from any known asteroid or
comet, it was nevertheless labeled as a "dark
comet", namely a comet without the unique signature that
would flag it as a comet:
a visible plume of gas and dust...
Given this definition, any object launched by
humans to space - which is pushed by rocket fuel or solar radiation
pressure, is a dark comet.
The best we can hope for is courageous
astronomers that would admit anomalies exhibited by outliers, namely
features that may fit better the description of a technologically
manufactured object than a natural rock.
Recently, I listed
the anomalies of the new interstellar object 3I/ATLAS. This
object is anomalously bright, implying a diameter of ~20 kilometers
for the typical reflectance of asteroids.
The implied diameter and detection rate are
untenable by the mass budget in interstellar asteroids, as I showed
in
a new paper - just published in Research Notes of
the American Astronomical Society.
If 3I/ATLAS ends up being a comet, its nucleus
must be an order of magnitude smaller.
But if it happens not to possess a large cometary
plume of dust or gas,
what is the nature of this object?
Without asking this question, humanity will
remain in the "stone age," regarding interstellar objects.
Even if 3I/ATLAS will show up as a genuine comet,
like 2I/Borisov, as it gets closer to the Sun and heats up, we
should always ask this question about future interstellar
objects...
An interstellar comet is easy to identify by its
tail.
But what are the markers that would
distinguish a technological interstellar object - a spacecraft,
from an asteroid?
Here is a list of some of them:
-
Propulsion:
a central engine or solar radiation
pressure (as I suggested in a paper
'Could
Solar Radiation Pressure explain 'Oumuamua's peculiar Acceleration?'
with Shmuel Bialy for 1I/'Oumuamua)
would cause a technological object to deviate from a
Keplerian hyperbolic orbit, dictated solely by gravity.
-
Trajectory:
the path of the object could selectively
target the inner planets in the Solar system.
For example, the orbital plane of
3I/ATLAS was within 5 degrees of the ecliptic plane for the
Earth's orbit around the Sun.
The likelihood for these orbital
angular momenta to be aligned so well is ~0.001, as I
had mentioned in my recent
anomalies essay.
-
Artificial lights:
Reflection of sunlight can be
distinguished from artificial light by its spectrum and
through its faster decline with increasing distance from the
Sun, as I discussed in a paper (Detection
Technique for Artificially-Illuminated Objects in the Outer
Solar System and Beyond)
with Ed Turner.
-
Shape:
An artificially-designed shape can be
inferred from the light curve of reflected sunlight as the
object rotates. This is how 1I/'Oumuamua was inferred in a
paper (Modelling
the light curve of 'Oumuamua - Evidence for torque and disc-like
shape) by Sergei Mashchenko to have a disk-like
shape.
-
Image from a
flyby:
Resolved details of the object's surface
could instantly distinguish a technological object from a
rock.
Such an image can be taken by a camera on
a dedicated intercept mission or in case the object happens
to be passing very close to Earth.
Landing on a technological object through
a rendezvous mission like
OSIRIS-REx would offer the benefit of a direct
inspection, including the privilege of pressing buttons on
it.
-
Surface
composition:
remote spectroscopy of the surface might
show signatures of bombardment by cosmic-rays, interstellar
dust particles and interstellar protons.
The energy deposition rate scales as
velocity cubed and travel duration. Faster or older objects
should be more scarred by interstellar damage.
-
Signals:
A functioning technological device might
transmit electromagnetic signs that terrestrial telescopes
could search for over a broad range of frequencies from the
radio to gamma-rays.
-
Launch of
mini-probes from a mothership:
An efficient way to seed habitable
planets with probes is to pass near them and release small
devices at the right time and place with the appropriate
velocity kick, so that they will intercept the planets while
the mothership continues on its journey to the next star.
Ironically,
3I/ATLAS
was discovered by the small
ATLAS telescope with an aperture diameter of half a meter,
during the same month that the 8.36-meter aperture of the
Rubin Observatory started to search for interstellar objects
from nearly the same location in Chile.
Over the next decade, the Rubin observatory is
expected to find tens of new interstellar objects.
My advocacy is simple.
We should study the Rubin data with an
open mind to the possibility that it may discover
technological objects from extraterrestrial civilizations.
If we insist that all interstellar objects are
asteroids and comets with the outliers catalogued as
'dark
comets,' then the answer to the question,
"Are we alone?" would be "Yes, by choice"...
Some of the loneliest people in the world are
those who stopped searching for a partner.
Their status is a self-fulfilling prophecy.
In order to find our cosmic partners,
we must allow them to exist in our mind as we inspect the
Rubin data.
Surely, interstellar objects were passing
overhead in the sky in 1950 when Enrico Fermi asked:
"where
is everybody...?"
As an experimental physicist, his oversight was
not to build a large telescope to search for them.
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