Astronomy
Detecting Interstellar Objects Passing Through the Solar System
Quick fact
In October 2017, the Pan-STARRS1 telescope in Hawaii detected 'Oumuamua, the first known interstellar object, as a faint point of light moving against the background stars. It was already speeding past Earth and out of the solar system, and astronomers had only a few weeks to observe it before it disappeared forever.
Why this is interesting
Imagine a rock from another star system zipping through our cosmic neighborhood at 200,000 mph—how could we ever catch a glimpse of it? In 2017, we did exactly that.
Read the full explanation
Understanding Detecting Interstellar Objects Passing Through the Solar System
To detect an interstellar object, astronomers don't aim a telescope at the sky and wait. Instead, they use surveys—large telescopes that repeatedly photograph wide areas of the sky. Each image captures millions of stars and galaxies, which appear stationary over short timescales. By comparing images taken minutes or hours apart, they look for objects that have moved. This is called difference imaging: they subtract one image from another, and any object that has changed position shows up as a pair of dots. The key is that interstellar objects move much faster than typical asteroids or comets in our solar system, because they are not bound by the Sun's gravity and are merely passing through. Their apparent motion across the sky is often on the order of arcseconds per hour, which is fast but still subtle. Detection requires careful filtering of data to rule out satellites, cosmic rays, and human error. Once a candidate is found, it triggers a series of rapid follow-up observations using other telescopes to confirm its trajectory and measure its properties.
A deeper explanation
The detection mechanism hinges on the physics of motion. Interstellar objects enter the solar system with a hyperbolic excess velocity, meaning they are not gravitationally bound to the Sun. Their paths bend as they pass near it, but they retain enough speed to escape. This makes their apparent trajectory across the sky different from solar-system objects, which move on elliptical orbits. Astronomers model candidate objects' paths using initial observations and solve for orbital elements. An object with a hyperbolic eccentricity greater than 1 is likely interstellar. However, outgassing from cometary activity can produce non-gravitational accelerations that complicate the trajectory; for 'Oumuamua, the non-gravitational acceleration was observed without an obvious comet tail, leading to debate. The challenge is the brevity of observation: these objects are small and faint, moving rapidly, so detection is possible only during a narrow window while they are close enough and bright enough. The most successful approach has been wide-field surveys like Pan-STARRS and ATLAS, and future facilities like the Vera C. Rubin Observatory's LSST will scan the entire southern sky every few nights, likely increasing detection rates significantly.