Did TESS Really Spot Interstellar Visitor 3I/ATLAS Months Before We Did?
One of the great advantages of having many telescopes scanning the sky is the treasure hidden in their archives. When astronomers discover something remarkable, chances are, earlier images of it already exist—just waiting to be noticed. That has proven true for our newest interstellar traveler, 3I/ATLAS. Although astronomers officially detected it in July, researchers have now revealed that it was visible as early as May in multiple telescope datasets. Even more surprising, the object was already showing signs of activity.
TESS Exoplanet Satellite Unintentionally Captures an Interstellar Object
The Transiting Exoplanet Survey Satellite (TESS) was never designed to hunt for faint interstellar visitors. Its primary mission is to monitor stars and detect the tiny dips in brightness caused by exoplanets crossing in front of them. Yet, data is data. By sheer chance, TESS was observing a region of the sky that 3I/ATLAS crossed earlier this year.
Intrigued, researchers Adina Feinstein and Darryl Seligman of Michigan State University, together with John Noonan of Auburn University, searched the telescope’s archives. Their efforts paid off—they found clear detections of the interstellar object going back to May 7, 2025, across two separate observing windows.
Shift-Stacking: How Astronomers Found a Needle in a Cosmic Haystack
Spotting 3I/ATLAS in TESS images was no easy task. Since TESS takes a picture every 200 seconds, background stars remain steady, but a fast-moving interstellar object quickly blurs away. To solve this, the team used a technique called “shift-stacking.”
By predicting where the object would appear in each frame, shifting the images accordingly, and then stacking them, they amplified the faint signal. What was nearly invisible in single frames suddenly became clear, revealing a distant traveler racing through our solar system.
A Mysterious Brightening: Outgassing Hypervolatiles?
At the beginning of TESS’s observations, 3I/ATLAS was about 6.35 astronomical units from the Sun. By June 2, it had moved closer, to around 5.47 AU. In that span, its brightness—or flux—increased fivefold.
Here’s the mystery: simple geometry predicts that moving closer to the Sun should have made the comet only about 1.5 times brighter. So why the dramatic spike?
The answer may lie in the object’s chemistry. Researchers suggest that 3I/ATLAS was outgassing hypervolatile compounds such as carbon monoxide and carbon dioxide. Unlike water ice, which sublimates closer to the Sun, these volatile materials evaporate much farther out, causing the comet to flare dramatically. If correct, this hints that comets from other star systems could have very different compositions than those born in ours.
What About the Nucleus and Its Rotation?
To probe deeper, astronomers tried to measure the rotation period of 3I/ATLAS’s nucleus. Was it tumbling like ‘Oumuamua, or spinning more stably like 2I/Borisov? Unfortunately, the TESS data offered no clear answer.
The culprit was likely the comet’s coma—a glowing cloud of gas and dust that masked the nucleus, preventing astronomers from detecting any regular changes in brightness that might reveal rotation.
Why Interstellar Visitors Matter
Every interstellar object that crosses our skies brings with it questions. How do they form? What materials are they made of? Could their differences reshape what we know about comets, planets, and even planetary systems themselves?
In the case of 3I/ATLAS, archival data shows us that by the time we spotted it officially, it had already been active for months. And with every discovery, astronomers are reminded that the cosmos holds more surprises than we can anticipate.
So the next question lingers: What will the next interstellar visitor teach us—and has it already passed through another telescope’s field of view, waiting for us to uncover it?
Source: Did TESS Really Spot Interstellar Visitor 3I/ATLAS Months Before We Did?
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