In a rare cosmic event, an interstellar comet made its closest approach to Mars last Friday, offering astronomers a fleeting opportunity to study a visitor from another star system. Two spacecraft in orbit around the Red Planet, the European Space Agency’s (ESA) ExoMars Trace Gas Orbiter and Mars Express, turned their cameras toward the distant object, with one successfully capturing an image.
A Close-Up from a Distant Origin
The interstellar comet, designated 3I/ATLAS, is a truly unique object. Unlike most comets that originate within our own solar system, it hails from another star system within the Milky Way galaxy. Astronomers believe the comet was ejected from its original system— perhaps by a giant planet or a passing star—and has been wandering the galaxy for hundreds of millions of years. Its entry into our region of space is a relatively recent occurrence.
A Brief Visitor, Traveling at Incredible Speed
The comet’s visit is brief, as it’s moving incredibly fast. Discovered on July 1, 2023, by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS), it was already traveling at an astonishing 137,000 mph at the time of detection. This velocity is far too great for the sun’s gravity to capture it; it’s essentially passing through our solar system.
“This was a very challenging observation,” said Nick Thomas, who leads the ExoMars camera team. “The comet is around 10,000 to 100,000 times fainter than our usual target.”
Imaging Challenges and a Successful Capture
Both orbiters attempted to photograph the comet, but their instruments were primarily designed to image Mars’ bright surface from relatively close range – not to detect faint, distant objects. Only the ExoMars Trace Gas Orbiter succeeded in capturing an image. The Mars Express’s shorter exposure times proved too restrictive for obtaining a clear signal. However, researchers are diligently working to analyze data from Mars Express, hoping to extract a faint image.
The resulting image from the ExoMars orbiter reveals a small, bright white splotch surrounded by a glowing cloud of gas and dust, known as a coma. Researchers were unable to directly observe the comet’s nucleus, and the tail was not visible. However, scientists suspect the tail will become brighter as 3I/ATLAS draws nearer to the sun and becomes more active.
Comets: Leftovers from Stellar Formation
Comets are composed of frozen mixtures of rock, ice, and dust. When exposed to sunlight, these ices sublimate (transition directly from solid to gas), releasing gas and debris that create the comet’s signature tails. Most comets observed reside within our solar system, believed to be remnants from the 4.6 billion years ago. Astronomers have identified over 4,000, with countless others suspected to lurk far beyond Neptune’s orbit.
A Clue to Planet-Building Elsewhere?
Observations of 3I/ATLAS revealed a dust composition similar to that found in local comets, suggesting that the basic recipe for planet formation may be universal across different star systems. This finding is a crucial point in understanding how planetary systems evolve across the galaxy.
A Rare Opportunity for Further Study
Following the close approach to Mars, ESA’s Jupiter Icy Moons Explorer (Juice) spacecraft will attempt to observe 3I/ATLAS in November. Researchers are optimistic that Juice’s observations will provide detailed images and data as the comet reaches its peak activity, exhibiting a bright halo and a large tail.
This interstellar visitor provides an incredibly rare opportunity to study a piece of another star system— a relic from the early universe— offering unique insights into the formation of planets and the potential for life beyond Earth. Despite its rapid passage, 3I/ATLAS presents a unique opportunity to learn more about the universe.
