Mars Orbiters Capture 3I/ATLAS as Swift Detects Water "Firehose"
Within 48 hours, ESA's ExoMars Trace Gas Orbiter and Mars Express produced the closest spacecraft imagery yet of interstellar comet 3I/ATLAS, while NASA's Neil Gehrels Swift Observatory confirmed the comet is blasting water into space at a rate of about 40 kilograms per second.
Closest View from Mars Orbit
ESA confirmed that both ExoMars TGO and Mars Express pivoted toward 3I/ATLAS as the comet swept past Mars. CaSSIS, TGO's colour and stereo imager, captured the interstellar visitor as a fuzzy core drifting against background stars while the spacecraft was still 30 million kilometres away. It is the tightest spacecraft view of 3I/ATLAS to date.
The observation pushed CaSSIS to its limits. The comet was 10,000–100,000 times dimmer than the camera's usual Mars targets, requiring 5-second exposures. Mars Express engineers are stacking multiple 0.5-second frames and combing through spectra from OMEGA, SPICAM and TGO's NOMAD instrument to determine whether any volatile signatures registered at Mars.
Observation Highlights
- Distance: 30 million km from the Mars orbiters on 3 October
- Exposure strategy: CaSSIS used 5 s integrations; Mars Express stacking 0.5 s frames
- Field result: Coma resolved a few thousand kilometres wide, nucleus unresolved
- Follow-up: Juice will attempt post-perihelion observations in early 2026
Swift Spots a Water Jet Far from the Sun
Hours after ESA released the CaSSIS animation, researchers led by Auburn University reported that NASA's Neil Gehrels Swift Observatory detected hydroxyl emission from 3I/ATLAS when the comet was still 2.9 AU from the Sun. Swift's ultraviolet/optical telescope measured a water production rate of roughly 40 kilograms per second--akin to a fire hose running at full blast.
The study, published in The Astrophysical Journal Letters, suggests sunlight is heating icy grains lofted from the nucleus, allowing them to sublimate and feed the coma even in the cold outer Solar System. Co-author Dennis Bodewits remarked that every interstellar object examined so far--'Oumuamua, 2I/Borisov and now 3I/ATLAS--behaves differently, underscoring how diverse planet-forming systems can be.
Key Numbers
Estimated water loss measured by Swift from 2.9 AU -- well beyond the usual solar ice line.
CaSSIS exposure length required to lift the comet above background noise.
Separation between the comet and Mars' orbital path during the 3 October sweep.
Upper bound on the nucleus diameter from recent Hubble observations.
Cross-Checks and Context
Independent coverage from ABC News highlighted the challenge of imaging the dim comet from Mars orbit, while Live Science and the Auburn University release echoed Swift's "fire hose" analogy. The alignment across mission teams and media confirms that 3I/ATLAS is entering a remarkably active phase as it races toward perihelion.
What Comes Next
The comet now accelerates toward its 29 October perihelion, when heating should intensify outgassing even further. ESA expects additional Mars Express products once stacked imagery and spectra are processed, and Juice will follow up after perihelion from the Jovian system. Swift and other UV-capable facilities will continue monitoring to see whether the water flux climbs.
Watchlist
- Oct 29: Perihelion at 1.357 AU--peak activity expected.
- Nov–Dec: Best Earth-based viewing window as the comet brightens.
- Early 2026: Juice returns post-perihelion observations from the Jovian system.
- Swift follow-ups: Continued UV monitoring to track changes in OH output.
Observer's Note
3I/ATLAS remains a telescopic target, but the Swift detection hints that activity is ramping up earlier than expected. Watch for increased coma size and possible tail growth as perihelion approaches.
If you are imaging the comet, consider longer integrations or stacking techniques similar to the Mars teams to bring out faint structures.