Hubble & JWST Launch Major Observing Campaign

Unprecedented Observing Campaign

On July 21, 2025, NASA's Hubble Space Telescope and James Webb Space Telescope (JWST) began a coordinated, intensive study of 3I/ATLAS. This marks the most comprehensive observational campaign ever conducted on an interstellar object while it's still approaching the Sun.

Unlike the previous interstellar visitors 1I/'Oumuamua and 2I/Borisov, which were discovered late in their passages, 3I/ATLAS was found early enough to allow detailed observation planning with the world's premier space observatories.

Hubble Estimates Nucleus Size

Hubble's high-resolution imaging has provided the first estimates of 3I/ATLAS's nucleus size. Despite the challenge of separating the nucleus from its bright coma, Hubble observations suggest:

"The comet's active coma makes it challenging to precisely measure the nucleus," explained Dr. David Jewitt, lead investigator. "But our best estimates suggest a relatively small nucleus, possibly comparable to 2I/Borisov."

JWST Reveals Chemical Composition

James Webb Space Telescope's powerful infrared instruments have detected multiple molecules in 3I/ATLAS's coma, revealing an unusual composition:

• Carbon dioxide (CO₂): Extremely abundant - unusually high compared to Solar System comets
• Water ice and vapor (H₂O): Present but in relatively small amounts
• Carbon monoxide (CO): Detected at moderate levels
• Carbonyl sulfide (OCS): Trace amounts detected

The CO₂-dominated composition is particularly intriguing. Most Solar System comets are water-dominated, making 3I/ATLAS's chemistry quite unusual and suggesting different formation conditions in its home star system.

Massive CO₂ Coma

JWST and NASA's SPHEREx mission have mapped the extent of 3I/ATLAS's carbon dioxide coma, finding it extends approximately 700,000 km in diameter - nearly twice the distance between Earth and the Moon!

The visible coma (dust and gas combined) measures approximately 26,400 × 24,700 km, about twice Earth's diameter. This extensive atmosphere is being created as the comet's nucleus heats up approaching the Sun, causing its ices to sublimate into space.

Rotation Period Determined

By monitoring brightness variations over multiple nights, astronomers have determined 3I/ATLAS's rotation period to be 16.16 ± 0.01 hours. This rotation causes the comet's brightness to vary slightly as different parts of its irregularly-shaped nucleus reflect sunlight.

Comparison to Previous Interstellar Visitors

3I/ATLAS is providing a stark contrast to the previous interstellar visitors:

• 1I/'Oumuamua: Showed no visible coma or tail; uncertain composition; discovered while leaving
• 2I/Borisov: Water-dominated composition like Solar System comets; discovered ~3 months before perihelion
• 3I/ATLAS: CO₂-dominated composition; discovered ~4 months before perihelion; most time for study

VLT Adds Spectroscopic Data

Complementing the space telescope observations, the Very Large Telescope (VLT) in Chile has obtained detailed spectroscopic data, detecting:

• Cyanide gas (CN): Common in comet comae, confirms active outgassing
• Atomic nickel (Ni I): Rare but detected in some Solar System comets

The presence of these molecules at concentrations similar to Solar System comets suggests that despite its interstellar origin, 3I/ATLAS shares fundamental similarities with comets formed around our Sun.

Ongoing Observations

Both Hubble and JWST will continue monitoring 3I/ATLAS throughout its passage, with additional observations planned around perihelion in late October 2025. These observations will track how the comet's activity evolves as it gets closer to the Sun.

"We're witnessing how a comet from another star system responds to our Sun," said Dr. Stefanie Milam, JWST deputy project scientist. "Every spectrum we obtain provides new clues about planetary system formation beyond our own."