NASA shares new pictures of interstellar comet that zoomed past Mars last month

Key Concepts

• Interstellar Comet 3I/2023 A3 (3I Atlas): A comet originating from outside our solar system, discovered by the NASA-funded ATLAS survey telescope.
• Comet: A small, natural, solid body composed of rocky and icy material that evaporates as it warms near a star.
• Interstellar Object: An object that originates from outside a star system and travels through interstellar space.
• Coma: A cloud of dust and ice surrounding a comet's nucleus, formed by evaporation.
• Nucleus: The solid, icy core of a comet.
• ATLAS Survey Telescope: A NASA-funded telescope in Chile that is part of NASA's planetary defense network and made the discovery of 3I Atlas.
• Planetary Defense Network: NASA's system of telescopes designed to detect and track near-Earth objects.
• Infrared Observations: Observations made in wavelengths of light invisible to the human eye, useful for determining the composition of celestial objects.
• Spectrography: A technique used to split light into its constituent colors (spectrum), revealing the chemical composition of an object.
• Non-gravitational Accelerations: Subtle changes in a comet's orbit caused by the expulsion of gas and dust, acting like small rocket engines.
• Perihelion: The point in an orbit where a celestial body is closest to the Sun.
• Citizen Science: Public participation in scientific research, such as identifying celestial objects.

Interstellar Comet 3I Atlas: A New Window into Other Solar Systems

This briefing from NASA's Goddard Space Flight Center details the ongoing observations and scientific findings regarding Comet 3I Atlas, the third known interstellar object detected passing through our solar system. The discovery, made by the NASA-funded ATLAS survey telescope in Chile on July 1st, 2025, has generated significant scientific interest due to its extraterrestrial origin. NASA experts, including Amit Shhatria (Associate Administrator), Nikki Fox (Associate Administrator for Science Mission Directorate), Shawn Domagal Goldman (Director for Astrophysics Division), and Tom Statler (Lead Scientist for Solar System Small Bodies), shared the latest images and insights.

Comet 3I Atlas: Origin and Nature

• Definition of a Comet: Comets are described as small, natural, solid bodies composed of a combination of rocky and icy material that evaporates when warmed by proximity to a star.
• Interstellar Origin: Comet 3I Atlas is confirmed to be a comet, behaving and appearing as such, but its origin lies outside our solar system. This makes it a scientifically significant discovery, being only the third such object ever found.
• Addressing Rumors: NASA acknowledged public speculation during a government shutdown period that prevented official comment, expressing excitement about the public's interest in space exploration and scientific discovery. They reiterated that 3I Atlas is definitively a comet, not an alien spacecraft.
• Comparison to Previous Interstellar Objects:
  • 'Oumuamua (2017): The first interstellar object, which behaved more like an asteroid, appearing elongated and mostly rocky, though with some indirect indications of gas evaporation. Observations were limited due to its outbound trajectory.
  • 2I/Borisov (2019): The second interstellar object, which behaved much like a typical solar system comet, with properties more in line with what astronomers are accustomed to.
  • 3I Atlas: Shares many similarities with 2I/Borisov but exhibits more pronounced differences, particularly in its carbon dioxide to water ratio.

Observational Campaign and Technological Capabilities

NASA's extensive fleet of spacecraft across the solar system has provided a unique opportunity to observe 3I Atlas throughout its passage, compensating for Earth-based observation challenges due to its trajectory behind the Sun. Nearly 20 mission teams have collaborated, pushing scientific instruments beyond their designed capabilities to capture detailed glimpses of this interstellar traveler.

• Timeline of Observations: Observations began with astrophysics missions and are continuing with planetary and heliospheric missions. The comet is expected to pass the orbit of Jupiter in spring 2026.
• Participating NASA Assets: Hubble Space Telescope, James Webb Space Telescope (JWST), TESS, Swift, SPHEREx, Perseverance Mars Rover, Mars Reconnaissance Orbiter (MRO), MAVEN, Europa Clipper, Lucy, Psyche, Parker Solar Probe, PUNCH, STEREO, and SOHO.
• Open Science Policy: NASA emphasizes its commitment to open science, making all data publicly available for analysis.

Key Findings from Astrophysics Missions

• Hubble Space Telescope:
  • Observed 3I Atlas when it was approximately 277 million miles from Earth.
  • Revealed a teardrop-shaped coma of dust emanating from the nucleus.
  • Estimated the nucleus size to be between 1,400 feet and 3.5 miles in diameter.
  • Confirmed the comet was losing dust at a rate consistent with solar system comets.
  • Provided improved orbital data to guide other observatories.
• James Webb Space Telescope (JWST) and SPHEREx:
  • Provided crucial infrared observations, allowing for the detection of molecular "fingerprints."
  • Detected an abundance of carbon dioxide gas in the coma, along with water ice.
  • The ratio of carbon dioxide to water was found to be higher than typically observed in solar system objects, with potential explanations including exposure to higher radiation levels or formation in a region with abundant carbon dioxide ice.
  • These observations help understand the comet's composition and formation environment.
• Swift Spacecraft: Studied the comet in X-rays and gamma rays.
• TESS Mission: Retrospective analysis of TESS data revealed observations of the comet as early as May, providing insights into its history before it approached closer to Earth.

Insights from Planetary and Heliospheric Missions

• Psyche Spacecraft: Acquired four black and white images of the comet over eight hours on September 8th-9th, 2025, when it was about 33 million miles away. These were the first images captured from this perspective.
• Lucy Spacecraft: Observed the comet on September 16th from the opposite direction. The image, spanning about one-third the width of the full moon, showed the coma and a tail extending away from the Sun. This "backlit" view, impossible from Earth, is crucial for understanding the dust's physical properties.
• MAVEN Spacecraft (at Mars):
  • When 3I Atlas passed within 20 million miles of Mars, MAVEN captured a spectrum of ultraviolet light.
  • The spectrum revealed the signature of hydrogen gas originating from the comet, confirming its presence and providing information about its chemical composition.
  • Combined with other observations, this data will help determine the comet's water production rate, offering insights into its formation and journey.
• SOHO Mission (ESA/NASA): Imaged 3I Atlas from October 15th-16th, approximately 222 million miles away, using detailed image processing to detect the faint object. This highlights the value of sun-observing spacecraft for detecting objects passing behind the Sun.
• Mars Reconnaissance Orbiter (MRO) - HiRISE Instrument: Provided an image on October 2nd, when the comet was 19 million miles away, showing it as a "fuzzy white ball" – its coma.

Scientific Questions and Future Discoveries

The scientific community is actively analyzing the collected data, with the understanding that definitive answers will emerge over time.

• Understanding Other Solar Systems: Interstellar objects like 3I Atlas offer a unique opportunity to study the composition and conditions of other solar systems, potentially predating our own.
• Origin of Planets and Life: The data contributes to the broader NASA mission of understanding the universe, the origin of planets, and the origin of life.
• Compositional Differences: The observed differences in elemental abundances and mineral compositions compared to solar system objects suggest a wide spectrum of conditions in different parts of the galaxy.
• "Frozen Fossils": These objects are viewed as "frozen fossils" from their formation moments, providing a snapshot of conditions in other stellar nurseries.
• Comet's Interaction with the Sun: The comet's rendezvous with our Sun is "waking it up," allowing for the release of gases and dust, which provides crucial compositional information that would otherwise remain hidden if it stayed frozen.

Addressing Specific Questions

• Shape and Size: The nucleus size is still being refined, with current estimates between a couple of thousand feet and a couple of miles in diameter. The shape is difficult to determine due to obscuration by dust, but current observations suggest it is not significantly elongated, appearing close to round.
• Origin Tracing: While tracing the incoming trajectory is complex due to galactic motion, the comet's high speed suggests it originated from an older stellar population, possibly a solar system older than our own.
• Alien Spacecraft Speculation: NASA experts definitively stated that all evidence points to 3I Atlas being a comet. No techno-signatures or evidence suggesting an artificial origin have been observed.
• Unique Differences: Beyond its trajectory, key differences include the carbon dioxide to water ratio, the nickel to iron ratio, and unusual polarization of light reflected off the dust. The appearance of a sunward tail early in its trajectory was also noted.
• Jets and Potential "Explosion": The observation of jets, particularly around perihelion, is common for comets and indicates active areas of volatile evaporation. This does not necessarily imply an explosion but rather active outgassing from specific surface regions, similar to observations of comet 67P by the Rosetta mission. The composition of these jets may change as more volatiles bake off.
• Non-gravitational Accelerations: These are being closely monitored and are currently consistent with those observed in solar system comets. Precise tracking is challenging due to the difficulty in directly observing the nucleus.
• Future Observations: Further observations are planned with JWST in December, along with ongoing analysis of data from various missions. The James Webb Space Telescope is expected to track the comet the longest as it exits the solar system due to its infrared capabilities.
• Image Processing: Images undergo standard calibration and normalization for scientific accuracy. Some initial releases may contain artifacts (e.g., scattered sunlight on spacecraft hardware) that will be addressed in subsequent, more refined processing. Stacking multiple images is sometimes necessary to make faint objects visible.
• Orbital Dynamics and Space Vastness: The probability of 3I Atlas hitting any planet is extremely low due to the vastness of space and orbital dynamics. The comet's trajectory is also slightly tipped relative to the ecliptic plane, further reducing collision probability.
• Age of the Comet: While precise age determination is difficult from a single object, the comet's high speed relative to its local stellar neighborhood suggests it originated from an older population of stars. Future discoveries of more interstellar objects may allow for better statistical analysis of their ages.
• Image Clarity: The clarity of images depends on factors like distance, instrument capabilities, and the object's brightness. While we can image grains of sand on Mars due to proximity, observations of 3I Atlas are from millions of miles away, with JWST providing compositional information and detailed images, while planetary spacecraft offer closer views as the comet passes by.

Conclusion

Comet 3I Atlas represents a rare and invaluable opportunity to study an object from beyond our solar system. The ongoing collaborative efforts of NASA missions and scientists are providing unprecedented insights into the composition, origin, and history of other stellar systems, contributing significantly to our understanding of the universe and our place within it. The data collected will be a rich resource for scientific analysis for years to come.