NASA Astronomy Picture (Video) of the Day

August 26, 2025

A Leaky Solar Prominence

What's hovering above the Sun? A solar prominence. A prominence is a crest of hot gas expelled from the Sun's surface that is held aloft by the Sun's magnetic field. Prominences can last for days, can suddenly explode into space, or just fall back to the Sun. What decides a prominence's fate is how the Sun's complex magnetic field changes the field's direction can act like an offramp for trapped solar particles. The 3-second (repeating) time-lapse featured video was captured earlier this month from Ottawa, Ontario, Canada. It shows the development of a larger-than-Earth prominence as it appears to leak solar plasma back to the Sun, over the course of an hour. What is unusual is that the prominence appears to hover more simple and typical prominences form magnetic loops that connect back to the surface. Many hours after this video ended, the hovering prominence disintegrated back into the Sun.

https://apod.nasa.gov/apod/astropix.html

tps://avi-loeb.medium.com/the-first-webb-telescope-observations-of-3i-atlas-8cd89e872870

https://science.nasa.gov/blogs/3iatlas/2025/08/25/nasas-webb-space-telescope-observes-interstellar-comet/

https://science.nasa.gov/blogs/3iatlas/2025/08/25/nasas-new-spherex-mission-observes-interstellar-comet/

https://www.space.com/astronomy/james-webb-space-telescope-takes-1st-look-at-interstellar-comet-3i-atlas-with-unexpected-results

The First Webb Telescope Observations of 3I/ATLAS

August 25, 2025

A few seconds before my flight to Copenhagen lifted off the ground at Boston’s Logan airport, I received an email with the paper reporting the first Webb telescope data from August 6, 2025 on 3I/ATLAS (accessible here).

The 15 minutes of wait for the onboard WiFi connectivity to show up felt like eternity. But the wait was worth it. The stunning Webb data from a 6.5-meter infrared telescope with unprecedented spectral sensitivity was worth the wait.

In short, the Webb data confirms the existence of a carbon dioxide (CO2) gas plume around 3I/ATLAS with significantly lower levels of water (H2O) and carbon monoxide (CO), as reported a few days earlier by the SPHEREx space observatory team (in a paper accessible here).

Whereas the Webb telescope has much better spectral and spatial resolution, SPHEREx mapped the spherically symmetric CO2 plume a hundred times farther from 3I/ATLAS and demonstrated that it extends beyond 348,000 kilometers.

3I/ATLAS does not feature a cometary tail that extends beyond the width of its coma, as was already evident from the higher resolution image taken by the Hubble Space Telescope (reported here).

That this tail is not seen suggests that 3I/ATLAS does not shed a lot of dust particles with a size comparable to the wavelength of sunlight, ~0.5 micrometer, and that the reflected sunlight originates from the surface of 3I/ATLAS.

This implies a diameter of up to 46 kilometers for an albedo of 5% according to the SPHEREx data.

Infrared spectroscopy of 3I/ATLAS at a heliocentric distance of 3.32 Earth-Sun separations was taken with the NIRSpec instrument onboard the Webb telescope.

The spectral images at wavelengths in the range of 0.6–5.3 micrometers reveal a prominent carbon dioxide (CO2) dominated coma, with enhanced outgassing in the direction of the Sun, as well as the presence of much less water vapor (H2O), carbon monoxide (CO), water ice and dust.

The derived ratio of CO2 to H2O output by number of molecules is 8, among the highest ever observed. The data implies an intrinsically CO2-rich nucleus. The low abundance of H2O vapor is surprising at the object’s distance from the Sun.

The spectrum of 3I/ATLAS shows a prominent CO2 gas emission feature along with weak H2O and CO emission features and a prominent water ice absorption feature.

The inferred mass loss rates from 3I/ATLAS are 130 kilograms per second for CO2, 6.6 kilograms per second for H2O and 14 kilograms per second of CO.

The H2O mass loss rate is only 5% of the CO2 output. This is 16 times more extreme than expected for a typical comet at the same distance from the Sun.

If the optically-thin dust plume makes a small contribution to the total reddened spectrum, the flux detected by SPHEREx at a wavelength of 1 micrometer from 3I/ATLAS suggests a nucleus with a diameter of 46 kilometers (as reported here).

This implies that the mass of the nucleus of 3I/ATLAS is a million times larger than that of the previous interstellar comet 2I/Borisov. This huge gap in mass is surprising since we should have discovered numerous objects of the size of 2I/Borisov before discovering a 46-kilometer interstellar object.

Moreover, as I noted in my first paper on 3I/ATLAS (accessible here), the amount of rocky material per unit volume in interstellar space is smaller by a factor of ten thousand than the value needed to deliver into the inner Solar system one giant rock of this size over the decade-long survey conducted by the ATLAS telescope.

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The Webb image at a wavelength of 1.2 micrometer shows no cometary tail behind 3I/ATLAS.

The glow around 3I/ATLAS could originate from the reflection of sunlight by fragments of CO2 ice that 3I/ATLAS sheds rather than dust. These icy fragments evaporate in the sunlight and create the extended spherically symmetric CO2 cloud around 3I/ATLAS.

The CO2 mass loss amounts to the ablation of a millimeter thick layer from the surface of a 46-kilometer object over a period of a few years. In other words, a relatively thin outer layer is sufficient to maintain the observed cloud of CO2 gas around 3I/ATLAS.

What lies under this outer skin remains unknown.

The high CO2 to H2O ratio is puzzling. There is only one previous comet with a similarly extreme CO2 to H2O ratio, named C/2016 R2, but its image shows a clear cometary tail that does not look anything like the plume around 3I/ATLAS.

The Webb team conjectures that the anomalous composition of the gas plume surrounding 3I/ATLAS might be the result of high reflectance or reduced heat penetration through its surface.

Increasing the albedo from 5% to the maximum value of 100% for a mirror, reduces the estimated diameter from 46 to 10 kilometers based on the SPHEREx data.

This still poses an untenable demand on rocky material in interstellar space.

A way to resolve the discrepancy between the mass reservoir of rocks in interstellar space and the unexpected discovery of a large object, is that 3I/ATLAS was not drawn from a population of rocks on random trajectories but instead — its trajectory was designed to target the inner Solar system.

This possibility is consistent with the alignment of this retrograde trajectory with the orbital plane of the planets around the Sun, a coincidence of 1 part in 500 for a random occurrence (as discussed here).

Just before I left for my trip, a humoristic news article appeared with the title “Asteroid the size of 59 Avi Loebs to pass Earth on Wednesday, Aug. 27 — NASA.” To put 3I/ATLAS in perspective, a human produces about 1 kilogram of CO2 per day.

The mass loss rate from 3I/ATLAS of 130 kilogram per second amounts to the CO2 output of about 10 million people.

A space platform which measures 46 kilometers in diameter could potentially host the needed population of biological passengers if they are packed as densely as humans are on Manhattan Island.

In the Uber drive towards the Niels Bohr Institute in Copenhagen, I was glad to see a new preprint this morning advocating the search for technological signatures from interstellar objects (accessible here).

Some of the authors of this paper criticized my advocacy to consider such signatures over the past month, but when a reporter with a filming crew asked me yesterday about my response — I told her that I avoid mud wrestling because it gets everyone dirty.

Instead, I prefer to play chess … and apparently, this approach appears to be paying off. As Oscar Wilde noted: “Imitation is the sincerest form of flattery.”

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