3I/ATLAS was spotted by NASA's TESS long before Atlas telescope. It saw no coma. Why?

Interstellar comet 3I/ATLAS has been a topic of discussion ever since it was discovered on July 1 this year. Scientists have been intrigued by the visitor from outer space because of its strange behaviour, such as its speed, light and size, all of which have baffled them. First spotted by the Atlas telescope in Chile, scientists also found observations taken by NASA's Transiting Exoplanet Survey Satellite (TESS) way back in May-June. A group of scientists from the American Astronomical Society got together to analyse the data of TESS on 3I/Atlas to look for an extended coma that would be a clear indication of cometary activity. However, they did not find a clear indicator. In a new paper, the researchers stated that "a faint extended coma" was seen in follow-up observations after July 1. Then, scientists started looking for pre-discovery images and found that TESS had recorded 3I/Atlas over 20 days between May 7 and June 1.

It was previously revealed that the object displayed sudden brightening by mid/late May. The researchers in the latest analysis also spotted the same, a behaviour spotted by other ground-based observations. They wrote, "This suggests that 3I experienced a notable increase in its activity when it was at a heliocentric distance ∼6 au." They added, "However, it is not known whether this change reflects a surge in production from a period of low-level, sustained activity, or if it represents the onset of activity from a bare nucleus."

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TESS data was used to create a deep image of 3I/ATLAS

To detect the coma, thousands of frames of images taken by TESS were co-added to produce a very deep image. This process can reveal features like the coma of a comet. They then measured the radial profile of the comet in each of the two coadded frames used and then applied the same procedures to four asteroids with comparable proper motions. They found that the profiles of comet 3I/ATLAS are comparable to those of the four asteroids, "indicating that there is no extended coma detected, even in the deep, coadded TESS data."

However, the researchers say that this does not mean that 3I/ATLAS was inactive at the time of the TESS observations since "any gas or dust emitted in late May would not be expected to produce an extended coma in the TESS observations." They added that any coma that was produced would have only become visible after the end of the TESS observations. The scientists stated that the lack of a coma detection in the TESS data means that "the observations from early May are consistent with either a bare nucleus, with very low levels of activity, or with very slow dust emission velocities."