Comet C/2025 R2 (SWAN) displays an anti-tail
ATel #17505; Federico Manzini (Stazione Astronomica di Sozzago), Virginio Oldani (Stazione Astronomica di Sozzago), Paolo Ochner (UniPD, INAF-OAPd), Andrea Reguitti (INAF-OAPd), Luigi R. Bedin (INAF-OAPd), Claudio Balcon (Stazione Astronomica di Belluno)
on 21 Nov 2025; 16:51 UT
Credential Certification: Andrea Reguitti (andreareguitti@gmail.com)
The long-period comet C/2025 R2 (SWAN) has already been the subject of two Astronomer's
Telegrams (ATel #17462 and #17469). It recently showed a significant increase in brightness
followed by signs of fragmentation. Orbital elements indicate a period of about 784 years (see JPL
Horizons system at https://ssd.jpl.nasa.gov/horizons/).
The geometric configuration between Earth, the comet, and the Sun determines the visibility of
particular structures in the coma and tail. Near the comet's orbital plane, the dimensions of the tail
can be observed in its perpendicular plane, where the dust is only minimally affected by the orbital
velocity vector. At particular elongation angles, when the Earth crosses or approaches the comet's
orbital plane and the line of sight is close to it, comets can also display the rare phenomenon of an
anti-tail. This feature is formed by larger dust particles, which are less affected by solar radiation
pressure and tend to remain confined near the orbital plane and project as a sunward-pointing
feature.
The Earth-Sun-comet geometry for C/2025 R2 has been favorable for the appearance of an anti-
tail since mid-November. Earth crossed the orbital plane of C/2025 R2 on September 10, 2025,
and will remain close to it (PlAng between -5° and -4°) until December 5, 2025.
Our monitoring began in early November, but no hints of anti-tail were detected in our observations
of November 3 and 9 (67/92-cm Schmidt telescope - Asiago Astrophysical Observatory, INAF-
OAPd, Italy), and November 4 (0.4-m telescope, Stazione Astronomica di Belluno). The first signs
of a presence of an anti-tail were visible in r-band images taken with the 1.82-m Copernico
Telescope (Asiago Astrophysical Observatory, INAF-OAPd, Italy) on November 10, 2025, when
Earth was at PlAng=-7.8°. A confirmation of the anti-tail was obtained on 18 November 2025,
using r-band images (14x180s) from the 0.4-m telescope at the Stazione Astronomica di Sozzago (Italy),
with Earth at PlAng=-6.2°.
Brightness measurements at distances of approximately 84,000 and 124,000 km from the nucleus
(performed on polar-projection processed images centered on the comet optocenter) showed two
intensity peaks at PA=75°±1° and PA=234°±1.5°, corresponding to the normal dust tail and the
anti-tail, respectively.
At the observation date, these angles are consistent with the Horizons ephemeris values:
PsAng=71.0°, the position angle of the Sun-comet vector, which approximates the gas tail direction
(anti-sunward); PsAMV=244°, which indicates the expected orientation of the dust tail (opposite to
the comet's heliocentric velocity vector), phase angle (STO) 36°.
The anti-tail of C/2025 R2 is expected to become more condensed in the near future, as the
viewing angle with respect to the orbital plane continues to decrease. However, the comet is also
moving rapidly away from Earth, and the feature may gradually fade.
Figures and captions are available at:
