C/2021 S1 (ATLAS)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	30 Dec 2021	18.5	6.143 AU	5.302 AU	06h23m	+57°00'	146.1°	5.1°	175°
Perihelion	2 Mar 2022	18.7	6.127 AU	5.776 AU	05h56m	+49°28'	106.2°	8.9°	98°
Today	27 Nov 2025	22.5	10.815 AU	10.880 AU	09h57m	-27°40'	83.6°	5.2°	291°

The interactive orbit chart above shows the comet's path through the solar system and its position at the given date. Green and blue lines are shown perpendicular to the ecliptic plane: Green if the path is above the ecliptic plane, blue if it is below. (Left-click and drag to rotate the view; Right-click and drag to move the view; Use scroll wheel to zoom in our out.)

Light curve

The light curve chart below shows the estimated development of the comet's magnitude. Blue and black dots are visual and photometric CCD observations from COBS. The light curve is based on the absolute magnitude and slope parameter as calculated from an MPEC, or the latest values provided by the minor planet center. (7.0 + 5 log[∆] + 10.0 log[r]). An additional green curve is displayed when an increase in apparent brightness is expected due to forward scattering of sunlight, which occurs when a dust-rich comet is located between the Earth and the Sun. (See Marcus 2007)

Charts

The all-sky chart below shows the path of the comet over the same period as the light curve. The comet's current position is marked yellow.

The following chart shows the short-term path of the comet in a field of view that is optimized for (~10x50) binoculars and finderscopes.

Orbital elements

The orbital elements of C/2021 S1 (ATLAS) are:

    e (Eccentricity)                : 1.0029620
    q (Perihelion distance)         : 6.1274380
    i (Inclination)                 : 52.03440
    Ω (Longitude of ascending node) : 299.85100
    ω (Argument of perihelion)      : 148.33930
    L (Longitude of perihelion)     : 279.07599
    B (Latitude of perihelion)      : 24.44448
    T (Time of perihelion passage)  : 2459640.63440

    Epoch                           : 2025 Nov 26
    Reference                       : MPEC 2023-J29

    Classification(s):              : Nearly isotropic; New (a > 10000 AU)

Ephemerides

Date       Time       RA (2000)    DEC (2000)    delta   radius  elong  phase   PA    magn
2025-11-26 00:00 UT   09 57 23.9   -27 32 44    10.893   10.807   82.4    5.2   290   22.5 
2025-11-27 00:00 UT   09 57 24.1   -27 37 32    10.885   10.812   83.2    5.2   290   22.5 
2025-11-27 13:30 UT   09 57 24.0   -27 40 14    10.880   10.815   83.6    5.2   291   22.5 
2025-11-28 00:00 UT   09 57 23.9   -27 42 19    10.876   10.817   84.0    5.2   291   22.5 
2025-11-29 00:00 UT   09 57 23.3   -27 47 04    10.868   10.822   84.7    5.2   291   22.5 
2025-11-30 00:00 UT   09 57 22.3   -27 51 47    10.859   10.827   85.5    5.2   292   22.5 
2025-12-01 00:00 UT   09 57 20.9   -27 56 29    10.850   10.832   86.3    5.2   293   22.5 
2025-12-02 00:00 UT   09 57 19.0   -28 01 09    10.842   10.837   87.1    5.2   293   22.5 
2025-12-03 00:00 UT   09 57 16.8   -28 05 47    10.833   10.841   87.8    5.2   294   22.5 
2025-12-04 00:00 UT   09 57 14.2   -28 10 23    10.825   10.846   88.6    5.2   295   22.5 
2025-12-05 00:00 UT   09 57 11.1   -28 14 57    10.816   10.851   89.4    5.2   295   22.5 
2025-12-06 00:00 UT   09 57 07.7   -28 19 29    10.808   10.856   90.2    5.2   296   22.5 
2025-12-07 00:00 UT   09 57 03.8   -28 23 59    10.799   10.861   91.0    5.2   296   22.5 

    Terminology:
            
    delta:  distance between comet and earth in AU
    radius: distance between comet and sun in AU
    magn:   magnitude (brightness) estimate    
    ra:     right ascension in hours (24h = 360deg)
    dec:    declination in degrees
    elong:  elongation in degrees (angle sun-earth-comet)    
    phase:  phase angle in degrees (angle sun-comet-earth)        
    AU:     Astronomical Unit (mean distance between earth and sun: 149597870.7 km      
    

Orbital elements usually provided by the MPC (Minor Planet Center).
Observations contributed by observers worldwide, via COBS (Comet Observation Database) or the MPC (Minor Planet Center)
Calculations by a modified version of AAPlus, a C# implementation of the AA+ project by PJ Naughter from the algorithms presented in the book "Astronomical Algorithms" by Jean Meeus.
NGC2000 dso catalog and star labels from VizieR as provided by the Strasbourg astronomical Data Center.
Tycho2 catalog from the ESO archive.