C/2021 S1 (ATLAS)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	30 Dec 2021	18.5	6.142 AU	5.302 AU	06h23m	+57°00'	146.1°	5.1°	175°
Perihelion	2 Mar 2022	18.7	6.127 AU	5.775 AU	05h56m	+49°29'	106.3°	8.9°	98°
Today	27 Oct 2025	22.5	10.663 AU	11.119 AU	09h54m	-25°03'	60.4°	4.6°	271°

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)                : 1.0029570
    q (Perihelion distance)         : 6.1269360
    i (Inclination)                 : 52.03480
    Ω (Longitude of ascending node) : 299.85170
    ω (Argument of perihelion)      : 148.33330
    L (Longitude of perihelion)     : 279.07241
    B (Latitude of perihelion)      : 24.44904
    T (Time of perihelion passage)  : 2459640.61580

    Epoch                           : 2025 Oct 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
2025-10-26 00:00 UT   09 54 07.8   -24 56 42    11.128   10.656   59.4    4.6   270   22.5 
2025-10-27 00:00 UT   09 54 19.5   -25 01 48    11.122   10.661   60.1    4.6   271   22.5 
2025-10-27 10:01 UT   09 54 24.2   -25 03 56    11.119   10.663   60.4    4.6   271   22.5 
2025-10-28 00:00 UT   09 54 30.8   -25 06 54    11.115   10.666   60.8    4.7   271   22.5 
2025-10-29 00:00 UT   09 54 41.8   -25 12 01    11.109   10.671   61.5    4.7   272   22.5 
2025-10-30 00:00 UT   09 54 52.5   -25 17 08    11.102   10.676   62.2    4.7   273   22.5 
2025-10-31 00:00 UT   09 55 02.8   -25 22 14    11.096   10.681   62.9    4.7   273   22.5 
2025-11-01 00:00 UT   09 55 12.8   -25 27 21    11.089   10.685   63.6    4.8   274   22.5 
2025-11-02 00:00 UT   09 55 22.4   -25 32 28    11.082   10.690   64.4    4.8   275   22.5 
2025-11-03 00:00 UT   09 55 31.7   -25 37 34    11.075   10.695   65.1    4.8   275   22.5 
2025-11-04 00:00 UT   09 55 40.7   -25 42 41    11.068   10.700   65.8    4.9   276   22.5 
2025-11-05 00:00 UT   09 55 49.3   -25 47 47    11.061   10.705   66.5    4.9   277   22.5 
2025-11-06 00:00 UT   09 55 57.5   -25 52 53    11.053   10.710   67.3    4.9   277   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.