C/2021 K2 (MASTER)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	13 Jul 2021	18.7	5.478 AU	4.560 AU	19h28m	-49°42'	152.1°	5.0°	1°
Perihelion	9 Sep 2021	18.9	5.462 AU	5.069 AU	18h25m	-41°04'	107.8°	10.1°	81°
Today	13 Oct 2025	23.9	11.427 AU	11.683 AU	16h50m	+47°12'	72.8°	4.8°	57°

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. (8.0 + 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 K2 (MASTER) are:

    e (Eccentricity)                : 1.0002970
    q (Perihelion distance)         : 5.4619810
    i (Inclination)                 : 100.87200
    Ω (Longitude of ascending node) : 282.34050
    ω (Argument of perihelion)      : 343.25550
    L (Longitude of perihelion)     : 285.58839
    B (Latitude of perihelion)      : -16.43534
    T (Time of perihelion passage)  : 2459466.61640

    Epoch                           : 2025 Oct 12
    Reference                       : MPEC 2022-O01

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

Ephemerides

Date       Time       RA (2000)    DEC (2000)    delta   radius  elong  phase   PA    magn
2025-10-13 00:00 UT   16 50 07.9   +47 14 27    11.675   11.423   73.0    4.8    58   23.9 
2025-10-13 20:16 UT   16 50 20.1   +47 12 17    11.683   11.427   72.8    4.8    57   23.9 
2025-10-14 00:00 UT   16 50 22.4   +47 11 53    11.684   11.428   72.7    4.8    57   23.9 
2025-10-15 00:00 UT   16 50 37.2   +47 09 22    11.693   11.433   72.5    4.8    56   23.9 
2025-10-16 00:00 UT   16 50 52.4   +47 06 54    11.701   11.439   72.3    4.8    55   23.9 
2025-10-17 00:00 UT   16 51 07.9   +47 04 29    11.710   11.444   72.1    4.8    54   23.9 
2025-10-18 00:00 UT   16 51 23.8   +47 02 07    11.718   11.449   71.9    4.7    53   23.9 
2025-10-19 00:00 UT   16 51 40.1   +46 59 48    11.727   11.454   71.8    4.7    52   23.9 
2025-10-20 00:00 UT   16 51 56.6   +46 57 32    11.735   11.459   71.6    4.7    51   23.9 
2025-10-21 00:00 UT   16 52 13.5   +46 55 20    11.743   11.465   71.4    4.7    50   23.9 
2025-10-22 00:00 UT   16 52 30.8   +46 53 10    11.751   11.470   71.2    4.7    49   23.9 
2025-10-23 00:00 UT   16 52 48.3   +46 51 04    11.759   11.475   71.0    4.7    48   23.9 

    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.