C/2017 K4 (ATLAS)
ephemeris	date	magn	radius	delta	ra	dec	elong	phase	PA
Nearest approach	3 Jul 2017	16.8	3.244 AU	2.316 AU	16h52m	-36°18'	151.2°	8.7°	71°
Perihelion	7 Jan 2018	16.9	2.664 AU	3.570 AU	20h32m	-15°28'	19.6°	7.1°	67°
Today	24 Jan 2026	28.7	18.321 AU	17.448 AU	06h29m	+29°41'	151.7°	1.5°	105°

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. This lightcurve is being recalculated every 6 hours based on the available COBS/MPC observations (currently 9.8 + 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/2017 K4 (ATLAS) are:

    e (Eccentricity)                : 0.9072110
    q (Perihelion distance)         : 2.6635700
    i (Inclination)                 : 16.65690
    Ω (Longitude of ascending node) : 298.92790
    ω (Argument of perihelion)      : 15.35860
    L (Longitude of perihelion)     : 313.67067
    B (Latitude of perihelion)      : 4.35404
    T (Time of perihelion passage)  : 2458126.44910
    P (Orbital period in years)     : 153.80

    Epoch                           : 2023 Jan 31
    Reference                       : MPC114598

    Classification(s):              : Nearly isotropic; Returning (a < 10000 AU); Halley type (P < 200 years)

Ephemerides

Date       Time       RA (2000)    DEC (2000)    delta   radius  elong  phase   PA    magn
2026-01-23 00:00 UT   06 30 02.8   +29 42 07    17.430   18.315  153.2    1.4   106   28.7 
2026-01-24 00:00 UT   06 29 50.4   +29 41 58    17.442   18.319  152.2    1.4   105   28.7 
2026-01-24 11:45 UT   06 29 44.4   +29 41 53    17.448   18.321  151.7    1.5   105   28.7 
2026-01-25 00:00 UT   06 29 38.2   +29 41 49    17.454   18.323  151.2    1.5   105   28.7 
2026-01-26 00:00 UT   06 29 26.2   +29 41 39    17.467   18.328  150.1    1.5   104   28.7 
2026-01-27 00:00 UT   06 29 14.3   +29 41 29    17.479   18.332  149.1    1.6   104   28.7 
2026-01-28 00:00 UT   06 29 02.6   +29 41 18    17.492   18.336  148.1    1.6   103   28.7 
2026-01-29 00:00 UT   06 28 51.0   +29 41 07    17.506   18.340  147.0    1.7   103   28.7 
2026-01-30 00:00 UT   06 28 39.6   +29 40 55    17.519   18.344  146.0    1.7   102   28.7 
2026-01-31 00:00 UT   06 28 28.4   +29 40 43    17.533   18.348  145.0    1.8   102   28.7 
2026-02-01 00:00 UT   06 28 17.3   +29 40 30    17.547   18.353  143.9    1.8   102   28.7 
2026-02-02 00:00 UT   06 28 06.4   +29 40 17    17.561   18.357  142.9    1.9   101   28.7 
2026-02-03 00:00 UT   06 27 55.7   +29 40 03    17.575   18.361  141.9    1.9   101   28.7 

    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.