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	18 Oct 2025	28.6	17.906 AU	17.636 AU	06h47m	+29°24'	104.2°	3.1°	273°

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
2025-10-17 00:00 UT   06 47 33.5   +29 23 59    17.650   17.902  103.0    3.1   273   28.6 
2025-10-18 00:00 UT   06 47 31.4   +29 24 16    17.638   17.906  104.0    3.1   273   28.6 
2025-10-18 03:27 UT   06 47 31.1   +29 24 19    17.636   17.906  104.2    3.1   273   28.6 
2025-10-19 00:00 UT   06 47 29.0   +29 24 34    17.626   17.910  105.0    3.1   273   28.6 
2025-10-20 00:00 UT   06 47 26.4   +29 24 51    17.614   17.914  106.0    3.1   273   28.6 
2025-10-21 00:00 UT   06 47 23.5   +29 25 09    17.602   17.919  107.0    3.0   273   28.6 
2025-10-22 00:00 UT   06 47 20.3   +29 25 27    17.590   17.923  108.0    3.0   273   28.6 
2025-10-23 00:00 UT   06 47 16.9   +29 25 45    17.578   17.927  109.0    3.0   272   28.6 
2025-10-24 00:00 UT   06 47 13.2   +29 26 03    17.566   17.931  110.0    3.0   272   28.6 
2025-10-25 00:00 UT   06 47 09.3   +29 26 22    17.554   17.936  111.0    3.0   272   28.6 
2025-10-26 00:00 UT   06 47 05.1   +29 26 40    17.543   17.940  112.0    2.9   272   28.6 
2025-10-27 00:00 UT   06 47 00.6   +29 26 59    17.531   17.944  113.0    2.9   272   28.6 
2025-10-28 00:00 UT   06 46 55.9   +29 27 18    17.520   17.948  114.0    2.9   272   28.6 

    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.