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C/2018 F4
C2018 F4-orbit.png
motion of comet through the solar system, with 30 day motion markers
Discovery [1]
Discovered byPan-STARRS
Discovery siteHaleakala Obs.
Discovery date17 March 2018
MPC designationC/2018 F4
hyperbolic comet[2]
Orbital characteristics[2]
Epoch 23 March 2018 (JD 2458200.5)
Observation arc27 days
Perihelion3.44584±0.0029 AU
Eccentricity1.00385±0.0072[a] (JPL)
1.0130±0.0046 (MPC)
Jupiter MOID0.60 AU (90 million km)
Physical characteristics
Dimensions10–20 km (assumed)[3]

C/2018 F4 (PANSTARRS) is a hyperbolic comet (previously classified as A/2018 F4, a hyperbolic asteroid).[2] It was discovered on 17 March 2018 when it was beyond the orbit of Jupiter, 6.4 AU (960 million km) from the Sun.[1] It was quite far from the Sun and turned out to simply be an asteroidal object that was discovered before cometary activity was noticeable. As perihelion (closest approach to the Sun) is inside the orbit of Jupiter, this object should become more active. In April 2018 it was determined to be a hyperbolic comet.[4] Given that the incoming velocity was similar to that of an Oort cloud object, we can very confidently say that it is not of interstellar origin.


Path of comet across sky, with 30 day markers, visible over the south pole near perihelion

It would be no problem to fit a parabolic orbit to the C/2018 F4 data,[2][a] as used to be done for most short arc comets. The orbital eccentricity is decently constrained at 1.0038±0.0072,[a] so it could even be a closed orbit with an eccentricity below 1.[2] The velocity of the object currently has an uncertainty of ±0.2 km/s. With a common Oort cloud velocity of roughly 3 km/s (6,700 mph) when inbound 200 AU from the Sun, there is no reason to think 2018 F4 is of interstellar origin.

It will come to perihelion around 3 December 2019 when it will be 3.4 AU from the Sun.[2]

Further observations will be necessary to determine if the orbit is hyperbolic. As a comet this object became known as C/2018 F4 (PANSTARRS).

The somewhat short observation arc of 35 days suggests that before entering the planetary region of the Solar System (epoch 1950), the comet had an orbital period on the order of a hundred thousand years. The heliocentric eccentricity became greater than 1 in November 2016 when the comet was 9.6 AU (1.44 billion km) from the Sun.

Inbound velocity at 200 AU from the Sun
compared to Oort cloud objects[5]
Object Year Velocity
# of observations
and obs arc
90377 Sedna 1746 1.99±0.00 196 in 9240 days
C/1980 E1 (Bowell) 1765 2.95±0.00 179 in 2514 days
C/1997 P2 (Spacewatch) 1779 2.96±0.01 94 in 49 days
C/2010 X1 (Elenin) 1798 2.96±0.00 2222 in 235 days
C/2012 S1 (ISON) 1801 2.99±0.00 6514 in 784 days
C/2008 J4 (McNaught) 1855 4.88±5.44 22 in 15 days
C/2018 F4 1799 2.89±0.79 144 in 35 days
C/1999 U2 (SOHO) 1947 17±447 41 in 1 day
1I/2017 U1 (ʻOumuamua) 1982 26.49±0.03 121 in 34 days

In the above table, the only outlier is ʻOumuamua with a confident 34 day observation arc. C/2008 J4 (McNaught) has a short observation arc with large uncertainties. C/1999 U2 (SOHO) has a meaningless 1 day observation arc. 2018 F4 has a common Oort cloud velocity when 200 AU from the Sun.


  1. ^ a b c A/2018 F4 could have an orbital eccentricity as low as 0.996. (Math: 1.0039-0.0072)


  1. ^ a b "MPEC 2018-F139: A/2018 F4". IAU Minor Planet Center. 2018-03-28. Retrieved 2018-03-28. (AK18F040)
  2. ^ a b c d e f "JPL Small-Body Database Browser: (A/2018 F4)" (2018-04-13 last obs.). Jet Propulsion Laboratory. Archived from the original on 28 March 2018. Retrieved 17 April 2018.
  3. ^ "Asteroid Size Estimator". Jet Propulsion Laboratory. Retrieved 28 March 2018.
  4. ^ "MPEC 2018-H21 : COMET C/2018 F4 (PANSTARRS)". IAU Minor Planet Center. 2018-04-17. Retrieved 2018-04-17. (CK18F040)
  5. ^ "HORIZONS Web-Interface". Solar System Dynamics Group. JPL. Results produced with JPL Horizons On-Line Ephemeris System using 2018-Apr-29. Observer Location: "@sun" / Table settings: "20. Observer range & range-rate", "39. Range & range-rate 3-sigmas". Columns: deldot, RNGRT_3sig

External links