BAA Comet Section : Comets discovered in 2021

Updated 2021 January 16


  • 2021 A1 (Leonard)
  • 2021 A2 (NEOWISE)
  • 2021 A3 (P/STEREO)
  • 2021 A4 (NEOWISE)
  • 2021 A5 (P/PanSTARRS)
  • 2021 A6 (PanSTARRS)
  • 2021 A7 (NEOWISE)

  • When observing a comet please try to forget how bright you think the comet should be, what it was when you last viewed it, what other observers think it is or what the ephemeris says it should be.

    The equations for the light curves of comets that are currently visible use only the raw observations and should give a reasonable prediction for the current brightness. If the comet has not yet been observed or has gone from view a correction for aperture is included, so that telescopic observers should expect the comet to be fainter than given by the equation. The correction is about 0.033 per centimetre. Values for the r parameter given in square brackets [ ] are assumed. The form of the light curve is either the standard m = H0 + 5 log d + K0 log r or the linear brightening m = H0 + 5 log d + L0 abs(t - T + D0) where T is the date of perihelion, t the present and D0 an offset, if L0 is +ve the comet brightens towards perihelion and if D0 is +ve the comet is brightest prior to perihelion.

    Observations of comets in 2021 are given in ICQ format. 

    Full details of recently discovered objects will not appear until they are available on the CBAT web pages. The actual accuracy of preliminary orbits is often (nearly always) much worse than the published accuracy implies.  In part this is because each orbital solution is treated as a mathematical construct and does not take account of observational error.  JPL does publish the errors, whereas the MPECs do not.


    2021 A1 (Leonard)
    Gregory Leonard discovered a comet of 19th magnitude in images taken with the 1.5m reflector of the Mt Lemmon Survey on 2021 January 3.54. It was placed on the PCCP as C4AGJ62. There were prediscovery observations from Catalina Sky Survey (2020 December, 2021 January), Mt Lemmon Survey (2020 April, November), PanSTARRS (2020 April, May, June, August) and the Szeged Asteroid Program, Hungary (2020 November). [CBET 4907, MPEC 2021-A99, 2021 January 10]. The comet is at perihelion at 0.6 au in 2022 January. The comet will pass 0.23 au from the earth on 2021 December 12 and 0.0286 au from Venus on 2021 December 18.  The comet could come within visual range in 2021 October and reach 4th magnitude in December during its approach to Earth.  It then goes into solar conjunction and becomes poorly placed for northern observers.  Southern hemisphere observers will see it fading from 5th magnitude after conjunction later in December.
    2021 A2 (NEOWISE)
    An approximately 15th magnitude comet was discovered from the NEOWISE satellite on 2021 January 3.02. It was posted on the PCCP as N00ht7m. Ground-based observers confirmed the cometary nature, with Michael Mattiazzo reporting it as bright as 12th magnitude in his 0.2 m reflector. [CBET 4908, MPEC 2021-A100, 2021 January 10] It is at perihelion at 1.4 au in 2021 January. A southern circumpolar object at discovery, it is rapidly moving north and could brighten by a further magnitude.
    2021 A3 (P/STEREO)
    An object was discovered by the Zwicky Transient Factory on January 5.1 and posted on the PCCP as ZTF01on. It was summarily removed on January 8 and noted to be P/2016 J3. The previous evening Maik Meyer had discovered the identity, computed a linked orbit and informed the MPC and CBAT.  Maik's linked orbit shows that the period is 4.67 years, with perihelion at 0.53 au on January 25.  Despite this it has not been designated 2021 A1.  Michael Jaeger has imaged it, finding it to be around 14th magnitude.

    Sam Dean notes:

    It's caught in a Kozai resonance with Jupiter that it's currently on the higher-e, lower-i leg of. I think that'll peak around 2400-2500 before cycling back. Like plenty of Earth-crossing Kozai oscillating objects, its orbit also crosses Earth sometimes, creating a potential for meteor showers. It last crossed Earth's orbit within 0.1 au in the 1300s, coming as close as 0.07 au - and it will next do so in the 2200s/2300s, coming less than 0.01 au around 2300, where it should create a fairly regular and impressive meteor shower considering that it would be even more active than it is now.
    Sam also suggests that the absolute magnitude is very faint, around 23 and that it brightens rapidly, perhaps at 20 log r.

    Rather belatedly the CBAT issued CBET 4911 on January 11, with a revision coming 15 minutes later.  This gives a similar account to that given above and notes that the comet will pass Jupiter at 0.9 au in 2031 December.  It also hints that either the comet shows strong non-gravitational forces or that the 2016 positions are somewhat out. The MPC finally issued MPEC 2021-A157 on January 12. This does not give elements for 2016, although the published elements do use observations from 2016, so it may just be an oversight.  Perhaps because of the delays it also briefly appeared on the PCCP as P11c5Dk.
    2021 A4 (NEOWISE)
    An approximately 20th magnitude comet was discovered from the NEOWISE satellite on 2021 January 3.92. It was posted on the PCCP as N00htdm. The MPEC suggests that it was reported by NEOWISE as being cometary, however the CBET suggests that it was reported because of interesting motion and that ground-based observers confirmed the cometary nature. There were prediscovery observations from PanSTARRS on January 3.6. [CBET 4914, MPEC 2021-A207, 2021 January 15] It is at perihelion at 1.1 au in 2021 March. It appears to be an intrinsically faint object and if so won't come within visual range. Discovery magnitudes of NEOWISE objects are often fainter than seen by visual observers.
    2021 A5 (P/PanSTARRS)
    PanSTARRS 1 discovered a 21st magnitude comet in images taken with the 1.8m Ritchey-Chretien on January 6.30. It was placed on the PCCP as P11bxkC. There were prediscovery Mt Lemmon observations from 2020 December 16. [CBET 4915, MPEC 2021-A209, 2021 January 15] The comet was at perihelion at 2.6 au in 2020 November and has a period of 5.3 years.
    2021 A6 (PanSTARRS)
    PanSTARRS 1 discovered a 21st magnitude comet in images taken with the 1.8m Ritchey-Chretien on January 8.60. It was placed on the PCCP as P11bKhA. There were prediscovery Mt Lemmon observations from 2020 November 26 and December 22. [CBET 4916, MPEC 2021-A210, 2021 January 15] The comet was at perihelion at 7.9 au in 2021 April.
    2021 A7 (NEOWISE)
    An approximately 20th magnitude comet was discovered from the NEOWISE satellite on 2021 January 9.11. It was posted on the PCCP as N00hue0. [CBET 4917, MPEC 2021-A211, 2021 January 15/16] It is at perihelion at 2.0 au in 2021 July.  The CBET suggests that there is a possibility that it may be an intermediate period comet.
    Ephemerides of current comets are available on the CBAT ephemeris page and positions of newly discovered comets are on the Possible Comet Confirmation Page.
    More information on LINEAR. A list of comets discovered by selected search programs.
    The Northumberland refractor is the telescope that was used in the search for Neptune. It now has a 0.30-m f20 doublet lens which gives a stellar limiting magnitude of around 15 at the zenith on good nights. The Thorrowgood refractor was built in 1864 and has a 0.20-m f14 doublet lens; it is currently out of action as the dome has stuck.
    Published by Jonathan Shanklin. Jon Shanklin - jds@ast.cam.ac.uk