C/2021 A1 (Leonard)
Comet 2021 A1 (Leonard).jpg
C/2021 A1 (Leonard) on December 28, 2021
Discovered byGreg J. Leonard
Discovery date3 January 2021[1]
Orbital characteristics A
Observation arc526 days
Orbit typelong period (inbound)
hyperbolic (outbound)
Aphelion≈3700 AU (barycentric epoch 1950)[2]
Perihelion0.6151 AU
Eccentricity0.99966 (barycentric epoch 1950)[2]
1.00004 (barycentric epoch 2100)
Orbital period≈80000 yr (inbound)[2]
Argument of
Earth MOID0.231 AU (34.6 million km)[3]
Jupiter MOID0.296 AU (44.3 million km)
Comet total
Last perihelion3 January 2022

C/2021 A1 (Leonard) was a long period comet[4] that was discovered by G. J. Leonard at the Mount Lemmon Observatory on 3 January 2021 (a year before perihelion) when the comet was 5 AU (750 million km) from the Sun.[1] It had a retrograde orbit. The nucleus was about 1 km (0.6 mi) across. It came within 4 million km (2.5 million mi) of Venus, the closest-known cometary approach to Venus.[5]

Observational history

The comet was discovered by G. J. Leonard at the Mount Lemmon Observatory on 3 January 2021 (a year before perihelion) when the comet was 5 AU (750 million km) from the Sun. The comet appeared to have a 10" coma and a 5" broad tail in the photographs taken as part of the Mt. Lemmon Survey and its magnitude was estimated to be 19.0. Precovery images dated back to April 11, 2020.[1]

On 10 October the comet showed a short but dense dust tail.[6] In early December the comet had an apparent magnitude (coma+nucleus) of around 6.[7] The first reports of naked-eye observations by experienced observers started coming in on 5 December 2021.[8] Much like observing Messier 33, the low surface brightness of the comet can make it difficult to observe near urban areas. On the morning of 6 December 2021 the comet was about 5 degrees from the star Arcturus. On 14 December 2021 the comet was 14.7 degrees from the Sun[9] and quickly became better seen from the southern hemisphere.

The forward scattering of light helped the comet to briefly brighten to as much as magnitude 2.5,[7][10][11] but was also enhanced by a modest outburst.[12] The comet experienced outbursts on December 15, 20 and 23, thus reaching third magnitude before dimming back to 4th magnitude. The ion tail of the comet appeared complex, with knots and steamers.[13] The comet's discoverer called the tails "some of the best ever observed".[14] In stacked photos the tail could be traced for 60 degrees in the sky.[15] While the comet was lower in the sky, atmospheric extinction offset much of the brightening.[citation needed] As of 22 December 2021, the comet was around apparent magnitude 4, making it a good binocular comet for the Southern hemisphere.[7][13] It was the brightest comet of 2021.[14]

On 23 February 2022 the comet was observed with the SLOOH telescope in Chile, operated by Martin Masek. The comet lacked a central concetration, which indicates that the nucleus of the comet disintegrated or evaporated completely. Further observations confirmed the lack of concetration.[14] In April 2022, the disintegrating comet was observed using the Hubble Space Telescope, with further observations planned for June 2022.[16]


Animation of C/2021 A1's orbit around Sun - 2021 close approach.mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{}   C/2021 A1 ·   Sun ·   Mercury  ·   Venus ·   Earth ·   Mars
Animation of C/2021 A1's orbit around Sun - 2021 close approach
   C/2021 A1 ·   Sun ·   Mercury  ·   Venus ·   Earth ·   Mars

C/2021 A1 has been inside of the orbit of Neptune since May 2009.[17] Using an epoch of 1950 which is well before the comet entered the planetary region of the Solar System, a barycentric orbit solution suggests the comet had an approximately 80,000-year orbital period.[2] Thus, the comet has spent the last 40,000 years inbound from approximately 3,700 AU (550 billion km) (0.06 light year). After perihelion, the comet will be ejected from the Solar System. The barycentric orbit will remain hyperbolic after September 2022.[2]

On 12 December 2021 the comet was 0.233 AU (34.9 million km) from Earth and on 18 December 2021 it was 0.028 AU (4.2 million km) from Venus.[3] It made its closest approach to the Sun on 3 January 2022.


See also


  1. ^ a b c "MPEC 2021-A99 : COMET C/2021 A1 (Leonard)". Minor Planet Center. Retrieved 2021-01-10.
  2. ^ a b c d e Horizons output. "Barycentric Osculating Orbital Elements for Comet C/2021 A1 (Leonard)". Retrieved 2021-01-13. (Solution using the Solar System's barycenter (Sun+Jupiter). Select Ephemeris Type:Elements and Center:@0) Epoch 1950 has PR= 2.91E+07 / 365.25 = 79700 years
  3. ^ a b "JPL Small-Body Database Browser: C/2021 A1". JPL. Retrieved 2021-09-29.
  4. ^ CBET 4907: original 1/a = +0.000575 (long-period) and future 1/a = -0.000030 (hyperbolic). The orbital energy is inversely proportional to negative semi-major axis. Objects in hyperbolic orbits have negative semi major axis, giving them a positive orbital energy.
  5. ^ Weule, Genelle (8 December 2021). "Astronomers anxiously watch Comet Leonard to see if it will live up to predictions as a Christmas treat over Australia". ABC News. Australian Broadcasting Corporation. Retrieved 8 December 2021.
  6. ^ "How Bright Will Comet Leonard Get?". Sky & Telescope. 2021-10-13. Retrieved 2021-10-20.
  7. ^ a b c "C/2021 A1 (Leonard)". Gideon van Buitenen. Retrieved 2021-11-16.
  8. ^ Martin McKenna naked eye
  9. ^ Horizons output. "Solar elongation Comet C/2021 A1 (Leonard) in mid-December 2021". Retrieved 2021-10-17. Sun-Observer-Target (S-O-T)
  10. ^ Magnitude estimate – Seiichi Yoshida
  11. ^ Qiсһеng Ζһаng from Pasadena on 14 Dec 2021
  12. ^ What happened to C/2021 A1 (Leonard)?
  13. ^ a b "Comet Leonard: A Gift at Christmastime". Sky & Telescope. 25 December 2021. Retrieved 17 January 2022.
  14. ^ a b c Machholz, Donald (21 March 2022). "Comet Leonard has disintegrated". earthsky.org. Retrieved 22 March 2022.
  15. ^ "APOD: 2021 December 30 - The Further Tail of Comet Leonard". apod.nasa.gov. Retrieved 17 January 2022.
  16. ^ Jewitt, David (7 March 2022). "Disintegration of Long-Period Comet C/2021 A1 - HST Proposal 16929". Space Telescope Science Institute.
  17. ^ Neptune has perihelion (closest approach to the Sun) at 29.8 AU. C/2021 A1 has been inside that distance since May 2009.