A comet from the "Jupiter family" has been discovered!

Author | Zhao Jingyuan

Review | Zhang Mi, Sun Guoyou, Zhou Wenjie, Gao Xing

Editor | Zhao Jingyuan

On March 9, 2024, in the NAS asteroid search project (NOWT Asteroid Survey) jointly conducted by the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences and the Xingming team, Liao Xi, a member of the Xingming team, found a moving target in the data taken by the 1-meter optical telescope (NOWT) at the Nanshan Observatory of the Xinjiang Astronomical Observatory on the evening of the 8th. At that time, its brightness was about 20 magnitude. Subsequently, Zhang Mi, Sun Guoyou, Yang Zesheng, Zhou Wenjie and others conducted further verification and submitted the data to the International Minor Planet Center (MPC). That night, the Xingming team used a half-meter telescope (HMT) to conduct follow-up observations of the target. Zhou Wenjie, Chen Zhuoyang, Zhang Mi, Yang Zesheng, Liao Xi and others conducted measurements, and the data obtained were also submitted to the MPC.

NAS discovery image (left) and HMT subsequent observation image (right)

On April 11, German amateur astronomer Robson Hahn independently discovered the object, and he noticed that its orbit was somewhat special, so he reported it to the MPC's Near-Earth Object Candidate Page (NEOCP). On April 14, the object received the temporary asteroid number 2024 FG9 awarded by the MPC. Because it crosses the orbit of Mars and its perihelion distance does not meet the requirements of a near-Earth asteroid, it is classified as a Mars-crossing asteroid.

Subsequently, Alan Hale, an American astronomer and one of the discoverers of the famous Hale-Bopp Comet, inferred from the orbit that 2024 FG9 might be a comet. With his reminder, some professional/amateur astronomers traced back historical survey data to analyze the orbital changes of the target. The orbit obtained according to these historical data shows that the aphelion of the target is close to the orbit of Jupiter, and the perihelion is near the orbit of Mars, which makes it very susceptible to the gravitational influence of Jupiter and Mars, and the orbit often changes significantly. These historical data can be traced back to 2018, in which American amateur astronomer Sam Deen noticed that the target showed obvious comet activity, with a coma with a diameter of 1.8 arc seconds and a tail about 3 arc seconds long.

On April 17, Sam Deen noticed in his images that day that 2024 FG9 showed cometary activity, with a coma of 3 arc seconds in diameter and possibly a tail of about 4 arc seconds in length. Since then, several observers, including Alan Hale, have obtained more images in which the object has more or less cometary characteristics, which further confirmed that it is a comet.

Image taken by Alan Hale on April 28

At 12:41 on June 1, MPC issued the 2024-L04 Electronic Notice (MPEC 2024-L04), officially announcing that 2024 FG9, which was previously believed to be an asteroid, is actually a comet. The NAS team that first discovered and reported it and Robson Hahn, who first reported its special orbit, shared this discovery and renumbered it and named it P/2024 FG9 (Nanshan-Hahn), which is translated into Chinese as Nanshan-Hahn Comet. That evening, the Central Bureau of Astronomical Telegrams (CBAT) issued the 5401 Electronic Notice (CBET 5401), which introduced the discovery process of the comet in more detail.

Partial screenshot of MPEC 2024-L04. For the full content, see Reference 1

Partial screenshot of CBET 5401, see reference 2 for full content

Comets are an important type of celestial body in the solar system, usually composed of ice, rock and dust. They are believed to be the material left over from the formation of the solar system, which helps to understand the early conditions of the solar system and is of great significance for studying the origin of the solar system. Comets with an orbital period of less than 200 years around the sun are usually called short-period comets, and those with an orbital period of more than 200 years are called long-period comets. The newly discovered Nanshan-Hahn Comet is a Jupiter-family comet. These comets are all short-period comets with an orbital period of less than 20 years, an aphelion near the orbit of Jupiter and an orbital inclination of less than 30 degrees. At present, the orbital period of Comet Nanshan-Hahn is about 5.88 years, and the orbital inclination is about 1.73 degrees. Its closest distance to the Sun is about 1.6 astronomical units (close to the average distance between Mars and the Sun, 1.52 astronomical units), and its farthest distance is about 4.92 astronomical units (close to the average distance between Jupiter and the Sun, 5.2 astronomical units). The eccentricity is about 0.51, and it orbits the Sun in an elliptical orbit. It is a very typical Jupiter-family comet.

Orbit diagram of Comet Nanshan-Hahn. Image credit: JPL

As mentioned earlier, since Comet Nanshan-Hahn is easily affected by Jupiter's gravity, its orbit often changes. On April 30, 2021, it passed by Jupiter at a distance of 0.2 AU. As a result, its perihelion shortened from 1.7 AU to 1.6 AU, and its orbital period changed from 6.00 years to 5.88 years. It is predicted that on April 16, 2033, Comet Nanshan-Hahn will pass by Jupiter at a distance of 0.2 AU again, at which time its perihelion will increase to 1.8 AU, and its orbital period will change to 6.22 years.

Comet Nanshan-Hahn passed perihelion on May 20 this year, reaching a maximum brightness of 17.5 magnitude, and will slowly dim. It is currently located in Cancer. Taking Wuhan as an example, its altitude is about 20 degrees after dark. We encourage all enthusiasts to observe it.

The trajectory of Comet Nanshan-Hahn in the sky from June 2 to August 1, 2024. Image source: Stellarium

Since December 2022, Xinjiang Astronomical Observatory has cooperated with the Xingming team to carry out the NAS asteroid search project, which is a professional-amateur cooperation project (Pro-Am). The project has previously discovered two near-Earth asteroids, 2023 DB2 and 2023 VB2. The discovery of Comet Nanshan-Hahn is another milestone in the project. This is the first comet named "Nanshan", making Xinjiang Astronomical Observatory the third Chinese Academy of Sciences unit to discover a comet; at the same time, the Xingming team discovered a comet again after 9 years, bringing the number of comets discovered by the Xingming team to 4. I would like to express my sincere congratulations to all the discoverers and the NAS team!

Those who participated in the discovery of Comet Nanshan-Hahn included Yumiti Mahmuti, Feng Guojie (all of whom are staff members of Xinjiang Astronomical Observatory), Liao Xi, Zhang Mi, Sun Guoyou, Gao Xing, Yang Zesheng, Zhou Wenjie (all of whom are members of the Xingming team) and the NAS asteroid search project team.

References

1. MPEC 2024-L04: Comet P/2024 FG9 (Nanshan-Hahn), MPC, https://minorplanetcenter.net/mpec/K24/K24L04.html

2. CBET 5401: 20240601: Comet P/2024 FG9 (Nanshan-Hahn), CBAT, http://www.cbat.eps.harvard.edu/iau/cbet/005400/CBET005401.txt

3. P/2024 FG9, MPC, https://minorplanetcenter.net/db_search/show_object?utf8=%E2%9C%93&object_id=P%2F2024+FG9

4. P/2024 FG9, JPL, https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=P%2F2024%20FG9&view=VOP

5. P/2024 FG9, http://astro.vanbuitenen.nl/comet/2024FG9

6. 2024 FG9, comets-ml mailing group, https://groups.io/g/comets-ml/topic/2024_fg9/105559594

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