Metadata

Parameter	Value
Designation	1P/Halley
Type	P
Perihelion (tp_cal, UTC)	1986-02-08T11:22:00.431040Z
Orbit solution date (soln_date, UTC)	2025-11-21T15:57:34Z
Obs used	8518
NEO	Yes
M1	5.500
K1	8.000
PC	0.030

Osculating Elements

Parameter	Value
Epoch (epoch_cal, TDB)	1968-01-20.0000000
e	0.96793600
q (au)	0.574864
a (au)	17.928635
i (deg)	162.190530
Ω (om, deg)	59.098947
ω (w, deg)	112.241431
M (ma, deg)	274.382337

Description

The first predicted periodic comet; the earliest known record appears in China’s Shiji (Records of the Grand Historian) in 240 BCE.

Magnitude

Scatter + Global Linear

$$ M_a = (3.661 + 3.876\cdot 2.5\log_{10} r) + \epsilon - 2.5\log_{10}\Phi(\theta) + 5\log_{10}\Delta $$ $$ \theta = 180^\circ - \alpha $$ $$ \Phi(\theta) = \frac{\delta_{90} D(\theta) + 1}{\delta_{90} + 1},\quad D(\theta) = k\cdot \Phi_{HG}(\theta; g_f) + (1-k)\cdot \Phi_{HG}(\theta; g_b) $$ $$ \Phi_{HG}(\theta; g) = \left(\frac{1+g^2}{1+g^2-2g\cos\theta}\right)^{3/2},\quad \Phi_{HG}(90^\circ)=1 $$

Model Parameters

Parameter	Value
H0	3.661
n	3.876
gf	1.000
gb	-0.500
k	1.000
δ90	10.000

Parameter	Symbol	Range	Description
Absolute magnitude at 1 AU from Sun and Earth	H0	-5 to 20	Intrinsic brightness of the comet
Photometric slope parameter	n	2 to 6	Rate of brightness change with heliocentric distance
Forward scattering asymmetry factor	gf	0.8 to 1	Degree of forward scattering
Backward scattering asymmetry factor	gb	-1 to -0.5	Degree of backward scattering
Partitioning coefficient	k	0.8 to 1	Ratio between forward/backward scattering contributions
Dust-to-gas brightness ratio	δ90	1 to 10	Relative contribution of dust scattering to gas emission

Reference: EPSC-DPS2025-160

Residuals (Piecewise)

$$\epsilon = a \log_{10} r_{\mathrm{au}} + b$$

Start (UTC)	End (UTC)	a	b
1981-12-02	1985-11-05	+6.7140	-0.431
1985-11-05	1986-01-28	+2.0298	+0.388
1986-02-15	1986-07-08	-2.7942	-0.455
1986-07-08	1987-12-12	+0.0819	-1.319
1987-12-12	2003-03-08	+3.7502	-4.319

log10 r = log10(heliocentric distance / AU), Δ = geocentric distance (AU)

Ephemeris

Date/Time (UTC)	RA (J2000)	Dec (J2000)	Nearest HIP	Dist from HIP (arcmin)
2026-09-20 00:00	08:19:40.02	+03:04:02.8	HIP 40893	28.61
2026-09-20 12:00	08:19:41.76	+03:03:45.6	HIP 40893	28.14
2026-09-21 00:00	08:19:43.49	+03:03:28.4	HIP 40893	27.67
2026-09-21 12:00	08:19:45.19	+03:03:11.2	HIP 40893	27.20
2026-09-22 00:00	08:19:46.87	+03:02:54.0	HIP 40893	26.74
2026-09-22 12:00	08:19:48.52	+03:02:36.8	HIP 40893	26.29
2026-09-23 00:00	08:19:50.15	+03:02:19.6	HIP 40893	25.83
2026-09-23 12:00	08:19:51.77	+03:02:02.4	HIP 40893	25.39
2026-09-24 00:00	08:19:53.35	+03:01:45.2	HIP 40893	24.94
2026-09-24 12:00	08:19:54.92	+03:01:28.0	HIP 40893	24.50
2026-09-25 00:00	08:19:56.46	+03:01:10.9	HIP 40893	24.07
2026-09-25 12:00	08:19:57.98	+03:00:53.7	HIP 40893	23.64
2026-09-26 00:00	08:19:59.48	+03:00:36.6	HIP 40893	23.22
2026-09-26 12:00	08:20:00.95	+03:00:19.4	HIP 40893	22.80
2026-09-27 00:00	08:20:02.40	+03:00:02.3	HIP 40893	22.38
2026-09-27 12:00	08:20:03.83	+02:59:45.2	HIP 40893	21.97
2026-09-28 00:00	08:20:05.23	+02:59:28.1	HIP 40893	21.57
2026-09-28 12:00	08:20:06.61	+02:59:11.0	HIP 40893	21.16
2026-09-29 00:00	08:20:07.96	+02:58:54.0	HIP 40893	20.77
2026-09-29 12:00	08:20:09.29	+02:58:36.9	HIP 40893	20.37
2026-09-30 00:00	08:20:10.60	+02:58:19.9	HIP 40893	19.99
2026-09-30 12:00	08:20:11.88	+02:58:02.9	HIP 40893	19.60
2026-10-01 00:00	08:20:13.14	+02:57:46.0	HIP 40893	19.22
2026-10-01 12:00	08:20:14.37	+02:57:29.0	HIP 40893	18.85
2026-10-02 00:00	08:20:15.58	+02:57:12.1	HIP 40893	18.48
2026-10-02 12:00	08:20:16.77	+02:56:55.2	HIP 40893	18.11
2026-10-03 00:00	08:20:17.92	+02:56:38.4	HIP 40893	17.75
2026-10-03 12:00	08:20:19.06	+02:56:21.5	HIP 40893	17.39
2026-10-04 00:00	08:20:20.17	+02:56:04.7	HIP 40893	17.04
2026-10-04 12:00	08:20:21.25	+02:55:48.0	HIP 40893	16.69
2026-10-05 00:00	08:20:22.31	+02:55:31.2	HIP 40893	16.34
2026-10-05 12:00	08:20:23.34	+02:55:14.5	HIP 40893	16.00
2026-10-06 00:00	08:20:24.35	+02:54:57.9	HIP 40893	15.67
2026-10-06 12:00	08:20:25.33	+02:54:41.2	HIP 40893	15.33
2026-10-07 00:00	08:20:26.28	+02:54:24.6	HIP 40893	15.01
2026-10-07 12:00	08:20:27.21	+02:54:08.1	HIP 40893	14.68
2026-10-08 00:00	08:20:28.11	+02:53:51.6	HIP 40893	14.36
2026-10-08 12:00	08:20:28.99	+02:53:35.1	HIP 40893	14.04
2026-10-09 00:00	08:20:29.84	+02:53:18.7	HIP 40893	13.73
2026-10-09 12:00	08:20:30.66	+02:53:02.3	HIP 40893	13.42
2026-10-10 00:00	08:20:31.46	+02:52:46.0	HIP 40893	13.12
2026-10-10 12:00	08:20:32.23	+02:52:29.7	HIP 40893	12.81
2026-10-11 00:00	08:20:32.97	+02:52:13.5	HIP 40893	12.52