| Parameter |
Value |
| Designation |
C/2021 A4 (NEOWISE) |
| Type |
C |
| Perihelion (tp_cal, UTC) |
2021-03-20T00:53:23.712000Z |
| Orbit solution date (soln_date, UTC) |
2022-03-08T19:43:19Z |
| Obs used |
683 |
| NEO |
- |
| M1 |
13.600 |
| K1 |
28.500 |
| PC |
0.030 |
| Discovery observation (MPC, UTC) |
2021-01-03T22:02:49.632000Z |
| Discovery magnitude (MPC) |
20.00 R |
| Discovery observatory (MPC) |
C51 · WISE |
Osculating Elements
| Parameter |
Value |
| Epoch (epoch_cal, TDB) |
2021-02-16.0000000 |
| e |
0.97324748 |
| q (au) |
1.145476 |
| a (au) |
42.817518 |
| i (deg) |
111.569834 |
| Ω (om, deg) |
307.084762 |
| ω (w, deg) |
204.665566 |
| M (ma, deg) |
359.887300 |
Magnitude
Scatter + Global Linear
$$
M_a = (14.236 + 6.000\cdot 2.5\log_{10} r) + \epsilon - 2.5\log_{10}\Phi(\theta) + 5\log_{10}\Delta
$$
$$
\theta = 180^\circ - \alpha
$$
$$
\Phi(\theta) = \frac{\delta_{90} D(\theta) + 1}{\delta_{90} + 1},\quad D(\theta) = k\cdot \Phi_{HG}(\theta; g_f) + (1-k)\cdot \Phi_{HG}(\theta; g_b)
$$
$$
\Phi_{HG}(\theta; g) = \left(\frac{1+g^2}{1+g^2-2g\cos\theta}\right)^{3/2},\quad \Phi_{HG}(90^\circ)=1
$$
Model Parameters
| Parameter |
Value |
| H0 |
14.236 |
| n |
6.000 |
| 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 |
| 2021-01-03 |
2021-03-19 |
+14.1648 |
-1.643 |
| 2021-03-21 |
2021-07-07 |
-3.2765 |
-1.725 |
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 |
20:35:36.03 |
-22:19:59.9 |
HIP 101658 |
25.20 |
| 2026-09-20 12:00 |
20:35:30.57 |
-22:20:02.6 |
HIP 101658 |
25.74 |
| 2026-09-21 00:00 |
20:35:25.17 |
-22:20:05.0 |
HIP 101658 |
26.32 |
| 2026-09-21 12:00 |
20:35:19.83 |
-22:20:07.1 |
HIP 101443 |
25.93 |
| 2026-09-22 00:00 |
20:35:14.55 |
-22:20:09.0 |
HIP 101443 |
24.76 |
| 2026-09-22 12:00 |
20:35:09.33 |
-22:20:10.7 |
HIP 101443 |
23.60 |
| 2026-09-23 00:00 |
20:35:04.18 |
-22:20:12.1 |
HIP 101443 |
22.46 |
| 2026-09-23 12:00 |
20:34:59.08 |
-22:20:13.3 |
HIP 101443 |
21.33 |
| 2026-09-24 00:00 |
20:34:54.05 |
-22:20:14.2 |
HIP 101443 |
20.23 |
| 2026-09-24 12:00 |
20:34:49.09 |
-22:20:14.9 |
HIP 101443 |
19.15 |
| 2026-09-25 00:00 |
20:34:44.19 |
-22:20:15.3 |
HIP 101443 |
18.08 |
| 2026-09-25 12:00 |
20:34:39.35 |
-22:20:15.5 |
HIP 101443 |
17.04 |
| 2026-09-26 00:00 |
20:34:34.57 |
-22:20:15.4 |
HIP 101443 |
16.02 |
| 2026-09-26 12:00 |
20:34:29.86 |
-22:20:15.1 |
HIP 101443 |
15.02 |
| 2026-09-27 00:00 |
20:34:25.21 |
-22:20:14.5 |
HIP 101443 |
14.05 |
| 2026-09-27 12:00 |
20:34:20.63 |
-22:20:13.7 |
HIP 101443 |
13.11 |
| 2026-09-28 00:00 |
20:34:16.11 |
-22:20:12.6 |
HIP 101443 |
12.19 |
| 2026-09-28 12:00 |
20:34:11.66 |
-22:20:11.3 |
HIP 101443 |
11.31 |
| 2026-09-29 00:00 |
20:34:07.27 |
-22:20:09.7 |
HIP 101443 |
10.47 |
| 2026-09-29 12:00 |
20:34:02.94 |
-22:20:07.9 |
HIP 101443 |
9.66 |
| 2026-09-30 00:00 |
20:33:58.69 |
-22:20:05.8 |
HIP 101443 |
8.91 |
| 2026-09-30 12:00 |
20:33:54.49 |
-22:20:03.5 |
HIP 101443 |
8.21 |
| 2026-10-01 00:00 |
20:33:50.37 |
-22:20:01.0 |
HIP 101443 |
7.58 |
| 2026-10-01 12:00 |
20:33:46.31 |
-22:19:58.2 |
HIP 101443 |
7.02 |
| 2026-10-02 00:00 |
20:33:42.32 |
-22:19:55.1 |
HIP 101443 |
6.56 |
| 2026-10-02 12:00 |
20:33:38.39 |
-22:19:51.8 |
HIP 101443 |
6.20 |
| 2026-10-03 00:00 |
20:33:34.53 |
-22:19:48.3 |
HIP 101443 |
5.96 |
| 2026-10-03 12:00 |
20:33:30.74 |
-22:19:44.5 |
HIP 101443 |
5.84 |
| 2026-10-04 00:00 |
20:33:27.02 |
-22:19:40.5 |
HIP 101443 |
5.84 |
| 2026-10-04 12:00 |
20:33:23.36 |
-22:19:36.2 |
HIP 101443 |
5.97 |
| 2026-10-05 00:00 |
20:33:19.77 |
-22:19:31.7 |
HIP 101443 |
6.20 |
| 2026-10-05 12:00 |
20:33:16.25 |
-22:19:26.9 |
HIP 101443 |
6.51 |
| 2026-10-06 00:00 |
20:33:12.80 |
-22:19:21.9 |
HIP 101443 |
6.90 |
| 2026-10-06 12:00 |
20:33:09.41 |
-22:19:16.6 |
HIP 101443 |
7.34 |
| 2026-10-07 00:00 |
20:33:06.09 |
-22:19:11.1 |
HIP 101443 |
7.82 |
| 2026-10-07 12:00 |
20:33:02.85 |
-22:19:05.3 |
HIP 101443 |
8.32 |
| 2026-10-08 00:00 |
20:32:59.67 |
-22:18:59.3 |
HIP 101443 |
8.85 |
| 2026-10-08 12:00 |
20:32:56.56 |
-22:18:53.1 |
HIP 101443 |
9.40 |
| 2026-10-09 00:00 |
20:32:53.52 |
-22:18:46.6 |
HIP 101443 |
9.95 |
| 2026-10-09 12:00 |
20:32:50.55 |
-22:18:39.9 |
HIP 101443 |
10.50 |
| 2026-10-10 00:00 |
20:32:47.65 |
-22:18:32.9 |
HIP 101443 |
11.05 |
| 2026-10-10 12:00 |
20:32:44.82 |
-22:18:25.7 |
HIP 101329 |
11.05 |
| 2026-10-11 00:00 |
20:32:42.05 |
-22:18:18.2 |
HIP 101329 |
10.57 |