from skyfield.api import load, EarthSatellite

ts = load.timescale(builtin=True)
conjunction = ts.utc(2021, 11, 25, 4, 18, [24 + 0.01*i for i in range(100)])
line1 = '1 25544U 98067A   21328.19951010  .00003407  00000-0  70845-4 0  9994'
line2 = '2 25544  51.6441 271.5215 0004374 235.8745 271.5482 15.48632762313390'
iss = EarthSatellite(line1, line2, 'ISS (ZARYA)', ts)

line1 = '1 44296U 19029BP  21327.89422035  .00005500  00000-0  11185-3 0  9993'
line2 = '2 44296  52.9987 137.7304 0006980 336.4135  23.6550 15.47973186142593'
deb = EarthSatellite(line1, line2, 'FALCON 9 DEB (44296)', ts)

iss_pos = iss.at(conjunction)
deb_pos = deb.at(conjunction)

min_dist, time = min(zip((iss_pos - deb_pos).distance().km, conjunction))
print(f"Minimum distance of {min_dist:.3f} km on {time.utc_strftime(format='%Y-%m-%d %H:%M:')}{time.utc.second:5.2f} UTC")