In telecommunication and astronomy, forward scatter is the deflection—by diffraction, nonhomogeneous refraction, or nonspecular reflection by particulate matter of dimensions that are large with respect to the wavelength in question but small with respect to the beam diameter—of a portion of an incident electromagnetic wave, in such a manner that the energy so deflected propagates in a direction that is within 90° of the direction of propagation of the incident wave (i.e., the phase angle is greater than 90°).
Forward scattering (back-lighting) can make a comet appear significantly brighter because the dust and ice crystals are reflecting and enhancing the apparent brightness of the comet by scattering that light towards the observer. Comets studied forward-scattering in visible-thermal photometry include C/1927 X1 (Skjellerup–Maristany), C/1975 V1 (West), and C/1980 Y1 (Bradfield). Comets studied forward-scattering in SOHO non-thermal C3 coronograph photometry include 96P/Machholz and C/2004 F4 (Bradfield). The brightness of the great comets C/2006 P1 (McNaught) and Comet Skjellerup–Maristany near perihelion were enhanced by forward scattering. During the night of 1861 June 30-July 1, the famed comet observer J. F. Julius Schmidt watched in awe as the great comet C/1861 J1 cast shadows on the walls of the Athens Observatory.
Forward scattering brightenings would have been an unrecognized agent in inspiring the awe and terror of comets recorded by earlier societies.
- "Comet Elenin as seen by STEREO-B, and what we think is going to happen next...". Sungrazing Comets. Retrieved 2011-08-05.
- Marcus, Joseph C. (2007). "Forward-Scattering Enhancement of Comet Brightness. I. Background and Model". International Comet Quarterly 29 (2): 39–66. Bibcode:2007ICQ....29...39M.
- Marcus, Joseph N. (October 2007). "Forward-Scattering Enhancement of Comet Brightness. II. The Light Curve of C/2006 P1" (PDF). International Comet Quarterly. pp. 119–130. Bibcode:2007ICQ....29..119M.