The comet Hale-Bopp had astronomers scrambling to modify their instrumentation to image and probe the heavenly wonder. NASA`s Polar spacecraft, used to image Hale-Bopp in the ultraviolet and visible regions during perihelion—when the comet was closest to the sun—is one such example. Polar normally flies in the Earth`s orbit above the north arctic regions. Its cameras are focused on the Earth`s northern and southern auroral zones, while other onboard instruments measure particles, fields, and wave phenomena in the Earth`s magnetosphere.
Two of the Polar cameras are designed to observe faint visible and ultraviolet light emissions of the Earth`s atmosphere in close proximity to the bright day side of the Earth. Because of this, Polar was the only spacecraft equipped to view the comet at perihelion.
Hale-Bopp`s sodium tail was an unexpected discovery for comet watchers. Images were obtained with a special 589-nm filter designed to accept light from sodium atoms while blocking most other light in Polar`s Visible Imaging System (VIS). This system was developed by Louis Frank at the University of Iowa (Iowa City, IA). The tail was first noted by the Isaac Newton Group in La Palma, Spain, and subsequently confirmed on images obtained about three weeks earlier.
This phenomenon, called a "neutral sodium tail," consists of electrically neutral sodium atoms. The tail glows with the yellow tint of the flame seen when ordinary table salt (which contains sodium) is tossed on a fire. In this image, both the sodium tail and the dust tail can be seen, though they extend beyond the field of view of the visible camera. The tail was about 800,000 km (500,000 miles) wide and at least 12 million km (7,500,000 miles) long as imaged by Polar on March 31 when Comet Hale-Bopp was near its closest approach to the sun. When the tail was discovered at the Observatorio del Roque de los Muchachos, La Palma, Canary Islands, Spain on April 16, the width had narrowed to about 660,000 km (410,000 miles).
In the image, the sun is located 40° down and 10° to the right of the nucleus, as indicated by the arrow. The force of the solar wind causes the sodium tail to be swept 7° clockwise from opposite the sun, or the anti-sunward direction. The dominant emission in the image is sunlight scattered off neutral sodium, with a lesser contribution from the continuum sunlight scattered by dust and NH2. Some of the dust grains in the dust portion of the tail are neutral sodium.