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Observing the Sky Solar System Satellites Navy Research Resources

Naval Research: Comets


Ultraviolet image of Halley's coma
Image of Comet Halley

In 1986 scientists were able to learn a lot about comets by observing Comet P/Halley using special cameras built by the Naval Research Laboratory. The cameras took pictures of the comet in ultraviolet light, light that is blocked by the Earth's atmosphere (some wavelengths of ultraviolet light do make it to the ground and cause tans and sunburns). To capture images in ultraviolet light, the cameras had to be lifted above the atmosphere on a sounding rocket from NASA.

The rocket launched early in the morning when Comet P/Halley was just above the horizon and the sun had not yet risen. When the rocket reached about 62 miles (100 km) altitude, doors protecting the payload opened and the cameras began taking images of the comet. Just ten minutes after launch the rocket reached its highest point, called apogee, at 198 miles (320 km) and then began to fall toward the Earth. When the payload had dropped back to 62 miles, the observations stopped and the protective doors closed for reentry into the atmosphere. At 16,000 feet a parachute opened and slowly lowered the payload onto the New Mexico desert.

Souding rocket payload in desert
Image of Souding rocket payload in desert

The ultraviolet images showed that the comet is surrounded by a cloud or coma of hydrogen, oxygen, carbon, and sulfur. Using the new images, scientists were able to calculate how much of these elements are released by the comet and how fast.

An image of the hydrogen coma around Halley showed that the coma is 12 to 19 million miles (20 to 30 million km) across. The hydrogen coma is much larger than the dust and ion tails we can see in visible light because hydrogen is the lightest element and the comet's gravity is not strong enough to hold it closer.

After the payload was picked up by a helicopter, it was flown back to the Naval Research Lab, refurbished and returned to the launch pad in New Mexico. Three weeks after the first flight, the cameras were launched a second time to catch the comet at a different location. The second view showed how the comet had changed as it sped by the Sun.