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.
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.