Remembering the Flybys

It’s hard to believe that 25 years have passed since Comet Halley swung around our way in its 75.3-year orbit. Right about now it’s heading out to the farthest point in its orbit — around 32.6 astronomical units from the Sun.  That’s farther away than Neptune’s average distance. In a few years, it will reach its most distant point (called aphelion) and then start its inward journey to round the Sun again in 2061.

In mid-March of 1986, a small armada of spacecraft flew near and through Comet Halley’s tail. One of those missions was the Giotto probe, which was nearly destroyed by its close passage to the comet.  But, it returned the first images ever seen of a comet’s nucleus and changed how we viewed these dirty snowballs.

The spacecraft was the European Space Agency’s first deep-space mission, and this year the agency has posted a “remembrance” of the night when the spacecraft approached the comet. Giotto was built to a design that drew on the Geos Earth-orbiting research satellites. It was fitted with shielding to protect it from the ‘sand-blasting’ it endured as it sped through the comet’s tail. The mission was originally conceived as a joint project with NASA, the Tempel-2 Rendezvous–Halley Intercept mission. When the United States pulled out after budget cuts, ESA decided to forge on, finding Japan and Russia willing to contribute their own missions. Together, they sent a flotilla, with the Russian missions serving as pathfinders to guide Giotto to its dangerous encounter.

There WAS another mission set to go to the comet — it was called Spartan Halley, or more technically, Spartan 203.  It was equipped with ultraviolet detectors to observe the glowing gases in the plasma tail of the comet. It was set for launch on Space Shuttle Challenger, and was lost when the shuttle was destroyed in the January 28, 1986 accident.

Comet Halley was a milestone of comet science in many ways. I was part of a team that studied the plasma tail of the comet as it traversed our point of view during the months of closest approach to and movement away from the Sun. We used images from the International Halley Watch, a ground-based effort undertaken by hundreds of observers to study the comet throughout the months it was visible to us on Earth. The images we were most interested in stretched from mid-1985 to mid-1986, the months when the plasma tail was “turned on” and we were able to see structure in it. The comet itself had been spotted in an image as early as 1982, but its tail structure had not yet formed, since it was too far from the Sun to do so.

We began studying the images (or at least my part of the project) began in 1988, when I went to work studying those images under the aegis of the Large-Scale Phenomenon Network of the International Halley Watch. My job was to take the images we had selected and pinpoint the exact location of the comet’s nucleus against the backdrop of the sky.  Of course, it was tough to SEE the actual nucleus, so we had to approximate the location very carefully and then use stars to triangulate the position.  From there, we could then figure out the relative position and distance of structures in the plasma tail. That, in turn, told us something about the speed and “loading” of the solar wind, since the solar wind directly affects a comet’s plasma tail.

In the following years after Halley’s appearance, we studied other comets, among them deVico, Borrelly, D’Arrest, Encke, Honda-Mrkos-Padjuakova, Mueller, and others. The goal was to observe the plasma tails as they turned on and were affected by their interactions with the solar wind. It was a great deal of work that added to the comet literature, and I’m pleased to have been a small part of it.  It’s just hard to believe it was 25 years ago!