A series of anniversaries

SkyCaramba blog for the week ending March 5, 2011

If you like anniversaries with certain numbers attached to them, here’s a good one for astronomy. It’s actually a series of 25th anniversaries. And you don’t need clear skies to enjoy them. In early March 1986, several spacecraft had close encounters with Halley’s Comet. Two were launched by Japan, two by the Soviet Union, and one by the European Space Agency.

Vega 1, the first of the Soviet craft, soared by on March 6. Nine months earlier, it dropped a balloon into the atmosphere of Venus. Then, it used the planet’s gravity to redirect itself to the comet. Vega 1 took pictures of Halley and determined the shape and temperature of the comet’s nucleus.

The Japanese mission Susei went by Halley on March 8. It spent a few months taking ultraviolet images of the comet’s tail.

Vega 2 was next on March 9. The companion to Vega 1 had also dropped a balloon over Venus. Then, it followed its brother to the comet for pictures and temperature measurements, etc.

Sakigake, from Japan, arrived March 11. It had equipment for measuring plasma wave spectra, solar wind ions, and interplanetary magnetic fields.

Last but not least, the European Space Agency was responsible for Giotto. On March 13, this spacecraft passed closest of all the “Halley Armada” and told the most about Halley. Giotto revealed the peanut shaped nucleus to be very rough, very black, and very old (about 4.5 billion years old).

Giotto’s measurements showed Halley’s tail was 80% water and 10% carbon monoxide. And folks in 1910 were worried about methane from the comet killing them as Earth passed through Halley’s tail!

Halley is the most famous comet. It was the first known to be a periodic comet, one that orbits the sun and thus makes subsequent appearances. Since its orbital period is 76 years, most people who see it find it a once in a lifetime experience. Since it hasn’t been around the sun as many times as closer comets, Halley has more material to lose. So, it usually puts on a better show with a longer tail and brighter apparition.

Scientists who study comets were excited about missions to study Halley in the 1980s. But they don’t want to wait until 2061 when Halley appears again to send the next fleet of spacecraft after it. Since the 1986 encounters, other comets have been photographed close up. Comet dust has been captured and returned to Earth. One probe, aptly named Deep Impact, punched a hole in a comet to make a dust plume to study.

Just a few weeks ago, the Stardust-NExT mission provided data about Comet Tempel 1. That’s the comet Deep Impact went to in 2005. The followup mission arrived February 14-15, 2011. Tempel 1 is the first comet to have been visited twice. NASA wanted to get better pictures of the crater Deep Impact made and see what other changes Tempel 1 has been through since its last orbit.