ESA's shortcut to a comet

by the European Space Agency

Night owls across Europe, get ready. On the night of 18-19 November 2002, you may see a spectacular sky show. As tiny particles in Comet 55/P Tempel-Tuttle's tail enter Earth's atmosphere, they will pierce through it, heating up, and finally explode. Welcome to one of the most spectacular natural fireworks displays of the year: a meteor shower called the Leonids.

Unlike in 2001, this year the Leonids will be very well visible from Europe. The meteor showers are expected every night between 13 and 21 November 2002. Scientists expect the Leonids meteor shower to reach its peak at 04:00 CET, in Europe, and 10:30 on the night of 18-19 November 2002.

Die-hards willing to travel each year all around the planet look for the best location to witness the shooting-star show. Some years that can be in the middle of a desert. This year, that will be on top of a cold mountain close to Pico Veleta in Granada, Spain. The scientists aim to obtain information about the meteor shower, the structure of our own atmosphere, and comets.

"It's like a cheap comet mission," says Detlef Koschny, a member of the ESA team. "The behaviour of the particles left in past orbits tells us things about comets, even without having to go there."

Koschny has led the ESA Leonids campaigns since 1998. The team has observed the meteors in The Netherlands, Spain, Germany, and Australia. This year the location is once again Spain. Why? Statistically it is one of the places where there are higher chances of good weather. Secondly, scientists can use a professional observatory, from which the visibility conditions are optimal. Lastly, technicians are at hand to help these Leonid-chasers solve possible engineering problems.

Scientific models predict the intensity and the maxima of the storms, and this year's shower appears promising. Scientists are expecting about 3000 events per hour in the stronger, first peak. However, the light of the near full moon will make observations more difficult and may reduce the apparent number by 10%.

The 2002 Leonids show could be the last one for many years to come. Calculations of the comet's path predict that the next perihelions, or closest points of approach, of the comet in 2031 and 2065 could be significantly less dramatic. Jupiter's gravitational pull is pushing the orbit of the comet further away from Earth's orbit.

The path of our planet intersects the comet's trails twice this year. Firstly, Earth's atmosphere will encounter the trail the comet left in 1767, seven comet journeys ago. These particles will create a shower storm visible from Europe. A few hours later, a second trail Comet Tempel-Tuttle left in 1866, about four cycles ago, will cause another shower. North American sky-watchers will benefit from this second shower.

Koschny is also involved in the Rosetta mission, the ESA spacecraft to Comet Wirtanen due for launch in January 2003. "The composition of the particles belonging to the two different trails can tell us a lot about the structure of the comet. If we observe different chemical properties, we can conclude that the particles are coming from different parts of its surface." Knowing whether a comet is homogenous or not is very useful since scientists have to decide on a landing site for the Rosetta lander touchdown on the comet in 2011.

The Leonids enter Earth's atmosphere at a speed around 70 kilometres per second - nearly twice as fast as other meteors. Why? Comet Tempel-Tuttle orbits the Sun roughly in the opposite direction than the Earth's orbit of the Sun. There are therefore almost head-on collisions between Earth and comet-trail debris. However, this is much less dangerous than it sounds.

"The biggest pieces can't exceed 0.5 metres in diameter," says Koschny. "The gases vaporising from the nucleus of the comet would not be able to lift anything bigger than that. These rocks are small enough to be vaporised by the Earth's atmosphere and really do not constitute a risk for the observers. More worrying is the effect they may have on satellites, which could be seriously damaged by such collisions."