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The Leonids - Questions and Answers
By Detlef Koschny, Space Science Department of ESA

Q: I keep hearing about the Leonids. What are they?

A: The Leonids are a meteor stream. If you go out during the nights 16/17 and 17/18 November after midnight, you will see many more meteors, or shooting stars, than normal. They will seem to come from the constellation of Leo, that's why they are called the Leonids.
What are they? Well, a meteor is the streak of light that occurs when a tiny dust particle flying around in the solar system enters the Earth's atmosphere. The Leonids are caused by dust particles ejected from comet P/55 Tempel-Tuttle. Many of these particles still follow the orbit of the comet. The Earth passes through the comet's orbit in the middle of November, that's why we see so many meteors in the nights between 15 and 18 November.

Q: There are other meteor streams, like the Perseids in August. So what's so special about the Leonids?

A: Indeed, there are almost 90 catalogued meteor streams during the year. Those with the highest meteor rates are the Perseids, the Geminids, and the Leonids. The Leonids showed a spectacular display of thousands of meteors per hour in 1966. Model calculations - which are confirmed by historical observations - show that the Leonids show extremely high activity roughly every 33 years. If the number of meteors goes as high as more than 1000 per hour, that's what we call a meteor storm. A meteor storm was predicted for the Leonids for 1998 and 1999. In 1998, we saw an activity similar to 1965, the year before the large storm in 1966. So we now expect a spectacular show again.

Q: I thought that the 'storm' was predicted for 1998. Last year, nothing much happened, except that the maximum occurred one night before the prediction. What was going on?

A: It is not true that nothing happened. There was indeed a very high activity of meteors in the night 17/18 November, at the predicted time of the storm, for about 30 minutes. Unfortunately, the brightness of the meteors was lower than expected, so that the peak was mainly observed by low light-level video cameras. With the naked eye, the fireball display the night before the predicted maximum was more spectacular. This was something that had not been modelled before. That's why the press reported that the maximum occurred 16 hours earlier. In reality, this was not true; it was just something unforeseen.

Q: So how come that this was not predicted?

A: Well, predicting the activity can be compared to the weather forecast. While we can make rough estimates on the activity, the details are hard to predict. Meteor astronomers are trying to predict the activity of a stream many months in advance - just imagine trying to predict the weather for your town 12 weeks in advance. While a general statement might be possible, the details really cannot be predicted.

Q: You are a scientist at ESA's Space Science Department, and you will be going out to observe the Leonids. Why are they so important for you?

A: There are two main reasons why ESA is interested:

1. The Leonids could be a threat for spacecraft. If a meteoroid particle hits a spacecraft, it might damage it permanently. This is not only a question of mechanical damage; in addition the generated plasma cloud might damage the sensitive electronics on board the spacecraft. The chances of that are very small, but the spacecraft operators do not want to take any risks. The Space Science Department of ESA is providing flux information to ESOC (ESA's operations centre in Darmstadt) every 15 minutes. With these data, together with radar data provided by other sources, they can decide whether they will issue a warning to spacecraft operators to switch off their spacecraft for security reasons.

2. Stream meteors are messengers from comets. In the case of the Leonids, comet P/55 Tempel-Tuttle ejects dust particles whenever it comes close to the sun. These are then visible in November when they enter the Earth's atmosphere. By studying these meteors we learn a lot about the physical and chemical parameters of the comet that ejected the particles. While the measurements are not as direct as for example those planned by the ESA's Rosetta mission to comet Wirtanen, observing meteors allows a cheap and quick overview over the properties of the dust from many different comets.