ESA Science & Technology - The Leonids are back: is a storm brewing?

What are the Leonids?

Each year, the night sky is illuminated by dozens of meteor showers. During these showers, pieces of cosmic debris - mostly coming from giant dirty snowballs called comets - leave glowing trails as they are incinerated during entry to the Earth's upper atmosphere. We see them as short-lived trails of light streaking across the sky.

One of the most famous meteor showers is known as the Leonids, so-called because their light trails all seem to originate from the constellation of Leo. The Leonid meteors are associated with dust particles ejected from Comet P/55 Tempel-Tuttle, which pays periodic visits to the inner Solar System once every 33.25 years.

Actually, the Leonids appear every year between November 15-20, when the Earth passes very close to the comet's orbit. However, the numbers on view vary tremendously. In most years, observers may see a peak of perhaps 5-10 meteors per hour around 17 November. But, roughly every 33 years, the Leonids generate a magnificent storm, when thousands of them illuminate the night sky: they are renowned for producing bright fireballs which outshine every star and planet. Their long trails are often tinged with blue and green, while their vapour trains may linger in the sky like enormous smoke rings for 5 minutes or more.

Although the incoming particles are small, ranging from specks of dust to the size of small pebbles, the Leonids glow brightly because they are the fastest of all the meteors. A typical Leonid meteor, arriving at a speed of 71 km/s (more than 200 times faster than a rifle bullet), will start to glow at an altitude of about 155 km and leave a long trail before it is extinguished.

The Unpredictable Leonids

Based on past behaviour, a meteor storm was predicted for 1998 or 1999. Last year, some very bright fireballs appeared unexpectedly 18 hours before the predicted maximum. They were associated with a previously unknown dust band which had been shepherded into a narrow stream by Jupiter's gravity. Unfortunately, although there was also a peak in meteor activity at the predicted time, their trails were not very bright and hard to see with the naked eye.

"In hindsight people found out that actually in 1965 we saw a similar display... these fireballs the night before and then not quite the activity that was expected at the real maximum and you might know that in 1966, that was when the real storm occurred" said Detlef Koschny, scientific coordinator of the ESA's Leonids observation campaign. "So that was one year after. So some scientists say, 'Ok, for this year we really expect a big storm or we hope; you can never be sure."

So what about this year?

In 1999, although the Earth will reach Tempel-Tuttle's orbit 622 days after the comet passed by, the distribution of its dust ribbons means that a notable display is still possible. One encouraging sign is that the 1998 shower was similar to that of 1965, the year before the storm of 1966. Most astronomers are not expecting a comparable display in 1999, but a spectacular show cannot be ruled out.

Activity will probably reach a peak on the night of 17 - 18 November, though earlier fireballs are always a possibility. Nothing will be visible until the 'sickle' of Leo rises above the eastern horizon around 22.30 GMT. At first, the fainter meteors will be swamped by light from the first quarter Moon, but once this sets soon after midnight, conditions should be ideal as long as the sky is cloud free.

The maximum activity should occur around 02.00 GMT, at the time when the Earth passes closest to the comet's orbit. At this time, Leo will be well above the horizon over Western Europe.

Some scientists predict that 2000 or 2001 may provide even better viewing opportunities for the Leonids, but no-one can be sure if these unpredictable cosmic travellers will live up to expectations.

"We just know from past history that, in the two years after the perihelion of Comet Tempel-Tuttle, there is enhanced activity," said Dr. Walter Flury of the European Space Operations Centre (ESOC).

"A storm is possible, but these things are very uncertain," he added. "Predictions are based on models of the way material is distributed along the comet's orbit. But the models are quite inaccurate. We just don't have enough information."

There are two main reasons why scientists study meteors: the clues they hold about the formation of the planets and the potential threat they pose to Earth-orbiting satellites:

1. ESA Scientists Seek to Study the Leonids

Since the Leonids ejected from a comet and since comets are thought to be left-overs from the formation of the planets, studying the Leonids helps scientists to learn more about the physical and chemical characteristics of their 4.5 billion year-old parents.

If the storm does materialise, scientists from ESA's Space Science Department intend to be ready. Armed with a variety of equipment, including image-intensifier video cameras, CCD cameras with wide-angle lenses and a spectrograph, they are planning an observational campaign at two observatories in southern Spain (Calar Alto and Sierra Nevada) from 11 to 19 November.

The main science goals are:

• determine the varying rates in the number of meteors and their magnitudes (visual brightness).
• study the physical properties of individual meteors by measuring their light output and changing velocity, then compare these to other meteor streams.
• use the 1.5 m telescope at the Sierra Nevada Observatory to perform spectroscopy of persistent trains and so determine their composition.

2. Hazards to spacecraft: how ESA is preparing

Although they are very small, the tremendous speed of the Leonids means they pack a mighty punch. Apart from knocking a spacecraft off alignment or causing physical damage in the form of an impact crater, such collisions can also generate a cloud of plasma (gas composed of neutral and electrically charged particles) which may cause electrostatic discharges or damage a spacecraft's sensitive electronics.

This threat is not simply theoretical. In 1993, an ESA's satellite called Olympus spun out of control due to an electrical disturbance caused by the impact of a particle from the Perseid meteor shower.

The situation is further complicated by the fact that there are currently more satellites in orbit around the Earth than ever before, all of which pose a tempting target for one of nature's miniature missiles. Despite this spacecraft population explosion, few, if any, satellites are likely suffer significant problems from meteors, even during a storm. Researchers estimate that the chance of one getting hit by a Leonid meteor is only about 0.1 percent.

This low hit rate was born out by an absence of damage during the 1998 Leonids event. Nevertheless, driven by uncertainty over the future of their high-tech hardware, satellite operators will once again be taking precautions to protect their multi-million Euros charges this November.

One of the largest targets, the NASA-ESA Hubble Space Telescope will be manoeuvred so that its mirrors face away from the incoming meteors and its solar arrays are aligned edge on to them. These precautions will continue for several Earth orbits, a duration of seven hours, during the Leonids' predicted peak.

Apart from reducing the exposed area of giant solar arrays, operators may shut off power to vulnerable electrical components of satellites, or switch them off entirely during the peak of the Leonid activity. Even spacecraft located some distance from the Earth may be at risk. ESA's Solar and Heliospheric Observatory (SOHO) studies the Sun from a vantage point 1.5 million kilometres away, but it, too, will roll so that its main navigational aid, the star tracker, is pointing out of harms way.

"There could be a lot of activity, but we just don't know for sure," commented Walter Flury. "It's better to take precautions now than be sorry later."

ESOC issued a warning to all satellite operators in August, explaining key protective measures to be taken such as switching off the payload and modifying the angle of the solar panels in relation to the Leonids storm.

By 17 November 1999 ESOC will have secured both earth observation satellites ERS-1 and ERS-2 by switching off the payload. The risk does not only lie in the damage caused by direct impact but rather in plasma discharge that could disrupt the functioning of the on-board electronics. Teams of operations experts will be on shift throughout the night to assess any potential damage and take necessary action.