Feature

It Comes From Beyond

Any housekeeper knows dust gets everywhere. Some bits get further than others

Veronika Meduna

Generations of inquisitive minds have been scouring the skies unsuccessfully in search of visitors to the Earth from outside the solar system, beyond the orbits of the known planets. Finally, evidence that planet Earth is indeed regularly visited by objects from outside the solar system has emerged from work carried out by a group of researchers, led by University of Canterbury physics and astronomy professor Jack Baggaley.

His findings bear no resemblance to the Larsonesque, bug-eyed sort of alien, but Baggaley's research into "dust from interstellar space" has nonetheless drawn an enthusiastic response from the international scientific community and brought science a significant step closer to understanding the evolution of our own solar systems and similar such star systems.

His data, accumulated over the last five years, show for the first time that a small percentage of the dust particles constantly piercing through the Earth's atmospheric layer have made their way to the planet from the far side of the Galaxy, originating in some other solar system within the Milky Way.

Baggaley says the publication of his results in the prestigious international science journal Nature in March caused a flood of letters of acknowledgment and congratulations. He and the co-authors, Drs Andrew Taylor [Catch A Falling Star, October 90] and Duncan Steel [Cosmic Collisions, July 94], who both completed their doctorates with Baggaley at Canterbury and are now working in Australia, have had numerous invitations to co-operate in international projects.

Baggaley and his team, including Dr Bob Bennet and Graeme Plank, have been tracking dust grains entering the Earth's atmosphere with the help of a highly sensitive radar system called Advanced Meteor Orbit Radar. He says most people imagine astronomers watching stars and planets with a telescope, but his work has been based on the use of radio waves emitted by the radar system, known internationally under the acronym AMOR. The instrument has been named after a type of near-Earth asteroid because it works accurately enough to pick up the dust debris associated with such heavenly bodies. Asteroids, rocky objects found in their thousands within a ring around the sun between the orbits of Mars and Jupiter, can be anywhere between one kilometre and 1,000km in diameter.

Collisions in this relatively densely populated asteroidal belt are frequent and often produce smaller particles called meteoroids which shoot off on their own trajectories and orbits at a speed reflecting the energy of the impact between the two parent rocks. The Canterbury radar facility, developed by the university team, has been recognised world-wide as the most sensitive and sophisticated in existence to monitor meteoroids and Baggaley says it can rapidly process information received from about 2,000 daily from its continuous surveillance.

These solid pieces of rock range in size "from that of a grain of sand to that of a car" and sometimes, when their trajectories meet the Earth on her pathway around the sun, they are pulled into the Earth's gravitational field and collide. More often than not, a meteoroid is too small to survive the fall through the Earth's atmosphere and actually hit the ground.

Baggaley says bodies larger than about a football are likely to survive the severe ablation and melting in the atmosphere and reach the ground, at which point they are referred to as meteorites. Smaller meteoroids will lose mass and velocity rapidly during their fall leaving a train of excited and ionised gas, an effect we see on clear nights as a shooting star, or meteor.

Some of these smaller bodies also reflect radar pulses and that is where Baggaley and his powerful radar telescope enter the equation. He has been tracking thousands of these small meteoroids from the moment they enter the Earth's atmosphere using radar antennas set up in Canterbury. Beaming regular pulses of radio waves into the atmosphere, the antennas also act as receivers for the reflected radio waves which allows for the exact monitoring of the particles' speed, accurate trajectories and orbits in the solar system.

Baggaley says the majority of the grain-sized bodies falling through the atmosphere behave as expected if one assumes either of the two commonly held theories about the origin of meteoroids.

"It was believed they were formed either as debris of collisions between the thousands of asteroids or as solid parts of a disintegrating nucleus of a comet. Because of the difficulties in measuring and monitoring meteoroids, the accepted wisdom until recently was that they are all gravitationally bound to the sun and are permanent members of the solar system."

However, about 1% of the over 500,000 individual meteoroid orbits Baggaley was able to monitor display a rather alien behaviour. They are small, between 15 and 40 mm (significantly smaller than a full stop on this page), and travel at speeds impossible to explain by either of those two theories. He says their speed indicates they are not bound in orbits around our Sun but have been kicked off somewhere else in the galaxy with enough energy to leave their place of origin and enter our solar system.

Baggaley explains that it is possible to calculate a minimum velocity at which everything falling to Earth will travel. Due to the planet's own gravity, anything entering the atmospheric layer travels at speeds of at least 11km/sec. Similarly to this "basic" speed, there is a speed which is the fastest a "home-grown" meteoroid, which originated within our solar system, can orbit around the Sun.

"For Earth-impacting meteoroids it is 42km/sec. Because in addition the Earth is moving around the Sun, the incoming speeds to be measured can exceed 73km/sec."

About 5% of Baggaley's meteoroids travel faster than that, and just under 1% exceed this speed limit by far. Previous attempts to detect these objects were unsuccessful; an innovative design was necessary to catch the galactic speedsters travelling at more than 100km/sec, which is significantly faster than what astronomers calculated as the maximum speed possible for bodies bound in closed orbits around the Sun.

"The maximum speed [possible for a meteoroid on a closed orbit around the Sun] is ten times greater than Earth-orbiting satellites and reliable detection of the trajectories of these minute, sand-grain-sized objects poses difficult technical problems."

Baggaley says it has been a matter of controversy for some decades whether any meteoroids reach the solar system from interstellar space, perhaps even from other star systems with planets.

"The detection of interstellar source objects entering the Earth's atmosphere hinges on accurate measurements of their trajectories and speed. Attempts to detect such meteoroids have been to date unsuccessful because they are expected to be very elusive; not only are they rare, but difficult to recognise."

He says when he first suggested in 1993 that his data pointed to a certain number of heavenly bodies from outer space reaching the Earth's atmosphere, his announcements were met with "some reservations by the international community".

"However, support came later from the inter-planetary probe Ulysses which -- though incapable of providing the accurate trajectories available from the Christchurch measurements -- did also suggest the existence of interstellar dust grains in the solar system."

Today, he is not only recognised for detecting and describing the out-of-space visitors but also for pointing to possible sources which might have produced and ejected the meteoroids. He says the project is a tribute to the expertise and support of the technical personnel in his department.

He says annual variations in the number of interstellar dust particles arriving in the atmosphere suggest they come from a number of discrete sources. One of these sources is likely to be specific young hot stars, essentially new stellar planetary systems in the making.

Veronika Meduna is a freelance journalist with an interest in science stories.