Catch a falling star

Observational astronomy normally conjures up images of sparkling white domes set on the edge of cliffs above ethereal views morphed out of cloud tops. For the Canadian Automated Meteor Observatory (CAMO), however, the reality ... is a pig farm. Literally. I couldn't help feel that southern Ontario had been a bit short changed. 

This particular swine-based observatory is one half of a twin set (the second is located a pig-free field 50 km away) of telescopes operated by the University of Western Ontario (UWO) that are used to measure the position and velocity of meteors. Paul Wiegert from UWO visited McMaster to give last week's Origins Colloquium and explained why they were searching for shooting stars.

A 'shooting star' or 'meteor' is the name of the flash of light in the sky caused by an extraterrestrial rock heating up as it falls through the Earth's atmosphere. Typically, a meteor will only be 3 mm in size but travelling at a notable speed of 30 km/s. Prior to its atmospheric entry, the rock is known as a meteoroid and if it actually reaches the Earth's surface before being completely burned up, it becomes a meteorite. To complete the journey, a starting meteoroid must be larger than sizeable 1 m in diameter. The biggest meteors are called fireballs, with the Peekskill fireball being one famous example. This fell over the United States in 1992 and was recorded on the camera of a woman who happened to be filming her son's football match at the time. The resulting meteorite struck a car that was for sale, raising its price from a few hundred to a quarter of a million dollars. A second example was the Grimsby fireball which fell in 2009 and was filmed here at McMaster by an instrument owned by Professor Doug Welch from the top of the Physics department.

Where, though, do these Earth-bound rocks come from? Professor Wiegert explained that there are three possible sources. The first is from comets in our Solar System; dirty snowballs that heat up as they approach the Sun, producing a tail of rocky particles. These objects are usually trapped by the Sun's gravitational pull in the same way that the Earth is, but are on highly eccentric orbits so their passes by the Earth can be up to thousands of years apart. Should the Earth's orbit intersect the orbital path of a comet, the extended trail of debris left in the comet's wake can produce a meteor shower, such as the annual Perseids shower each summer: a more attractively stunning version of the trail of slime left by a departed slug.

A second source of meteors is a collision between two asteroids or a planet with an asteroid; rocky bodies that are smaller than planets, but also orbit around the Sun. The larger meteors that survive their decent to become meteorites typically come from this source. Contrary to what you might expect, a meteorite is typically cold when it reaches the ground. This is because it consists of the core of the meteor that is not burned up travelling through the atmosphere. The end part of the meteorite's journey is known as the 'dark flight' where it falls from the average height of a commercial aircraft without burning. The fact that this rock is not heated to high temperatures has important implications for the transmission of life across space; it could be possible that microbes shielded safely in the cold core of meteors might survive to populate a new world.

This consideration leads to the question as to whether there is a third source of meteorites: solid particles that originate from outside our Solar System. Since we find evidence of asteroid collision debris from the Moon and Mars, might we not also have received material from further away? From star systems billions of kilometres from our own?

Material from Mars found on Earth is not uncommon. It is estimated that around 500 kg or 15 individual meteorites per year hit the Earth from the red planet. However, the vastness of space makes interstellar rock transfer a whole new game. Approximately 100 rocks with diameters greater than 10 cm (the minimum size needed to protect biological molecules in space) are estimated to leave our Solar System from a terrestrial planet every year. Unfortunately, that only equates to the chance of striking another terrestrial planet to be a minute 10-4 per billion years. Even if you were to lower your requirements and ask what the probability was of such a rock merely being captured by another star system, in the hope that this would eventually lead to a collision with a planet, the chance is only 1 per billion years. This makes the odds of us receiving biological material from another star system incredibly unlikely.

What though, asked Professor Wiegert, about smaller meteorids? Ones that are too small to be biological carriers but might still arrive in the Earth's atmosphere? Are we able to detect these and get a handle on how many we receive? It was this goal of finding such tiny object that CAMO was designed. Such small pieces of material will be very hard to detect, especially since they will burn up long before you have the chance to hold them in your hand and put them under a microscope. However, if they come from outside the Solar System, their expected velocity is very high, equal to around 20 km/s on arrive in our Solar System which will be accelerated to 46.6 km/s as they are drawn towards the Sun.

So has CAMO detected any of these interstellar visitors?

Possibly; but it's very hard to confirm. One of the problems is the giant planets in our Solar System, namely Jupiter and Saturn, are able to sling shot rocks to much higher speeds that they would otherwise obtain, making it look like they originate from further afield. It is important for CAMO to get an accurate measurement for both the meteor's position and velocity to rule out this possibility.

CAMO began taking data only last summer but perhaps soon we will know if our atmosphere is receiving the most distant of visitors.