Phew. That was close. Sort of ...

Globe & Mail (Toronto, Canada), Jan 12, 2002 

Science: astronomy: Phew. That was close. Sort of: this weeks's asteroid 2001 YB5 didn't pose any danger to Earth: it wasn't very big and it was far away. Strauss, Stephen.

Noooo!!! Kabooooom!!! Silence.

The above are the sounds of what did not happen on Monday night when a smallish asteroid passed near, in astronomical terms, to Earth.

No collision occurred; no craters were dug; no cities were obliterated; no dust clouds blotted out the sun; no species went extinct; and no huge waves lapped at any mountain sides.

It wasn't even the closet near-miss or the biggest thing to fly by in recent years.

However, with at least six comet/asteroid movies, such as Armageddon and Deep Impact, having been made in the past five years, people's minds are full of images of disaster raining down from space -- whether any of it ever really happens.

And what asteroid 2001 YB5, the object that some newspapers described as missing our planet by "a stellar hair's breath," did was focus attention on the uncertainties surrounding the potential of collisions between the pieces of rock and ice that float around the solar system and Earth.

Everything related to what some typify as a "doomsday event" is awash in qualifications.

For example, the nearness of Monday's near-miss was extremely relative. Part of what probably caught the attention of reporters was that YB5 was described as missing Earth by an apparently minuscule 0.0056 Astronomical Units. But an AU is the average distance of Earth to the sun, or 149,597,871 kilometres.

YB5 was originally predicted to pass by 600,000 kilometres away, but the closest it came was roughly 830,000 kilometres, a length that was metaphorically translated as more than twice the distance from Earth to the moon.

The 230,000-kilometre difference underscores a central difficulty in collision prediction: You can't tell where they are going if you don't know where they are.

Tracking asteroid paths simply from light readings taken after only a few nights -- YB5 was first sighted in late December -- produces a significant possibility of error. To reduce it, astronomers have begun using radar readings, which have permitted them to position asteroids within a few tens of metres of their actual locations. In so doing, it has been revealed that previous light-based measures of orbits were in error by distances ranging from 100 kilometres to 100,000 kilometres.

Even if orbits are accurately calculated, other factors make predicting the effects of any possible collision more than murky. For example, while YB5 was generally described in the media as being 300 metres across, its precise size, shape and composition were, in fact, unknown.

This is because asteroid measurements are made obliquely as a function of the sunlight that the projectile reflects. Generally speaking, the bigger the asteroid, the less light it reflects.

However, astronomer Peter Brown of the University of Western Ontario, who is part of a committee that advises the Canadian Space Agency on asteroid and meteorite collisions, points out that there are a multitude of confounding factors.

Shape is one, but more directly what the asteroid is composed of dramatically affects how much light is reflected. A metallic body will reflect more brightly than one that is largely composed of carbon -- no matter what its size. "We can't simply detect how much sunlight is reflected," Brown says.

Size and composition have a huge effect when trying to compute what will happen when rogue rocks and ice actually strike Earth. Once the stuff of convoluted equations, the effect of changing asteroid sizes, speeds and composition on collision scenarios is now something that ordinary people can calculate for themselves.

Anyone with an access to the Internet can use the Solar Systems Collisions Calculator -- janus.astro.umd.edu/astro/impact.html -- created by University of Maryland astronomer Doug Hamilton. It not only gives you a sense of collision magnitudes and consequences, it also allows you to construct your own disaster scenarios.

For example, if you plug in the approximate size and speed -- 108,000 kilometres an hour -- of YB5, the calculator informs you that if the asteroid was largely rock, it would make a crater five kilometres across and half a kilometre deep. Upon impact, it would release the energy equivalent of 3,764 megatons of TNT, a figure roughly equal to the explosion of a third of all of the world's nuclear weapons at once.

If the impact's energy release was translated into an earthquake scale, it would be a Magnitude 8 -- with the largest quake ever recorded measuring a 9.5. A collision that size would probably occur only once in every 28,000 years.

However, if you change parameters only a little, you end up with some very different results. If the asteroid was actually an icy comet, the energy released would fall to 1,186 megatons and the crater diameter would shrink to 3.4 km, but a collision this large would occur roughly once every 11,000 years.

The fallout effects of these "events" depend on where the asteroids hit. Again, a slew of variables are at work. Roughly 70 per cent of Earth is covered with water so the greatest likelihood is that incoming bodies from outer space will hit with a splash.

However, where the splash occurs makes a big difference. If YB5 had hit the ocean near a continent, a tsunami tens of metres high would have rolled ashore over hundreds of kilometres. However, the same asteroid hitting in the middle of the Pacific might produce a wave only a few metres high.

Non-watery collisions have their own issues. Hitting an ice cap would have a different effect than hitting a bit of Canada's rocky north.

All of which raises the larger question of how many collisions are likely to occur any time soon.

The Lincoln Near Earth Asteroid Research Project recently reported that after three-year period, it had identified 606 near-Earth satellites. Based on where it hadn't looked and nights it couldn't look, LINEAR scientists estimated that there were 1,157 to 1,397 near-Earth satellites of greater than a kilometre in diameter.

Through the Internet, everyone gets to know what is coming and when.

Harvard University has mounted a Potentially Hazardous Asteroids Web site that describes all of the asteroids scientists know about that are going to pass anywhere near Earth in the next 176 years. The list includes roughly 600 bodies.

In terms of foreseeable close calls, the closest will occur in December of 2140 when asteroid 2000 W0107 will pass about .0007998 AU -- approximately 120,000 kilometres -- from Earth.

Even if we have identified half of the biggest, most dangerous objects floating near us -- what Brown calls "planet busters" -- everyone accepts that there are a lot more small things than big things out there. The exact number is difficult to determine because the co-ordinated detection efforts have focused on big hits. (For anyone interested in seeing how the search is going daily, they need only go to Harvard University's Minor Planet Web site, cfa-www.harvard.edu/iau/mpc.html and read daily updates).

However, some estimates placed the number of near-Earth asteroids over 100 metres in diameter at 300,000.

Even if the general numbers of smaller asteroids are vague, their risk has been quantified by astronomers. Alan Hildebrand, a University of Calgary planetary scientist who is on the same advisory committee as Brown, says it has been computed that on average 15 one-metre-sized chunks of space rock crash into Earth's surface each year. Unless they land on you directly, their effects are likely to be minimal.

But as size mounts, consequences increase. On average, every four years a rock two metres in diameter will strike and produce a thud equivalent to a Magnitude 5 earthquake. Every 15 years, a four-metre-wide rock will hit and release the equivalent of 4,500 tons of TNT -- an amount equal to the largest chemical explosion ever. Every 35 years, a collision by a six-metre-wide asteroid will release the energy equivalent of an atom bomb; every 370 years, a 23-metre-wide asteroid with hydrogen bomb-like power will strike.

Things get really frightening as one moves toward the largest recent collision, the asteroid or comet that blew up over Tunguska in Siberia in 1908, releasing the energy equivalent of the volcanic blast that blew off the top of Mount St. Helens. Luckily, something like that is thought to hit Earth only every 1,900 years.

All of which still leads to the question of what one would do if the real kaboom actually happened. While the possibility of its impact forms the dramatic sinew of movies such as Armageddon, Hildebrand suggests that two quite different strategies are possible.

If, as with YB5, people have only a few days' or weeks' notice of the asteroid's arrival, there would be little to do except try to remove as many people as possible from the likely area of impact.

If you had 10 years' warning, however, you could mount a concerted effort whose goal would be to slow down the potentially killing asteroid by a tiny, tiny bit. If you brake its speed by only two centimetres a second, over hundreds of millions of kilometres, that difference would be enough to make it miss Earth.

How do you slow an asteroid even a bit?

"We could easily send up some nukes, and put them alongside, and start popping them. Crudely, that would start working on some level," Hildebrand says.

But if you want to be much less high-tech, maybe all you would have to do is send up some astronauts with paint cans to coat the asteroid with a reflecting paint. What that would do is cut down on a small but significant energy source -- the recoil that occurs when the energy from sunlight is re-radiated. Changing that gentle push even slightly might be enough to save Earth, Hildebrand says.

Crater creator

Meteors, asteroids and other space debris that collide with Earth are small compared with the size of the craters and impact energy they create. Below, the force of these impacts is compared to historical events on Earth.

 
Projectile diameter            2 m
 
Crater diameter                35 m
Energy (TNT equivalent)        500 tons
Average frequency of impact    4 years
Comparable terrestrial event   Minimum damaging earthquake (M=5)
                               Largest chemical explosion experiment
                               ("Snowball", Canada, 1964)
 
Projectile diameter            6 m
 
Crater diameter                120 m
Energy (TNT equivalent)        20,000 tons
Average frequency of impact    35 years
Comparable terrestrial event   Atomic bomb explosion (Hiroshima, Japan,
                               1945)
 
Projectile diameter            90 m
 
Crater diameter                1.8 km
Energy (TNT equivalent)        60 MT
Average frequency of impact    4,400 years
Comparable terrestrial event   San Francisco earthquake, 1906 (M=8.4)
                               Largest hydrogen-bomb detonation (68 MT)
 
Projectile diameter            155 m
 
Crater diameter                3.1 km.
Energy (TNT equivalent)        310 MT
Average frequency of impact    12,000 years
Comparable terrestrial event   Mount St. Helens, Washington eruption, 1981
                               (total energy, including thermal)
 
Projectile diameter            350 m
 
Crater diameter                6.9 km.
Energy (TNT equivalent)        3,600 MT
Average frequency of impact    51,000 years
Comparable terrestrial event   Largest recorded earthquake, Chile, 1960
                               (M=9.6)
 
Projectile diameter            1.5 km
 
Crater diameter                31 km.
Energy (TNT equivalent)        310,000 MT
Average frequency of impact    720,000 years
Comparable terrestrial event   Total annual energy release from Earth
                               (heat flow, seismic, volcanic)
 
Projectile diameter            10 km.
 
Crater diameter                200 km
Energy (TNT equivalent)        8.7 E + 7 MT
Average frequency of impact    150 million years
Comparable terrestrial event   Largest known terrestrial impact structures
                               (original diameters 200-300 km)
                               Sudbury, Canada; Vredefort, South Africa;

Chicxulub, Mexico

Source: Traces of Catastrophe