Date:31 December 2006
More than 100 000 asteroids hurtle past our planet. But only one – that we know of – may hit us in the next 30 years
Friday the 13th of April 2029 could be a very unlucky day for planet Earth. At 4:36 am Greenwich Mean Time, a 25 million-ton, 250 m-wide asteroid called 99942 Apophis will slice across the orbit of the Moon and barrel toward Earth at more than 45 000 km/h.
The huge pockmarked rock will pack the energy of 65 000 Hiroshima bombs – enough to wipe out a small country or kick up a 240 m tsunami. On this day, however, Apophis is not expected to live up to its namesake, the ancient Egyptian god of darkness and destruction. Scientists are 99,7 per cent certain it will pass at a distance of 30 250 to 33 500 km. In astronomical terms, 30 250 km is a mere stone’s throw, shorter than a round-trip flight from New York to Melbourne, Australia, and well inside the orbits of Earth’s many geosynchronous communications satellites.
For a couple of hours after dusk, people in Europe, Africa and western Asia will see what looks like a medium-bright star creeping westward through the constellation of Cancer, making Apophis the first asteroid in human history to be clearly visible to the naked eye. And then it will be gone, having vanished into the dark vastness of space. We will have dodged a cosmic bullet.
Maybe. Scientists calculate that if Apophis passes at a distance of exactly 30 405 km, it will go through a “gravitational keyhole”. This small region in space – only 600 m wide, or just over twice the diameter of the asteroid itself – is where Earth’s gravity would perturb Apophis in just the wrong way, causing it to enter an orbit seven-sixths as long as Earth’s. In other words, the planet will be squarely in the crosshairs for a potentially catastrophic asteroid impact precisely seven years later, on 13 April 2036.
Radar and optical tracking during Apophis’s fly-by last summer put the odds of the asteroid passing through the keyhole at about 45 000 to 1. “People have a hard time reasoning with low-probability/high-consequence risks,” says Michael DeKay of the Centre for Risk Perception and Communication at Carnegie Mellon University in the US. “Some people say, ‘Why bother? It’s not really going to happen’. But others say that when the potential consequences are so serious, even a tiny risk is unacceptable.”
Former astronaut Rusty Schweickart, now 71, knows a thing or two about objects flying through space, having been one himself during a spacewalk on the Apollo 9 mission in 1969. Through the B612 Foundation, which he co-founded in 2001, Schweickart has been prodding Nasa to do something about Apophis – and soon. “We need to act,” he says. “If we blow this, it’ll be criminal.”
If the dice do land the wrong way in 2029, Apophis would have to be deflected by some 8 000 km to miss the Earth in 2036. Hollywood notwithstanding, that’s a feat far beyond any current human technology. The fanciful mission in the 1998 movie – to drill a hole more than 240 m into an asteroid and detonate a nuclear bomb inside it – is about as technically feasible as time travel. In reality, after 13 April 2029, there would be little we could do but plot the precise impact point and start evacuating people.
According to projections, an Apophis impact would occur somewhere along a curving 48 km-wide swath stretching across Russia, the Pacific Ocean, Central America and on into the Atlantic. Managua (Nicaragua), San Jos (Costa Rica) and Caracas (Venezuela) would all be in line for near-direct hits and complete destruction. The most likely target, though, is several thousand kilometres off the US West Coast, where Apophis would create an 8 km-wide, 2 700 m-deep “crater” in the water. The collapse of that transient water crater would trigger tsunamis that would hammer California with an hour-long fusillade of 15 m waves.
But don’t evacuate just yet. Although we can’t force Apophis to miss the Earth after 2029, we do have the technology to nudge it slightly off course well before then, causing it to miss the keyhole in the first place. According to Nasa, a simple 1-ton “kinetic energy impactor” spacecraft thumping into Apophis at 8 000 km/h would do the trick.
We already have a template for such a mission: Nasa’s Deep Impact space probe – named after another 1998 cosmic-collision movie – slammed into the comet Tempel 1 in 2005 to gather data about the composition of its surface. Alternatively, an ion-drive-powered “gravity tractor” spacecraft could hover above Apophis and use its own tiny gravity to gently pull the asteroid off course.
In 2005, Schweickart urged Nasa administrator Michael Griffin to start planning a mission to land a radio transponder on Apophis. Tracking data from the device would almost certainly confirm that the asteroid won’t hit the keyhole in 2029, allowing everyone on Earth to breathe a collective sigh of relief.
But if it didn’t, there would still be time to design and launch a deflection mission, a project that Schweickart estimates could take as long as 12 years. It would need to be completed by about 2026 to allow enough time for a spacecraft’s tiny nudge to take effect.
Nasa, however, is taking a wait-and-see attitude. An analysis by Steven Chesley of the Near Earth Object programme at the Jet Propulsion Laboratory (JPL) in Pasadena, California, concludes that we can safely sit tight until 2013. That’s when Apophis swings by Earth in prime position for tracking by the 305 m-diameter radio telescope in Arecibo, Puerto Rico. This data could also rule out a keyhole hit in 2029.
But if it doesn’t, the transponder mission and, if necessary, a last-resort deflection mission could still be launched in time, according to Chesley. “There’s no rush right now,” he says. “But if it’s still serious by 2014, we need to start designing real missions.”
In 1998, the US Congress mandated Nasa to find and track near-Earth asteroids at least 1 km in diameter. The resulting Spaceguard Survey has detected, at last count, about 75 per cent of the 1 100 estimated to be out there. (Although Apophis was nearly 760 m short of the size criterion, it was found serendipitously during the search process.) Thankfully, none of the giants so far discovered is a threat to Earth.
“But any one of those couple of hundred we haven’t found yet could be headed toward us right now,” says former astronaut Tom Jones, an asteroid-search consultant for Nasa and a Popular Mechanics editorial adviser. The space agency plans to expand Spaceguard to include asteroids down to 140 m in diameter – less than half the size of Apophis, but still big enough to do serious damage. It has already detected more than 4 000 of these; Nasa estimates that approximately 100 000 exist.
Predicting asteroid orbits can be a messy business, as the history of tracking Apophis in its 323-day orbit demonstrates. Astronomers at Arizona’s Kitt Peak National Observatory discovered the asteroid in June 2004. It was six months before additional sightings – many made by amateurs using backyard telescopes – triggered alarm bells at JPL, home to the Sentry asteroid-impact monitoring system, a computer that predicts the orbits of near-Earth asteroids based on astronomical observations.
Sentry’s impact predictions then grew more ominous by the day. On 27 December 2004, the odds of a 2029 impact reached 2,7 per cent – a figure that stirred great excitement in the small world of asteroid chasers. Apophis vaulted to an unprecedented rating of 4 on the Torino Impact Hazard Scale, a 10-step, colour-coded index of asteroid and comet threat levels. But the commotion was short-lived. When previously overlooked observations were fed into the computer, it spat out reassuring news: Apophis would not hit the Earth in 2029 after all, though it wouldn’t miss by much. Oh, and there was one other thing: that troublesome keyhole.
The small size of the gravitational keyhole – just 600 m in diameter – is both a blessing and a curse. On the one hand, it wouldn’t
take much to nudge Apophis outside it. Calculations suggest that if we change Apophis’s velocity by a mere 0,00016 km/h, or about 79 cm per day, in three years its orbit would be deflected by more than 1,6 km – a piddling amount, but enough to miss the keyhole.
That’s easily within the capabilities of a gravity tractor or kinetic energy impactor. On the other hand, with a target so minuscule, predicting precisely where Apophis will pass in relation to the keyhole becomes, well, a hit-or-miss proposition. Current orbit projections for 2029 have a margin of error – orbital scientists call it the error ellipse – of about 3 200 km.
As data rolls in, the error ellipse will shrink considerably. But if the keyhole stubbornly stays within it, Nasa may have to reduce the ellipse to 1,6 km or less before it knows for sure whether Apophis will hit the bullseye. Otherwise, a mission risks inadvertently nudging Apophis into the keyhole instead of away from it.
Can we predict Apophis’s orbit to the sub-1,6 km level far enough in advance to launch a deflection mission? That level of forecasting accuracy would require, in addition to a transponder, a vastly more complex orbital calculation model than the one used today. It would have to include calculations for such minute effects as solar radiation, relativity and the gravitational pulls of small nearby asteroids, none of which are fully accounted for in the current model.
And then there’s the wild card of asteroid orbital calculations: the Yarkovsky Effect. This small but steady force occurs when an asteroid radiates more heat from one side than the other. As an asteroid rotates away from the Sun, the heat that has accumulated on its surface is shed into space, giving it a slight push in the other direction. An asteroid called 6489 Golevka, twice the size of Apophis, has been pushed about 16 km off course by this effect in the past 15 years.
How Apophis will be influenced over the next 23 years is anybody’s guess. At the moment we have no clue about its spin direction or axis, or even its shape – all necessary parameters for estimating the effect.
If Apophis is indeed headed for the gravitational keyhole, ground observations won’t be able to confirm it until at least 2021. By that time, it may be too late to do anything about it. Considering what’s at stake – Chesley estimates that an Apophis-size asteroid impact would cost about R3 trillion in infrastructure damage alone – it seems prudent to start taking steps to deal with Apophis long before we know whether those steps will eventually prove necessary.
When do we start? Or, alternatively, at what point do we just cross our fingers and hope it misses? When the odds are 10 to 1 against it? A thousand to one? A million?
When Nasa does discover a potentially threatening asteroid like Apophis, it has no mandate to decide whether, when or how to take action. “We’re not in the mitigation business,” Chesley says. A workshop to discuss general asteroid-defence options last June was Nasa’s first official baby step in that direction.
If Nasa eventually does get the nod – and more important, the budget – from the US Congress, the obvious first move would be a reconnaissance mission to Apophis. Schweickart estimates that “even gold-plated at JPL”, a transponder-equipped gravity tractor could be launched for R1,8 billion. Ironically, that’s almost precisely the cost of making the cosmic-collision movies and . If Hollywood can conjure up that much money in the name of defending our planet, why can’t Congress?
How to head off an Asteroid
Fortunately, Apophis needs to be nudged only about 1,6 km to avoid a gravitational “keyhole” in space – a region that would send the asteroid on a collision course with Earth. Otherwise, it would have to be diverted 8 000 km for it to miss our planet.
This reduces the energy required to deflect Apophis by a factor of about 10 000 – making it theoretically possible using current technology. A number of methods have been proposed to do the job.
(1) Bump it
A simple 1-ton “kinetic energy impactor” spacecraft that slams into Apophis at 8 000 km/h would theoretically change the velocity of the 50 million-ton asteroid by about 0,00016 km/h. Over three years, that’s a drift of several kilometres. UPSIDE: We already know how to do this; Nasa’s Deep Impact probe hit a comet last summer. DOWNSIDE: An impact could break off new asteroids, and an off-centre hit would impart spin instead of drift.
(2) Thrust it
A nuclear- or solar-powered ion-drive rocket engine on Apophis’s surface could generate enough thrust over a period of weeks (and we’re talking hundreds of grams here) – to accelerate the asteroid by the necessary 0,00016 km/h. UPSIDE: Ion-drive technology has already been proved on Nasa’s 1998 Deep Space 1 mission. DOWNSIDE: The rocket would have to be “soft-landed” and firmly attached to unknown surface material. Because of the asteroid’s rotation, the rocket would require a complex control system, so that thrust would be applied in only one direction.
(3) Tug it
A 1-ton “gravity tractor” could use solar ion-drive or hydrazine thrusters to hover about 240 m above the asteroid’s surface. The spacecraft’s gravity would slowly pull the asteroid off course, in effect transferring the engines’ tiny thrust – for about a month – to the asteroid. UPSIDE: The deflection could be monitored or even modified as it happens. A gravity tractor would also avoid the rotation problems of a surface thruster. DOWNSIDE: A hover position is unstable, and extra fuel must be burned to maintain it.
(4) Blast it
A thermonuclear bomb buried deep within Apophis could theoretically turn it into a swarm of smaller asteroids. UPSIDE: The visceral satisfaction of blasting Apophis to (we hope) smithereens. DOWNSIDE: Deep drilling in space is far beyond current technology. Plus, many smaller radioactive asteroids may be worse than one big one.
(5) Nuke it
A better place for a nuclear explosion would be just above the surface. Vaporising surface material would propel Apophis in the other direction. UPSIDE: The rotation of the asteroid doesn’t matter. DOWNSIDE: Nuclear weapons in space are currently forbidden by international law, and stockpiling nukes for an asteroid mission might hinder nuclear disarmament efforts.
Earth’s greatest hits
About 100 tons of interplanetary material drifts to the Earth’s surface on a daily basis. Occasionally, an object hurtles with enough force to leave a mark. Asteroids are large rocky or metal bodies that originate in the