AS CELESTIAL events go, the night of 13 April 2029 will be pretty special. It will probably be the closest near miss by a dangerous asteroid for over a millennium.
The celestial oddity won’t look like much, just a point of light moving slower than a satellite. But depending on where it passes this time – and the difference could come down to a few metres – it could conceivably hit Earth on a subsequent orbit a few years later. The race is on to find out what our chances are, and to work out what to do if the asteroid really is on a collision course with Earth.
The asteroid 2004 MN4 was discovered in June 2004, and led to a few nail-biting days for astronomers last December (see “Countdown to all-clear”). At the time, NASA’s web page on asteroid impact hazards gave this object the highest ever odds of hitting the Earth – more than 1 in 50. “I certainly don’t expect to see anything like this in the rest of my career,” said Steve Chesley of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, who saw the saga of 2004 MN4 unfold first-hand.
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Christmas holidays and worldwide attention to the tsunami meant there was little media coverage at the time, and by early January the odds of an impact had fallen away to nearly negligible. Earth seemed to be safe.
But the close call leaves astronomers with a new question: what will happen to MN4 after its fly-by? At around 300 metres across, the asteroid is capable of devastating a large city, making it one of this century’s most serious foreseeable dangers.
“The asteroid will pass well within the orbit of ordinary communications satellites”
The miss will be exceptionally close. The latest estimates put closest approach at around 25,600 kilometres from Earth’s surface, well inside the orbit of communications satellites. If you are in Europe, Africa or central Asia on the night it passes, MN4 will appear as a fairly bright star drifting slowly across the sky.
And the asteroid will be affected by its brush with Earth. When an asteroid comes very close to a planet, gravity very slightly changes its orbit and speeds it up, and the closer it passes the bigger the change. This phenomenon has been the basis for many of the unmanned space missions over the last few decades. By including a few “gravity assist” fly-bys on the flight plan, mission controllers have often sped probes on their way to outer space. But until recently, no one had thought through the implications for asteroids.
That changed in the aftermath of another asteroid, XF11, discovered in 1997. It began with an announcement from Brian Marsden of the International Astronomical Union and Smithsonian Astrophysical Observatory’s Minor Planet Center in Cambridge, Massachusetts. The report said that XF11 would come close to Earth in 2028, that it would probably pass a little closer than the moon, but also stated that the possibility of a collision was “not entirely out of the question”. There were a spate of news stories, but the next day, Paul Chodas and Don Yeomans of JPL said that the probability of impact was essentially zero. Eleanor Helin of the Near-Earth Asteroid Tracking Project in Haleakala, Hawaii, found images of the asteroid taken in 1990, seven years before its official discovery. When connected to the later images, they showed that XF11 will not hit the Earth for many centuries, and follow-up observations confirmed this.
“What matters is often where a dangerous asteroid isn’t”
But weeks later, Marsden went back over the calculations for the orbit of XF11, and realised that, based on the data they had before the 1990 images, Earth might not have been as safe as Chodas and Yeomans had claimed. The 2028 pass would be a miss all right, but it was close enough to change XF11’s speed just a little, and make its subsequent path more uncertain. Over the coming years, there would have been subsequent near misses with odds of impact as high as 1 in 50,000, far higher than anything else found up to that point.
The saga showed Marsden that an apparent all-clear might not always be as clear as it seems.
It also pointed Marsden and others to a new way of assessing near-misses. Today, when a possibly dangerous asteroid is discovered, Marsden starts by assuming that it is going to hit Earth on near passes in the future, then uses this to map what exact trajectory the asteroid has to take between now and then for this to happen. In such calculations, there are fairly well defined “keyholes” of space through which an asteroid must pass during a near miss if it is going to hit Earth in future.
The keyhole idea has proved useful, because astronomers checking the position of an asteroid now need not cover the entire area of uncertainty, they can just scan the keyhole. If the asteroid is not in it, the Earth is safe, even if the asteroid’s exact location is not known for sure. What matters is often where a dangerous asteroid isn’t.
And that’s the scary thing about MN4. Even though we now know it will miss us in 2029, it has a chance of passing through one of several keyholes and swinging round to hit Earth on 13 or 14 April 2035 or 2036. These possible collision dates have high-enough risks to be given a 1 on the Torino scale.
The Torino scale is an attempt to quantify the risk of potential impacts, with a rating of 0 indicating a harmless asteroid, and 10 signifying a certain collision capable of causing a global catastrophe. Asteroid MN4 was the first object ever to rise above a 1 on the scale, and last December it briefly shot all the way to 4, though luckily that proved to be a false alarm. “None of us expected to have a case go that high,” Binzel says.
In the case of MN4, there are other, less likely, possible strike dates in 2034 and 2037, 2046, 2048, 2054 and 2055, culminating in a total risk of 1 in 6000 for an impact over the next century. Until we see what keyholes MN4 passes through in 2029, the risks are very hard to define further.
As scary as the brief high rating was, there is also something unnerving about the all-clear. In January MN4 came close enough for radar observation, which is the most accurate way of determining position and orbit. The new measurements confirmed a miss, but showed that the actual miss distance was much smaller than had been predicted by a long way – well beyond the expected margin of error. Astronomers still don’t understand the reason for this. “How could we have gotten it so wrong?” asks Binzel, who was not involved in the calculations. “We have to understand that. Our most important currency is our credibility.”
Estimates of the asteroid’s size have also come down since it was first discovered. It was initially assumed that the asteroid was of the most common form, known as C-type or carbonaceous. These asteroids reflect just 3 per cent of any light reaching them, and using this information MN4 was estimated to be around 500 metres across. But later measurements taken by Binzel and colleagues showed that MN4 is actually one of the much rarer Q-type asteroids. These are much brighter, reflecting around 35 per cent of incident light, meaning MN4’s size was revised down to around 320 metres. That is big enough to take out a city if the asteroid hit land (see Map), or several coastal cities if it landed in the ocean and caused tsunamis.
“There is a never-ending supply of dangerous asteroids out there”
MN4 will be coming fairly close again next year, providing an opportunity for fresh observations. If they show that the possibility of impacts in 2034 and beyond still looms, we might want to start doing something about it.
The first thing would be to try to pin down MN4’s exact position, and the best way to do that, according to several researchers, would be to attach a device such as a radio transponder to it. Just this May former Apollo astronaut Rusty Schweickart told the US Congress such a mission should be a high priority.
Even if MN4 is not on course to hit us, researchers still feel it is worthwhile taking the near miss seriously, if only to get a better idea what asteroids are made of. “One of the big unknown is the internal structure of asteroids,” says Bill Bottke of the Southwest Research Institute in Boulder, Colorado.
This information will be vital in understanding exactly what happens to an asteroid as it goes past a planet. Astronomers know that Earth’s gravity will create a stretching effect on the asteroid, attracting the side nearest to the planet more strongly than the far side, but contrary to some early speculation, specialists in asteroid dynamics now say there’s no real chance an object would be pulled apart by these tidal forces.
But it will undoubtedly undergo tidal stresses that will send probing shock waves through it, so astronomers want to equip MN4 with a few simple seismometers. “Possibly we could learn something about its internal structure by how it is affected,” says Bottke. “It’s a great opportunity to wire this thing up,” agrees Dan Durda, also of SWRI Boulder. “Nature has given us an experiment,” he says. “It’s probably going to creak and groan as it goes by. We can localise where the creaks and pops in the interior are coming from, and delineate any boundaries that may be in there.”
We don’t really know whether most asteroids are mostly solid, riddled with cracks or voids, or made up of lots of loose rubble, so any attempt to push one out of the way, whether with rocket blasts or bombs, might shatter it instead and just lead to even more impacts. And its hard to learn about these internal properties – any experimental perturbation would be difficult and expensive to produce, and fraught with uncertainties.
“We still don’t know whether asteroids are solid, riddled with cracks, or made of loose rubble”
The Hollywood-style assumption is that nuking an asteroid could be the right way to go, but as we know so little about their internal structure, the ideal may be to find ways to push them gently out the way (żěè¶ĚĘÓƵ, 19 April 2003, p 36). One way to do this might be to spray them with white paint. This would increase their reflectance, so changing the balance of force they feel from solar radiation. Done just right, the effect could be to make them drift gradually off course, leading to a miss instead of a hit.
This means a trip to MN4 would not be wasted, even if it turns out not to be on target for Earth. Asteroid researchers argue that planting a radio beacon and other instruments on MN4 would provide all kinds of extra information about the possible killers we might face in future. This could help our chances of designing a deflection strategy.
After all, there is a never-ending supply of dangerous asteroids out there. And, as Marsden points out, this is the closest that one is likely to come in our lifetimes. So if we don’t get blown to smithereens, we might as well take advantage of it.
Countdown to all-clear
ALTHOUGH 2004 MN4 never made the headlines, the countdown to the all-clear provided a thrilling few days. The asteroid was discovered in June 2004, but was quickly lost from view. A combination of a number of sightings by amateur, professional and military telescopes then made it possible to compute steadily better estimates of the asteroid’s orbit.
On 20 December, Steve Chesley of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, realised that MN4 might be a serious risk to Earth. “I think I was the first one to notice,” he says. Images taken in December put the chance of an impact as high as 1 in 5000.
Then Brian Marsden of the Minor Planet Center in Cambridge, Massachusetts, combined the June sightings with the December ones. Usually, the more observations, the faster impact odds fall, but this time they got worse, down to 1 in 2000. “Then we started really looking at those observations in June,” says Chesley. It turned out the June images were very unclear, and there were clock problems that affected the position calculations. For an object that was at the time thought to be a half-kilometre across, the odds were significant enough to require extra work, and so co-discoverer David Tholen of the University of Hawaii on Manoa took another close look at his June images. He was able to get much better measurements to plug into the JPL software. “It got dramatically worse at that point,” Chesley recalls. By the morning of 27 December the reported odds went as high as 1 in 38 and kept climbing, at one point reaching around 1 in 17. The worst odds ever encountered before had been only about 1 in 660, for asteroid 1950 DA, which will hopefully sail past Earth in 2880.
Then came the big break. Jeff Larsen and Anne Descour, two astronomers with the Spacewatch Project at Kitt Peak, Arizona, found some early images of MN4 from March 15 2004, predating its official discovery. While the sightings were not very clear, they were enough to extend the known trajectory of the asteroid by three months. This was enough to change everything. Later on the 27th December, Chesley posted the official all-clear on the JPL website, stating “an Earth impact in 2029 is ruled out.”
