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Articles: Too Close for Comfort

By Seanie Morris, MAC

It was not long ago that the world was treated to Hollywood style asteroid impacts on Earth. Then, even more recently was the discovery that Earth could be hit for real on April 13th 2029 by asteroid 99942 Apophis. It was soon discovered however that Earth may be spared one of the ultimate means of destruction (just look at the dinosaurs). It now means that we will see just how close an asteroid can get – and what effect Earth will have on it.

On that day in 2029, much of Asia and Africa will get a glimpse of the brightest asteroid pass in history. While the 1,000feet in diameter asteroid is by far NOT the largest known, its close proximity will stir both fear and awe in equal amounts as scientists watch closely how Earth’s gravity will affect it, and what it may look like up close. Will Earth change the asteroids course? Will it cause it to break up? Will pieces fall off and hit us? Such questions get a very rare opportunity for such close inspection. Such occurrences happen on average once every 1,500 years. Scientists could not ask for a better opportunity than Apophis.

As Earth’s gravitational field will churn the insides of Apophis, scientists plan to use radar to track its movements inside, and telescopes to observe changes on the outside and monitor its rotation. But even the most sophisticated ground-based observations won't be sufficient for gathering detailed information about the interior of Apophis. That kind of detail would require closer inspection, such as space probes capable of measuring acceleration and seismic activity, probably embedded in the asteroid's surface. Another possibility would be to place a probe in orbit around the asteroid in order to keep tabs on it and to map its surface. There is plenty of time for all this to be planned – or so it seems.

 

The object in question

Apophis was discovered in the summer of 2004. It is appropriately named after the snakelike Egyptian god of darkness and chaos. For a brief period of time after Christmas, scientists had given Apophis, then known as 2004 MN4, a 1-in-40 chance of colliding with Earth in 2029.

Additional observations ruled out the 2029 impact after more accurate readings of its orbit were observed, and scientists now predict there is about a 1-in-10,000 chance that the asteroid will hit Earth in 2036, on yet another of its trips around the Sun on a course that crosses the orbit of Earth.

A large part of the uncertainty surrounding Apophis' movements is due to something called the Yarkovsky Effect. When rotating bodies (like an asteroid) pass through the Solar System, they absorb solar radiation from the Sun that they then re-radiate, mostly in the form of heat and light. This tiny but persistent pressure from this re-radiation can cause the body to speed up or slow down, and thus change its flight path.

After the recent success of missions like Stardust and Deep Impact, some scientists think it prudent to launch a space mission to determine whether Apophis poses a significant threat. With all the attention this asteroid is bound to receive, perhaps it is a good idea to start this sooner rather than later.

Interest has been so keen, that even former Apollo astronaut Rusty Schweickart sent a letter to NASA administrator Mike Griffin urging the agency to investigate whether Apophis might be affected negatively by Earth’s gravity in a manner that could put it on a more certain collision course at a future date.

In order to more accurately track its movements, Schweickart also proposed launching a space mission to place a radio transponder on Apophis by 2014, and that an intervention mission, should it prove necessary, be launched prior to 2029. It is not yet known what NASA’s response to Schweickart’s letter is.

As was seen in July’s cometry collision by the Deep Impact mission, it is possible to strike a fast moving body in space using current technologies. Already, subtle changes in the orbit of the comet that was hit, Temple 1, have been observed, but only on a miniscule scale. Over time, the actual change would be more noticeable. In order to affect Apophis’ orbit so as to make sure it doesn’t pose a threat, action must be taken much sooner than later.

Scientists urge caution, however, and say that we shouldn't rush into action. We don't want to nudge it until it is known what the nudge is going to do. The worst thing that could happen, of course, would be to nudge the asteroid in the wrong direction, based on the incomplete data now in hand, and actually cause a future collision.

 

And if there was a collision?

So what would happen if there was a collision? In 1998 at Sandia National Laboratories in New Mexico, scientists using virtual reality techniques, decades of experience in shock physics, advanced computer programs, and the world's fastest computer at the time, completed one of the largest hypervelocity impact physics calculations ever performed.

In the latest computing scenario, an asteroid 1 kilometres in diameter strikes the Atlantic Ocean 25 miles south of Brooklyn, New York. To model the event, the scientists broke up a 120-square-mile space that roughly approximates the New York City metropolitan area, the air above, and the water and earth below, into 100 million separate cubes, or grids. Sandia's teraflops supercomputer then calculated what happened inside each cube as the asteroid splashed down. The cubes were reassembled to produce a three-dimensional moving picture of the collision. The teraflops, the world's fastest computer at the time, performs more than one trillion mathematical operations per second.

The simulation is no video game; the calculations take into account the real world laws of physics governing time, temperature, pressure, gravity, the densities of water and earth, and hundreds of other considerations to create an accurate prediction. What's more, the resulting computer simulation can be explored using interactive virtual reality techniques. For instance, scientists can "fly through" the 3-D movie to get a better idea of what's happening on Coney Island if they want.

The computer-generated images by Sandia National Laboratories' scientists showed the impact of the 1km test asteroid hitting in the open ocean. The asteroid and 300 to 500 cubic kilometres of ocean water would be vaporized nearly instantaneously by the tremendous energy of the impact. The impact energy of about 300 gigatons of TNT would be equivalent to about 10 times the explosive power of all the nuclear weapons in existence in the 1960s at the height of the Cold War. What happens in the following tens of seconds after impact is, needless to say, frightening.

For an object the size of Apophis, it would pose a local threat with an aftermath affecting most of the world, but not on a catastrophic scale. If it landed in the ocean, tsunamis would rage across the waters, devastating low lying lands somewhat worse than what was seen in the Sumatran earthquake on December 26th last. If it struck land, local devastation would ensue, as an explosion at worst of about 250,000 tonnes of TNT would flatten any large city, immerse the surrounding area in a choking cloud of dust and smoke, and level anything in its path for miles around. Daylight would darken and become hazy for weeks afterwards around the world, the stars would be hard to see at night, plants and animals would be erratic in behaviour. Thus, using Apophis to study what could be in store in the future, is something not to be taken lightly.

 

What next?

Prior to Apophis’ climatic pass, it will veer within an observable distance of Earth twice in the meantime - once in 2013 and again in 2021. Based on data collected from those two flybys, scientists and NASA should be able to conclude with 99.8% accuracy whether a future impact scenario should be ruled out or not. Even though such a mission may be held back as it would cost hundreds of millions of dollars, it might be too late to find out if it was necessary, and the clean-up bill in the aftermath will be far higher.

 

Postscript – hot off the press from Australian Antarctica Division (www.aad.gov.au) on August 26th 2005:

Cosmic hole-in-one captured over Antarctica

What at first looked like an electronic glitch turned out to be a significant event in space, in fact, a cosmic hole-in-one. What a powerful telescope had picked up as it stretched towards the night sky over Antarctica was the trail of dust left in the wake of the death of an asteroid. The remarkable story features in the latest edition of the prestigious international science journal, Nature.

Lead author, Dr Andrew Klekociuk, from the Australian Antarctic Division said that early last September 2004, a physicist at Australia's Davis station in Antarctica had prepared his monitoring instrument, known as LIDAR, for keeping watch on atmospheric activity during the long night ahead.

"Just as observation of the stratosphere began a strange signal was recorded from 30 km overhead. Our physicist thought his preparation of the optics may have been amiss so fitted a filter but the signal persisted for another 30 minutes. What he didn't know at the time was that seven hours earlier an asteroid had crashed to Earth in another part of Antarctica, about 1500 km west of Davis. The closest it got to human habitation was around 900 km west of Japan's Syowa station," Dr Klekociuk said.

Shortly after the LIDAR observations it was revealed that the event had also been picked up by the global network of satellites and a range of other instruments. But the most detailed evidence of the trail of dust, carried by strong winds around Antarctica, has been captured by the LIDAR at Davis station.

Dr Klekociuk said that it was thought that the asteroid had come from what is known as the Aten group somewhere between Venus and Earth, ranging anywhere up to 46 million km from the sun. Measuring roughly 10 metres it is the biggest body to enter Earth's atmosphere in the past decade. Its travel time from entering Earth's atmosphere 75 km up until it landed was just five seconds.

Scientists believe that the asteroid's original size was close to that of a small house weighing a thousand tonnes and that if it had not broken up on entry into the atmosphere its effect on impact would have been that of the bomb dropped on Hiroshima. The size of the dust cloud in the stratosphere was 200 km by 75 km. Had a cloud that size passed over the sun the light would have dimmed by around 20 per cent. Inevitably particles contained in the dust cloud have fallen to Earth and samples from all three Australian Antarctic continental stations - Davis, Casey and Mawson -- have been retrieved for analysis at the Australian Antarctic Division."

Dr Klekociuk said that these analyses will enable scientists to validate models of atmospheric circulation. The timing and location of the event will also allow for testing theories relating to the impact of large meteorites on ozone and climate. While there were no obvious short-term associated changes in regional climate or ozone levels, the longer term implications are still being evaluated. The LIDAR, developed by the Australian Antarctic Division in collaboration with the Adelaide University, was installed during the summer of 2000.

 

If you thought being hit by bird poop was lucky…

On 30th November 1953 in Sylocuga, Alabama, Mrs. Hewlett Hodges was struck on the hip by a meteorite that had crashed through her ceiling. It is reported that after penetrating the roof and ceiling of her home, it first bounced off the radio, and then hit her when she was lying on the couch. She suffered only minor bruising and had to rest for a few days.

Another near miss occurred in 1991 at around 11:30am on 5th May. Mr Arthur Pettifor was busy gardening at his Glatton village home in Cambridgeshire, England, about 70 miles North of London. Suddenly, he heard a loud whining noise and saw something crash through his conifer hedge, just a few feet away from where he stood. Below the conifers, he found a black rock sitting in a shallow depression and assumed that local "hooligans had thrown the stone" into his garden. Within a few seconds of it's fall, he picked it up, noticing that the stone felt warm to the touch, and then quickly guessed that this black crusted rock was something much more extraordinary. Sure enough, the stone was soon confirmed as a meteorite by visiting meteoriticists.

Despite a thorough search of the surrounding area, no more meteorites were recovered - and if Mr Pettifor hadn't been out in his garden at the time, the Glatton meteorite would not have been recognised at all!

Mr Pettifor kept the meteorite for a few months (carefully wrapped in plastic Cling Film on the advice of the Natural History Museum) and displayed it at his local Summer fete, charging a small fee (donated to the church fund) to view the meteorite. Later, the little space rock was sold to the museum, where it still remains today.

 

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