On June 17, the LINEAR Spaceguard system discovered Near Earth Asteroid (NEA) 2002 MN, which had passed the Earth on June 15 at a distance of only 120,000 km, one of the closest asteroid fly-bys on record. Based on its brightness, 2002 MN has a nominal diameter of about 100 m, large enough to penetrate through the atmosphere to the surface if it struck the Earth. In March, another asteroid, 2002 EM7, passed within 463,000 km. This asteroid also was not found until after its flyby of Earth. Considerable press interest in this objects has been evident, as well it might be. The June 15 approach of 2002 MN was among the closest on record. Unfortunately, however, some of the press coverage has been sensationalistic. Some stories either decry that these NEAs were found after closest approach (rather than before) or express concern about the blind spot otherwise commonly known to astronomers as the daytime sky. It is quite true that an asteroid close to the Sun in the sky cannot be seen. However, if an NEA is approaching Earth from the daytime sky, it is quite likely to pass into the night sky, where it can be discovered, as these two asteroids were. Far from being a cause of concern, the discovery of NEAs 2002 EM7 and 2002 MN is an example of the success of the Spaceguard program; there is no cause for doom and gloom in either of these asteroids. Presumably a part of the problem is that many people do not understand the Spaceguard Survey strategy to discover and catalog NEAs long in advance of any possible threat, providing decades (or more) of warning if any NEA is currently on a collision course. It makes no difference if a NEA is discovered on approach or departure from the vicinity of the Earth. We don't give extra points for an approaching NEA or demerits for one that has already passed the Earth at discovery. The only effect of "blind spots", whether they be due to sunlight or moonlight or bad weather or lack of a southern hemisphere survey telescope, is to slow down the completion of the NEA catalog. Objects in blind spots will be missed until they move into a more favorable geometry, sometimes within a few days, otherwise usually within a few years. Both of these asteroids were successfully found, although they are well below the 1-km diameter that is emphasized by the current Spaceguard effort. To put the latest asteroid (2002 MN) in perspective, a 100-m asteroid hits the Earth at an average interval of several millennia. One passes within the orbit of the Moon, however, at least once per year. This has been happening throughout history. What is new is that we are now beginning to discover these objects, whereas previously they would have sped past undetected and unheralded. David Morrison
PRESS COMMENTS ON 2002 EM7, THE "BLIND SPOT" ASTEROID NEAR MISS WENT UNDETECTED UNTIL AFTER IT HAPPENED From MSNBC, 19 March 2002 By Alan Boyle An asteroid as wide as a Boeing 747 narrowly missed Earth this month - and we never knew it was coming. The case of asteroid 2002 EM7 has drawn attention to the gaps in the planet's infant system for monitoring potential threats from space. At its closest, the space rock was about 288,000 miles (463,000 kilometers) from Earth on March 8, according to asteroid-watchers at NASA's Jet Propulsion Laboratory and Italy's University of Pisa. That's just a bit farther away than the moon - spitting distance in astronomical terms. But it wasn't detected until four days later, by the Massachusetts Institute of Technology's Lincoln Laboratory. There are two reasons why it was completely missed, scientists say. For one thing, the rock came at us literally "out of the blue," from the big blind spot on Earth's sunward side. Objects that pass through Earth's orbit almost always have to be spotted in the night sky first.
ASTEROID BUZZES EARTH, HIGHLIGHTING COSMIC BLIND SPOT From Space.com, 19 March 2002 By Robert Roy Britt Senior Science Writer An asteroid large enough to have flattened a city buzzed Earth earlier this month and was not seen until after if flew harmlessly by. The space rock approached Earth in the glare of the Sun, a blind spot that made it impossible to see during the day or night from any terrestrial vantage point. The event illustrates the potential of a surprise hit by an asteroid, astronomers said.
WHEW! STEALTH ASTEROID NEARLY BLINDSIDES EARTH From CNN, 19 March 2002 By Richard Stenger (CNN) -- A sizable asteroid zipped near our planet this month without anyone noticing because it traveled through an astronomical blind spot, scientists said. The space boulder passed Earth within 288,000 miles (461,000 kilometers) -- or 1.2 times the distance to the moon -- on March 8, but since it came from the direction of the sun, scientists did not observe it until four days later. "Asteroid 2002 EM7 took us by surprise. It is yet another reminder of the general impact hazard we face," said Benny Peiser, a European scientist who monitors the threat of Earth-asteroid collisions.
PRESS COMMENTS ON 2002 MN, WHICH CAME WITHIN 120,000 KM ASTEROID 2002 MN GIVES EARTH ITS CLOSEST SHAVE IN YEARS Near Earth Object Information Centre National Space Centre, Exploration Drive, Leicester Press Release, Thursday 20th June 2002: Asteroid 2002MN On Friday 14 June, an asteroid the size of a football pitch made one of the closest ever recorded approaches to Earth. Astronomers working on the LINEAR search programme, near Socorro, New Mexico first detected the giant rock on 17 June, a few days after its close approach. The Near Earth Object, known to astronomers as '2002MN', was travelling at over 10 km/s (23,000 miles per hour) when it passed Earth at a distance of around 120,000 km (75,000 miles), bringing it well inside the Moon's orbit. The last time a known asteroid passed this close was back in December 1994. Asteroids are typically too small and distant to measure their size directly from Earth, so scientists use the amount of light they reflect, along with a basic understanding of the materials they are made of, to estimate their size. With a diameter between 50-120 metres, 2002 MN is a lightweight among asteroids and incapable of causing damage on a global scale, such as the object associated with the extinction of the dinosaurs. However, if it had hit the Earth, 2002MN may have caused local devastation similar to that which occurred in Tunguska, Siberia in 1908, when 2000 square kilometres of forest were flattened. Whilst the vast majority of NEOs discovered do not come this close, such near misses do highlight the importance of detecting these objects. This reminder comes in a week when the UK telescopes on La Palma are being tested to search for NEOs. Brief Description of Object
Object Designation: 2002MN
Date of First Observation: 17/06/02
Number of Observations: 14
Search Team: LINEAR (Lincoln Near Earth Asteroid Research)
Date of Closest Approach: 14/06/02
Closest Approach Distance: 119,229 km (0.3 Lunar Distances)
Asteroids Velocity Relative to Earth at Closest Approach: 10.58 km/s
Estimated Diameter of Asteroid: 50-120 metres
Orbital Period: 894.9 days
CLOSE ASTEROID FLYBY From Sky & Telescope, 19 June 2002 On June 17th, astronomers from the Lincoln Laboratory Near Earth Asteroid Research project (LINEAR) discovered a new Earth-crossing asteroid. Designated 2002 MN, the object is approximately 100 meters across and flew by us on June 14th. What is most shocking is just how close it came to Earth. This is only the sixth known asteroid to penetrate the Moon's orbit, and by far the biggest. According to Brian G. Marsden (Harvard-Smithsonian Center for Astrophysics), the object came within 120,000 kilometers (0.0008 astronomical unit) of impacting Earth. Though the exact details of an impact scenario depend on the rock's composition, had it hit Earth the event would have been been "Tunguska-like," with a force rivaling the largest H-bombs. The object was too small, however, to be classified as a potentially hazardous asteroid (PHA). Nor does it qualify for the Torino scale used to predict the devastation caused by an impacting asteroid. A disturbing detail is that 2002 MN was discovered three days after its closest approach. Though we are almost certainly out of harm's way from this near Earth object (no potential impacts are forecast until at least 2050), its late detection may be telling. Currently there is no dedicated Southern Hemisphere NEO search program, and NASA is currently focused on finding bodies greater than 1 kilometer across. Regardless of whether or not it should have been seen, "it was a close shave," says Marsden. Copyright 2002 Sky Publishing Corp.
ASTEROID PASSED EARTH (Associated Press) An asteroid the size of a football field hurtled past the Earth a week ago, missing what could have been a catastrophic collision by a mere 75,000 miles -- less than a third of the distance to the moon. The miss was one of the nearest ever recorded for an object of that size, scientists said Thursday. "It was a close shave," said Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The asteroid was not detected until three days after it sped past Earth on June 14. When such asteroids are detected, they are usually spotted far from Earth, when they are approaching or on their way out. "Looking statistically at the asteroid population, maybe 50 times a year a 100-meter-class asteroid passes within a lunar distance of Earth," Grant Stokes said. "But only a handful of such asteroids that have penetrated the Moon's orbit have been spotted by asteroid search programs." Currently, there is no program dedicated to searching for objects of 2002 MN's size. NASA concentrates its efforts on bodies bigger than .62 miles across, which would cause worse devastation. "NASA has a goal of discovering and obtaining good orbits for all the near earth objects with diameters larger than 1 kilometer," said Thomas Morgan, a scientist at NASA headquarters in Washington. "Asteroids of this size could potentially destroy civilization as we know it." Such asteroids could theoretically hit Earth every million years, or at longer intervals. Asteroids the size of 2002 MN are estimated to hit the Earth every 100 to several hundred years, causing local damage but no disaster to civilization or the planet's ecosystem, Stokes said. "It's something the public should know about, but shouldn't get nervous about," he said. "Civilization has to get used to them on some level." Copyright 2002, The New York Times
ROCK PASSES WITHIN 75,000 MILES, DETECTED ONLY AFTERWORD By Alan Boyle, MSNBC June 20 - An asteroid roughly as wide as a football field gave Earth its closest call in eight years last week, astronomers say. They say asteroid 2002 MN passed within 75,000 miles of Earth - closer than the moon last Friday, but it was not detected until three days afterward. At an estimated 50 to 100 yards across, the asteroid wasn't big enough to cause an "Armageddon"-style catastrophe, but in a worst-case scenario, it could have laid waste to a city. The close shave on June 14 was similar to an asteroid encounter reported in March - but this time the space rock came much closer. In fact, NASA says this was the closest known approach of such an object since 1994, when a much smaller asteroid came within 65,000 miles (105,000 kilometers). As was the case for the March encounter, 2002 MN came from Earth's sunward side, where daylight obscures visual observations of the sky. It was only after the object made the switch to the night sky that astronomers with the LINEAR asteroid-tracking effort in New Mexico could gather enough data to compute its orbit. "Things of this size come by every few weeks," Don Yeomans, who heads NASA's Near-Earth Object Program at the Jet Propulsion Laboratory, told MSNBC.com. "It's just a matter of finding them." "In all likelihood, a small Tunguska-size object will occur over uninhabited areas of the earth," said Benny Peiser, an anthropologist at Liverpool John Moores University who is an expert on the social impact of cosmic collisions. "In that respect, the damage will be limited, but the psychological effects will be dramatic and global, because it would convey the message that we are living in an extremely dangerous universe -- which is at the same time true and not true." The two close calls reported within just four months demonstrate how quickly astronomers are adding to their capability to detect asteroids, he told MSNBC.com. "It's just because we are enhancing our detection capability all the time that we now realize what is going on in our immediate environment," Peiser said. "If we were actually observing all the objects that are shaving us every day, I think people would be quite concerned." |
