From the meteor that exploded over eastern Russia in February to the asteroid 1998 QE2 that squeaked by the Earth at the end of May 2013, meteoroids and other near-Earth objects (NEOs) have garnered increasing attention recently. NEOs, which include comets and asteroids, are objects whose closest approach to the Sun is less than 1.3 times the mean distance of Earth’s orbit around the Sun []. Such close proximity means that some NEOs have the chance to impact Earth, causing anything from small craters to mass extinctions like the one believed to have wiped out the dinosaurs. Most small meteoroids burn up harmlessly in the atmosphere and so are not causes for much concern, but the larger asteroids are much more worrisome. So, do NEOs pose an imminent threat to the survival of our species, or is it an unnecessary worry about our crowded interstellar neighborhood that is no more urgent than Chicken Little’s fear of the sky falling? While this question is still debated, both sides agree that many NEOs remain undiscovered and are not well understood. Moreover, any technology used to deflect a threatening NEO requires us to first identify and track it. Below I will discuss the details of NEOs, their impact risk, and ongoing and proposed projects to catalog and study them.
Potentially hazardous asteroids
Meteoroids are small fragments of asteroids and comets. They are called “meteors” as they burn up in the Earth’s atmosphere, and “meteorites” if they manage to land on Earth’s surface. Potentially hazardous asteroids (PHAs), a subset of NEOs, are defined as asteroids that are bigger than about 500 feet in diameter and pass within 4.7 million miles of Earth (approximately 20 times the distance between Earth and the Moon) []. By comparison, the meteoroid that exploded over Chelyabinsk, Russia on February 15th was estimated to be 60 feet in diameter and exploded about 15 miles above the surface of Earth []. Nonetheless, the explosion was estimated to contain more than 20 times the energy of the atomic bombs detonated in World War II []. Such precise measurements were generated from infrasound signals (sound that is less than 10 Hz in frequency) collected by stations normally devoted to monitoring similar signatures from nuclear weapons testing []. Despite the immense energy of the explosion, there was no way to track the Chelyabinsk meteoroid to provide any advanced warning prior to the explosion because of the meteoroid’s small size and its particular entry trajectory [].
According to NASA’s Near Earth Object Program, as of June 19, 2013, there were 1409 known PHAs with well-catalogued orbital data []. These asteroids were discovered as part of the Spaceguard Survey, launched by NASA in 1998, which has tracked approximately 90% of all estimated near-Earth asteroids (NEAs) larger than one kilometer (0.62 miles) in diameter (as inferred from average brightness) []. Such asteroids, some of which have the potential to cause globally catastrophic events (e.g., atmospheric change or mass extinctions), constitute 99% of the impact threat from large NEOs because of their density and proximity compared to other objects such as comets []. However, it is estimated that there are more than one million smaller NEAs with diameters larger than 40 meters (130 feet) []. Also, as explained by former astronaut Ed Lu in a congressional testimony, these asteroids pose a serious risk to densely populated areas, with an estimated 3-in-10 chance of an impact event within the next century []. However, less than 1% of these objects have been tracked [] (Figure 1). This has led to proposals for international government programs and privately funded groups to extend the scope of NEA tracking efforts.
Figure 1. Small NEOs pose a greater Earth impact risk and are less catalogued than large NEOs. The estimated Earth impact rate for small NEOs (with diameter, D, between 40 and 1000 meters) is 1,500 times greater than that for large NEOs (D>1000m). The number of NEOs shown here represents the impact rate per 500,000 years (e.g., for small NEOs, the rate is 15×100 (or 1,500) per 500,000 years). Scientists estimate that ~90% of large NEOs and <1% of small NEOs have been discovered, as represented by the proportion of NEOs marked off by the dotted white line and highlighted in gray [6,9]. (Illustration by Hannah Somhegyi)
A Sentinel for Earth
One such privately funded group is the B612 Foundation, which plans to track more than 90% of the NEOs larger than 140 meters (460 feet) in diameter []. In the congressional testimony described earlier, Ed Lu, as CEO of the B612 Foundation, described plans to construct and launch a solar-orbiting, infrared space telescope, called the Sentinel, by 2018 to carry out this task []. This satellite will be built using existing technology from the Kepler and Spitzer space telescopes and will be operated through a partnership with NASA []. The Sentinel will be free of the many limitations of ground-based telescopes (e.g., atmospheric distortion and absorption, light pollution) and is expected to track approximately one million NEOs, which includes 50% of those over 50 meters (165 feet) in diameter, over its 6.5-year operating time []. While the Sentinel is designed only for detection of NEOs, several other spacecraft have or are planned to explore and study these objects in greater detail.
Not just dangerous neighbors
Although the popular press often emphasizes the threats posed by NEOs, they are also an incredibly rich source of scientific information. For instance, some of the objects that impacted early Earth were believed to have ferried many of the ingredients for life to our planet. Additionally, asteroids provide insight into the formation of our solar system and could potentially serve as sources of raw materials for technological development []. Hence, there have been many recent space missions to investigate asteroids, including the Hayabusa mission, which successfully returned a surface sample from an asteroid in 2010. Planned missions include the OSIRIS-Rex mission in 2016, which will send a robotic spacecraft to NEA 101955 Bennu to better understand its composition and high potential impact risk []. More recently, NASA has announced plans to robotically capture and bring a small asteroid (25 feet in diameter) closer to Earth for human exploration [].
From previous and ongoing research on NEOs, we have gained a better understanding of such objects, from their relation to the formation of the solar system, to the estimated impact risk of the known PHAs. But as recent events have shown us, many undiscovered NEOs could still surprise us. The diverse population of NEOs of all sizes in our local planetary environment can pose serious risks to civilization as we know it, but also present much opportunity for exploration and research. Continued study of such objects is crucial, not only in areas such as tracking in order to move beyond statistical knowledge of impact risks, but also unmanned and eventually manned missions that could potentially use NEOs as stepping stones for missions to nearby planets. One thing is for certain: there are countless NEOs awaiting discovery and much more to learn about these fascinating neighbors of our solar system.
Alex Zhang is a PhD student at the School of Engineering and Applied Sciences at Harvard.
Interested in finding out more about the cutting edge in space science and space exploration? Check back to the SITN Flash at the end of July for our Special Edition on Space!
Link of Interest:
Updated discovery statistics for NEOs can be found at the NASA Near Earth Object Program website: http://neo.jpl.nasa.gov/stats/
References:
[] Yeomans, Don. “Frequently Asked Questions.” Near Earth Object Program. National Aeronautics and Space Administration. n.d. Web. 25, May, 2013. http://neo.jpl.nasa.gov/faq/
[] Fountain, Henry. “A Clearer View of the Space Bullet That Grazed Russia.” The New York Times. 25 Mar. 2013. Web. 30 May, 2013.
[] Phillips, Tony. “What Exploded over Russia?” NASA Science: Science News, n.p. 26 Feb. 2013. Web. 2 June, 2013. http://science.nasa.gov/science-news/science-at-nasa/2013/26feb_russianmeteor/
[] Thunborg, Annika. “Russian Fireball Largst Ever Detected by CTBTO’s Infrasound Sensors. ” 18 Feb. 2013 . CTBTO Preparatory Commission. n.p. Web. 5 June, 2013. http://www.ctbto.org/press-centre/press-releases/2013/russian-fireball-largest-ever-detected-by-ctbtos-infrasound-sensors/
[] Morrison, David. “FAQs About NEO Impacts.” May 2013. NASA Ames Research Center: Asteroid and Comet Impact Hazards. n.p. Web. 7 June, 2013. http://impact.arc.nasa.gov/intro_faq.cfm
[] Lu, Edward. “Testimony to the U.S. Senate Committee on Commerce, Science and Transportation” 20 Mar. 2013. n.p. Web. 7 June, 2013. http://www.hq.nasa.gov/legislative/hearings/2013%20hearings/3-20-2013%20LU.pdf
[] “Asteroid 1998 QE2 to Sail Past Earth Nine Times Larger Than Cruise Ship” 15 May 2013. n.p. Web. 12 June, 2013. http://www.jpl.nasa.gov/news/news.php?release=2013-163
[] “Senator: NASA to lasso asteroid, bring it closer” 5 April. 2013, Politico. Associated Press. Web. 14 June, 2013.
[] National Aeronautics and Space Administration Near-Earth Object Science Definition Team. “Study to Determine the Feasibility of Extending the Search for Near- Earth Objects to Smaller Limiting Diameters.” 22 Aug. 2003. n.p. Web 25 June, 2013. http://neo.jpl.nasa.gov/neo/neoreport030825.pdf