Near-Earth objects (NEOs) are asteroids and comets whose highly elongated orbits periodically bring them close to Earth—sometimes dangerously so. In June 1908, for example an object about 70 meters across exploded in midair over Tunguska, in eastern Siberia, flattening 2,000 square kilometers of forest. In February 2013 a 17-meter–wide near-Earth asteroid disintegrated in a fireball over the city of Chelyabinsk, also in Russia. And this past December, an object estimated at 11 meters across blew up over the Bering Sea, releasing the energy equivalent of 10 Hiroshima-size atomic bombs.
These objects shattered before they could strike the Earth itself, but large asteroids occasionally do crash into the planet, blasting out craters and affecting life on our planet. One example: an impact site off the coast of the Yucatán Peninsula in Mexico is believed to be a record of the event that led to the extinction of the dinosaurs some 65 million years ago. Although big NEO impacts are rare, their likelihood motivates us to identify all the potentially hazardous objects (PHOs), thus taking the first step in protecting our planet from a cosmic catastrophe.
To date, 18,911 NEOs have been discovered, with 893 of them being over one kilometer in size. However, even smaller objects are a concern. If a 140-meter NEO were to strike the Earth, it would release the energy equivalent to 100 megatons of TNT, inflicting severe damage to entire regions or even continents. Today we know only 8,343 or a third of the estimated population of 25,000 NEOs 140 meters and larger—and completeness of the known NEO population drops rapidly for smaller objects.
The existing population of NEOs smaller than 50 meters is largely unexplored. To date, only a small number of such objects have been discovered, with over 90 percent of the estimated 250,000 50-meter–class NEOs having not yet been found. And of those that have been found, nearly all of them have been subsequently lost because of insufficient knowledge of their orbits.
The possibility that some NEOs could impact the Earth has motivated astronomers to systematically search, catalog and study the NEO population. These efforts were intensified in 2005 when the U.S. Congress charged NASA to find 90 percent of NEOs with sizes 140 meters or larger by the year 2020. However, that goal turned out to be too ambitious. The NEO discovery rate has improved since 2005, but it appears that, even under an optimistic scenario, the congressional mandate will be unmet even by 2040.
The good news is that this estimate relied on using large astronomical facilities on the ground and/or large infrared telescopes in space. Today, however, we can do much better! Here we describe a radically different approach that benefits from two emerging technologies: the technique of synthetic tracking and a new generation of small and capable spacecraft. Together these technologies are poised to revolutionize NEO search, making it possible to meet the congressional mandate in only two and a half years. Here we discuss how that is possible.
Finding a small, dark and cold object in space is hard—very hard. If this object is moving very fast, the problem becomes even harder. On average, NEOs move across the sky so quickly that an exposure longer than seven seconds results in a streaked image. For faint moving objects, streaking spreads the few detected photons across a large number of pixels where they are overwhelmed by background noise. As a result, the sensitivity of most NEO surveys relying on exposures of 30 seconds or longer is much degraded for fast-moving NEOs as compared to that for the slower moving objects.
Synthetic tracking is a technique that de-streaks optical images of NEOs by taking multiple fast exposures. It relies on a combined use of a new generation of low-noise, fast-frame cameras and high-performance processors capable of executing hundreds of gigaflops. De-streaking the image puts all the photons detected from an NEO in one spot, thus increasing the sensitivity.
Existing facilities were designed around an older generation of sensors that can’t benefit from synthetic tracking. But our technique enables even a small telescope to be as sensitive in detecting dim and fast-moving objects as a much larger facility, making for instance a 35-centimeter telescope comparable to its two-meter counterpart.
In addition, the availability of wide-field small telescopes and a new generation of large format focal planes make such systems extremely competitive. With telescopes like these, one could think of using many of them to track NEOs from different locations worldwide, which is exactly what we are starting to do.
Given the properties of the NEO population, however, the sky around the Earth changes very slowly. Adding another large search facility in the Earth’s vicinity, will not result in a significant number of new detections. This is known as the saturation effect. Take, for instance, the Large Synoptic Survey Telescope (LSST) that is currently being built in Chile. LSST is a major astronomical facility featuring an 8.4-meter primary mirror with an exceptionally wide field of view.
When completed by 2023, it will be by far the most powerful tool for ground-based NEO searches; if used only for this purpose, it would take LSST over 15 years to find 90 percent of NEOs larger than 140 m. Because of the saturation effect, building an LSST-twin elsewhere on Earth would increase the yield of unique NEO finds only by 2 percent. And putting a moderate-size half-meter telescope in space in the vicinity of Earth wouldn’t improve that significantly: over 90 percent of the NEO discoveries reported by both of these facilities will be virtually identical.
A natural solution to this problem is to uniformly distribute several telescopes along the Earth’s orbit around the sun. However, with moderately sized telescopes one is forced to use a custom-built spacecraft, which come at a prohibitively high cost.
That’s where microsatellites (microsats) armed with small telescopes and equipped with synthetic tracking come in. Modern microsats use commercial space-qualified hardware with costs dramatically lower than those of a conventional spacecraft. With lifetimes extending beyond five years, costing less than $10 million, microsats open new opportunities for astronomy. The radically lower cost enables mission architectures that otherwise would be dismissed out of hand, especially those involving multiple spacecraft.
A constellation of microsats offers the most interesting solution for planetary protection. Our analysis shows that a constellation of five 35-centimeter synthetic tracking telescopes uniformly distributed in Earth’s heliocentric orbit can find 90 percent of 140-meter or larger NEOs in 2.5 years. For finding NEOs down to 50 meters in size, such a constellation of microsats has even greater advantages, both in the cost and in the time to complete the search, thus providing a valuable input for the emerging asteroid mining community.
At first, our constellation would yield precise orbits to identify potential impactors. Realistically, there will be just a handful of such NEOs that may pass by the Earth at alarmingly close ranges. Then, armed with the orbital data, a dedicated smallsat could rendezvous with these few PHOs to directly measure their mass properties and study their compositions.
Accordingly, synthetic tracking enables breakthrough capabilities in the investigation of NEOs, providing a major alternative to many ongoing and planned NEO-search efforts.