One hundred ten years ago tomorrow, a meteoroid exploded in the air above Tunguska, Siberia, and flattened more than 2,100 square kilometers of unpopulated forest. The interplanetary invader released enough energy upon explosion to flatten any major city in the modern world.
The world marks the 30 June anniversary of this event—the largest in Earth’s recorded history—with Asteroid Day. Established in 2016 by the United Nations, Asteroid Day aims to raise global public awareness about the hazards of asteroid impacts.
Will we know ahead of time if a devastating asteroid is on a collision course with Earth, and if we find one, what do we do?“It is a very exciting time within the field of planetary defense,” said Ed Lu, a former NASA astronaut and executive director of the Asteroid Institute. “There is about to be an awful lot more data and observations on asteroids.”
Ahead of Asteroid Day 2018, Lu joined a panel of other asteroid scientists and risk experts on Thursday to discuss ongoing and future asteroid missions and the future of asteroid risk mitigation. Their big question was simple in its ask but complex in its answer: Will we know ahead of time if another similar object from space is on a collision course with Earth, and if we find one, what do we do?
A “Flood of Data”The panelists expressed their excitement about the slew of asteroid science missions that are currently under way or in development, including the upcoming Large Synoptic Survey Telescope (LSST). Although LSST will not primarily be an asteroid telescope, scientists expect the future ground-based observatory to be a boon to the asteroid community.
LSST “is going to increase our discovery rate of asteroids by perhaps a factor of 10,” Lu said. “That means that we’re going to be discovering tens of thousands of new asteroids in orbits that could bring them close to Earth.” With LSST expected to come online in 2021, asteroid scientists are “preparing for this flood of data,” Lu said.
The possible range of asteroid sizes, how often they hit Earth, how much energy they release on impact, how many of each size likely exist, and how many of them we have found. Small asteroids are common and not well sampled but not very dangerous. Large asteroids are rare, very dangerous, and mostly accounted for. Credit: ESA, CC BY-SA 3.0 IGO. Click image for larger version.
Asteroid Day Expert Panel chair Mark Boslough explained that in the past 2 decades, scientists have cataloged and plotted the orbits of more than 95% of all the near-Earth asteroids larger than 1 kilometer in diameter. None of the detected large asteroids, capable of globally catastrophic destruction, are on paths likely to hit Earth. Scientists are currently working to track down the other 5%.Multiple space agencies are also developing complementary missions to deflect an asteroid for the first time.
But what remain mostly undetected are the smaller asteroids, which are more numerous and harder to detect but individually present a much lower risk. LSST will be able to find and track many of these smaller objects, Boslough explained.
“I remain very optimistic,” Boslough said. “If we do find something on a collision course, I am confident that it will be discovered in time to take action and deflect it.” Thanks to LSST, “there is a very low probability that we will discover it too late to do anything about it,” he added.
Sample Returns and Deflection MissionsThe panelists also expressed their excitement about ongoing missions to rendezvous with near-Earth asteroids and return with samples from their surfaces.
“This week we’ve seen the Japanese Hayabusa2 mission arrive at Ryugu,” said Alan Fitzsimmons, an astronomer at the Queen’s University Belfast Astrophysics Research Centre in the United Kingdom. The carbonaceous asteroid 162173 Ryugu is approximately 1 kilometer in size, has an orbital path around the Sun similar to Earth’s, and is about 280 million kilometers away. “Ryugu is a fascinating object.…We’re seeing on its surface a lot of complex terrain.”
“We’re going to get a lot more information from Hayabusa2 at Ryugu over the coming months,” he continued, “particularly after it deploys its landers onto the surface and then attempts the sampling of the surface to return in 2020.”
Fitzsimmons also explained that the results we will get in the coming weeks from Hayabusa2 will give us a preview of NASA’s Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer (OSIRIS-REx) rendezvous with the asteroid Bennu at the end of this year.
Multiple space agencies are also developing complementary missions to deflect an asteroid for the first time: NASA’s Double Asteroid Redirection Test (DART) and the European Space Agency’s Hera. DART will impact the smaller object in the Didymos binary asteroid system, and Hera will follow a few years later to measure any change in the asteroid’s orbit.
“DART will be humanity’s first attempt at moving a small asteroid in its orbit to check whether or not we can actually deflect an asteroid,” Fitzsimmons said. Neither Didymos asteroid is likely to impact Earth.
Remaining RisksDespite the future trove of data from LSST and valuable science from asteroid rendezvous missions, some level of risk from asteroid impacts may never vanish.
“The residual risk that I don’t think will ever go away is short-warning objects—Chelyabinsk- and Tunguska-like objects—that are very small and that we’re not going to discover,” said Boslough. The 15 February 2013 Chelyabinsk meteor impact over Russia, like the 1908 Tunguska impact, was caused by a small, undetected object that struck with no warning. It also struck at a low angle from the direction of the Sun, which would have made it difficult to see ahead of time even had scientists been monitoring the airspace. Chelyabinsk’s associated shockwave is estimated to have damaged more than 7,200 buildings and indirectly injured around 1,500 people.
If scientists get enough data about asteroids’ orbits, they could predict possible impactors with up to 10 years’ warning.“There are far more of the smaller objects which are currently harder to detect,” said Debbie Lewis, “which, from a crisis and consequence management perspective, makes the job very difficult.” Lewis is specialist in asteroid risk management and communications.
Leaving aside these short-warning, low-angle objects, the experts saw a pathway to a target goal: If scientists get enough data about asteroids’ orbits, they could predict possible impactors with up to 10 years’ warning.
“Having a 10-year notice time would certainly allow the emergency management communities to start to think about the types of arrangements that would need to be put in place,” Lewis said.
For a full listing of Asteroid Day 2018 events, see this year’s Asteroid Day website. The website also features a live broadcast, currently under way, filled with presentations by expert scientists from around the world, films and documentaries about asteroid risks, and information about how to survive an asteroid strike.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
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