If theorists are right, next month observers should spot a cluster of potentially hazardous asteroids as they cruise past Earth.
It's been 35 years since Victor Clube and William Napier tossed out a radical idea: Periodic Comet 2P/Encke, unusual for its short, 3.3-year orbit, is more than just the source of the annual Taurid meteor shower. It's actually the fragment of a much larger comet — roughly 100 km across — that broke apart some 20,000 years ago and left huge chunks of debris coursing through the inner solar system.
The idea seemed audacious, even preposterous. But at the time, orbital dynamicists and paleontologists were buzzing about the realization that a devastating impact 65 million years ago had led to the near-eradication of life on Earth. Emboldened by that discovery, Clube and Napier pointed out that the Taurids are actually a complex of discrete particle streams that pepper Earth over several months, and that the Tunguska impact on June 30, 1908, involved a 100-meter-ish object with a Taurid-like orbit.
Rather than fade away, as so many notions of cosmic cataclysm do, this one has grown more legit with time. Later work by dynamicist David Asher (Armagh Observatory) and others showed that a concentrated swarm of Taurid objects orbits in resonance with Jupiter, completing seven elongated loops around the Sun for every two by Jupiter. (Like 2P/Encke, these objects have highly elongated paths that range roughly from Mercury's orbit nearly out to Jupiter's.) In 1975, seismometers left on the Moon by Apollo astronauts recorded a spike in tremors just as this "Taurid resonant swarm" (TRS for short) was nearing Earth, and a score of asteroids have been discovered in suspiciously Encke-like orbits.
Most recently, an observing team led by Pavel Spurný (Czech Academy of Sciences) recorded more than 100 bright fireballs traveling en masse within a particular branch of the TRS, and several near-Earth asteroids have matching orbits.
Truth Test Soon
Why am I telling you all this? Because this month and next, the core of the TRS will pass closer to Earth than it has since 1975. Although dynamicists aren't expecting any Tunguska-like encounters, the close passage will offer observers the chance to discover sizable near-Earth asteroids with advance knowledge of where and when to look.
David Clark, Paul Wiegert, and Peter Brown (University of Western Ontario) have simulated the swarm's arrival in a paper for the Monthly Notices of the Royal Astronomical Society. Starting with orbits like those of the 2015 fireballs, Clark's team has identified three "windows" that give observers their best chance to discover big TRS objects — "big" meaning at least 100 m across, assuming they are only 5% reflective (which is typical of comets and carbon-rich asteroids).
The swarm crossed Earth's orbit inbound back in April, when Earth itself was nowhere nearby, and it swung through perihelion weeks later. Now they're coming at us on the outbound leg, still hidden from view in the daylight sky. Observers will have to wait until just after they've passed by to spot them.
The first window falls between June 25th and July 11th, corresponding to the weak-moonlight periods on either side of July 2nd's total solar eclipse. Individual objects will be their brightest, some around magnitude 21 or better, but their motion across the sky will be most rapid — up to ½° per hour. They'll be moving through declinations of –40° to –60°, with the brightest objects the farthest south.
A second opportunity falls in the dark-sky window between full moons from July 21st to August 10th. Objects in the concentrated TRS core will appear fainter by 3 or 4 magnitudes, but their sky motion will be slower. The target area is not quite so far south.
The third opportunity, from July 5th to 11th, is a dynamical "sweet spot" during which individual objects are still relatively close by. But they'll be moving across the southern sky at less than 2¼° per day — slow enough to have a good chance of being swept up by telescopic surveys. As the plot at right shows, the simulation suggests that the brightest bodies will be around magnitude 22 — a tall order for amateur-scale telescopes but easy pickings for professional instruments. Many observers will concentrate their search in the area several degrees south of the tail of Scorpius.
So stay tuned for the results! The June-August encounter provides a "unique opportunity" to identify large bodies in the swarm, conclude Clark and his colleagues, "helping to either substantiate or refute the giant comet hypothesis of Clube and Napier and the Taurid Complex hypothesis of Asher." At the very least, they add, telescopic surveys should be able to determine how concentrated large objects are near the swarm's center.