As fighter pilots in World Wars I and II were acutely aware, attacking an enemy aircraft from the direction of the sun was a very effective tactic to catch your target by surprise. The glare of sunlight provided cover until it was too late for the opponent. While asteroids don't consciously have this tactic in mind (we hope!), astronomers are extremely mindful that the sun may be hiding a cache of undiscovered and potentially hazardous asteroids within its glare.
This concern was highlighted by the July 8, 2019 announcement that a surprisingly big asteroid with the shortest "year" was discovered by the Zwicky Transient Facility (ZTF), a powerful camera at the Palomar Observatory in California. The 0.6-mile (1-kilometer) wide asteroid, designated "2019 LF6," orbits the sun entirely inside Earth's orbit, completing one orbit every 151 days. It zooms within the orbit of Mercury (that orbits the sun every 88 days) and swings as far out as Venus (which has a 225-day orbit) in a wonky trajectory that flings it out of the orbital plane, a sign that it was once gravitationally disturbed by one of the two planets in the past.
Why 2019 LF6 Is so Rare
The rare space rock belongs to a very exclusive group of asteroids. Known as Atira asteroids, there are only 20 known to exist and all orbit the sun closer than Earth. This makes them uniquely difficult objects to detect. But even for Atira asteroids, LF6 is a unique challenge.
"Thirty years ago, people started organizing methodical asteroid searches, finding larger objects first, but now that most of them have been found, the bigger ones are rare birds," Quanzhi Ye, postdoctoral researcher and discoverer of LF6 who works at the California Institute of Technology (Caltech), said in a statement. "LF6 is very unusual both in orbit and in size — its unique orbit explains why such a large asteroid eluded several decades of careful searches."
Asteroid 2019 LF6 was detected as a part of the "Twilight" campaign. As the name suggests, the best time to observe asteroids such as these is during the short period of twilight, shortly after sunset and just before dark. The campaign — which was developed by Ye and Wing-Huen Ip of the National Central University in Taiwan — discovered another Atira asteroid designated 2019 AQ3 in January 2019, which has a 165-day orbit around the sun. In addition, the ZTF has bagged an impressive haul of 100 Near-Earth Objects (NEOs), plus around 2,000 asteroids living in the main asteroid belt between Mars and Jupiter.
In addition to the Twilight campaign, the proposed NASA Near-Earth Object Camera (NEOCam) spacecraft will also be able to study the inner solar system for more Atira asteroids by seeking out their heat signature.
"Because Atira asteroids are closer to the sun and warmer than other asteroids, they are brighter in the infrared," George Helou, also at Caltech and member of the discovery team, said. "NEOCam has the double advantage of its location in space and its infrared capability to find these asteroids more easily than telescopes working at visible wavelengths from the ground."
Is LF6 a Threat to Earth?
Since NASA began its NEO Observations program in 1998, the agency estimates that it has discovered more than 90 percent of near-Earth asteroids (NEAs) measuring 0.6-mile (1-kilometer) and larger. While LF6 has been classified as an NEA, and is therefore part of a dwindling group of undiscovered objects of this size, it's not considered a threat to Earth.
LF6 is therefore not a "potentially hazardous asteroid," or PHA, as computer simulations of its future orbits indicate no imminent likelihood of a future collision. However, it's a reminder that these substantial asteroids are still out there and projects like ZTF can probe the inner solar system where the sun may be hiding them.
So, for now, while Earth is safe from being smashed by large space rocks that could cause global damage, astronomers are on high alert to ensure we don't get blindsided by the glare of the sun.