Incredibly, every week about 40 new Near-Earth Objects (NEOs) are discovered. NEOs are objects that pass near the Earth as they orbit the sun. We know of more than 20,000 NEOs at the moment, the vast majority of which are small asteroids. Even if they were to cross Earth’s orbit one day and crash into our planet they’d break apart in the atmosphere and at worst drop a few meteorites.
But NEOs that pass within 4.6 million miles (7.5 million km) of Earth’s orbit and are larger than 460 feet (140 meters) are a different breed. These are classified as a potentially hazardous asteroids or PHAs. That’s how large an impacting asteroid would have to be to cause major regional destruction. It wouldn’t be the end of life as we know it, but the result would be devastating to the country struck by the impactor. Thankfully, such events are rare, occurring about once every 10,000 years. I should note that “potentially hazardous” doesn’t mean an asteroid will necessarily strike the Earth only that there is a small probability it could do so.
Astronomers have already discovered 156 “big ones” — PHAs 0.6 miles (1 km) across or larger. That’s about 90 percent of the estimated total. The largest include 1999 JM8 (4.3 miles: 7 km) and Phaethon (3.6 miles: 5.8 km), the parent asteroid of the Geminid meteor shower. If one of those smashed into the planet humanity might not recover.
But when it comes to the “run of the mill” PHAs, we’ve only found and tracked about 35 percent of the total. That sounds a little scary but consider that only a tiny percentage of those have a minute chance of colliding with Earth in the future. Clearly, there are thousands more potentially harmful asteroids we’ll need to discover if we’re going to get an accurate inventory the neighborhood and access future impact risks.
Many of the “weekly 40” NEOs are found by robotic asteroid surveys like the phenomenally successful Catalina Sky Survey, located near Tucson, Arizona and the Pan-STARRS survey atop Mt. Haleakala in Hawaii. But they’re not enough. Astronomers can’t see asteroids approaching from the direction of the sun and only visible in bright or twilight skies. For that you need to loft a telescope above the glare of the atmosphere into the blackness of space.
To that end the University of Arizona is spearheading work that would begin efforts to construct an orbiting infrared telescope called the NEO Surveyor that would provide a new way to net potentially frightening asteroids. The Near-Earth Object Surveillance Mission consists of a spacecraft that would continuously collect infrared images of near-Earth space, and the investigation team which will process and analyze the data.
The spacecraft will use highly sensitive heat-sensing cameras to detect the infrared glow from asteroids and comets that are warmed by the sun as they get close to the orbit of Earth. Searching for asteroids by sensing the heat they release allows astronomers to both track them and measure their sizes. Using heat instead of light also means that even the darkest, carbon-rich asteroids won’t escape notice.
“This mission would answer a fundamental question: “Are there asteroids or comets out there that can cause harm to the Earth over the next century?” said professor Amy Mainzer of the Lunar and Planetary Laboratory at the University of Arizona. Mainzer will lead the projected mission in partnership with NASA’s Jet Propulsion Laboratory.
The NEO Surveyor telescope could be launched as soon as 2025 if work begins next year and would survey the skies for at least five years. Its mission goal is to find more than 90 percent of NEOs larger than 460 feet as directed by the United States Congress back in 1998. The telescope will be equipped with two 16-megapixel infrared cameras capable of searching the twilight and dawn skies, and it will be parked at a gravitationally stable point between Earth and the sun called the L1 Lagrange point. L1 is located about 900,000 miles (1.5 million km) in front of Earth in the direction of the sun. The location is ideal for watching for approaching asteroids from the sun’s direction; it also helps the telescope cool to minus –390° F (–234° C) — far from the heat emitted by the Earth — without any cryogenic coolant. Observing in infrared means keeping heat to a minimum.
I find all of this not only exciting but long in coming. We’re smart enough to know that Earth is vulnerable to cosmic forces and to do something about it.
