by Edna DeVore - Deputy CEO
"Why aren't you using this telescope to search for asteroids? Don't you know that near-earth asteroids are the biggest threat to the survival of everything, including us, on this planet? Why is it that you astronomers don't focus on what's important?" an earnest man asked at the recent American Astronomical Society (AAS) meeting in Albuquerque, pointing at a model of the SOFIA observatory. He was a local citizen with a passion for saving the human race from the sure destruction of asteroid impact. He wanted to know how NASA's future infrared airborne observatory, SOFIA, would be used to find NEOs (Near Earth Objects), a collection of boulder- to mountain-sized rocks that travel around the Sun, passing through our immediate neighborhood. In his opinion, all astronomers should be working on one project: the discovery and tracking of asteroids and comets that pass near Earth. He was convinced that professional astronomers were on the wrong track if they studied anything else.
Indeed, there is a genuine reason to be concerned. Earth has been hit by asteroids and comets many times over the past 4.5 billion years since its formation. There's a big hole in the Arizona desert, Meteor Crater, a.k.a. Barringer Crater, where a rock about 35 meters (38 yards) across plowed into the Earth approximately 40,000 years ago. The energy of that impact has been estimated as equivalent to 300 Hiroshima-type atomic bombs -- a big explosion with lots of damage.
More famously, the death of the dinosaurs is attributed to a worldwide catastrophe caused by the impact of an object estimated to be about 12 kilometers (7 miles) across on the Yucatan Peninsula 65 million years ago creating the Chicxulub Crater -- a much bigger explosion with global consequences.
Anyone looking at the Moon can see lots of craters, uneroded evidence of impacts collected over billions of years. And there have been two near misses this year. In March 2002, a big rock (estimated 40-80 meters, or 44-87 yards) swung by Earth only a little farther away than the Moon, and on June 14, an asteroid the size of a soccer field buzzed the planet with closest approach a mere 75,000 miles, 1/3 the distance to the Moon. That was a close call. There is reason to be concerned.
So, why not use the biggest (largest diameter) telescopes on the ground, in space, or in airplanes like SOFIA to discover and track asteroids? Isn't bigger always better? Not necessarily.
I explained to the earnest citizen that SOFIA would be very useful in determining the size of asteroids and comets via occultation research and even the composition via spectroscopy, but not particularly useful in making initial discoveries. The same is true of the great Hale Telescope on Mt. Palomar in California, the Keck Telescope in Hawaii, and even the Hubble Space Telescope in orbit about Earth. The reason is simple. These telescopes "see" objects with exceptional clarity because they can collect a great deal of light from even the dimmest objects in the sky such as distant galaxies or nearby small objects like asteroids. All this light allows for superb resolution, capturing the exquisite details invisible to smaller diameter telescopes.
But none of these big telescopes "see" very much of the sky at a time. They have a small field of view (FOV). For example, the FOV for SOFIA is about 8 minutes of arc (astronomers use "arcminutes"). If you hold a penny at arm's length, the eye of Abraham Lincoln is about 8 arcminutes across. For comparison, the Moon is 1/2 degree, or 30 arcminutes in diameter. The Palomar telescope's FOV varies according to the instrumentation used; at a maximum it's about the size of the full Moon, but more typically just a couple of arcminutes. A large diameter telescope is good for details, but not the right tool for observing larger areas of the celestial sphere in a single image.
To find asteroids or comets, it's much more useful to survey a "big" piece of the sky, to look for the tracks they make speeding through the neighborhood. You want a telescope with a wide FOV that is designed to see a big piece of the sky at once, not a telescope with a large diameter mirror. A quick survey of NEO observatories confirms this. The telescopes used by the LONEOS and Catalina projects see 2.9 x 2.9 degrees at a timea block of sky equivalent to 36 full Moons lined up edge to edge in a 6 x 6 array. Others see a smaller portion of the sky, as little as a 38 arcminute FOV, but run automatically to survey the sky continuously. Of the telescopes dedicated to NEO searches, the primary mirrors vary from 0.5 to 1.2 meters in diameter. They're moderate-sized telescopes with wide FOVs.
An NEO can be discovered as it makes a streak on the image or appears in different places on successive images because it moves across the field of view as it orbits the Sun. By comparing several different images of a piece of the sky over time, the orbit of the NEO can be derived. We'll know where it is, predict where it is going, and understand whether we need to worry about that object or not.
So far, the NEO searches have turned up more than 1,900 objects tracked by the Minor Planet Center (MPC). About 600 of these are larger than 1 km in size, and about 440 are watched as potentially hazardous objects. You can see an up-to-date map of the asteroids and comets at here.
The man from Albuquerque knew about the NEO projects, but during our conversation came to understand that wide FOV telescopes are best for finding NEOs and that the "biggest" telescopes aren't necessarily the right tool for the job. Sometimes, bigger isn't better.