In Hot Pursuit of Asteroids
The whole idea,” says Brian Marsden, director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, “is to try and find these things before they find us.” Given the potential for asteroids to literally and figuratively impact life on Earth in a profound way, asteroids have been quite sought after since the first and largest one, Ceres, was discovered in 1801. After 1898, when astronomers discovered that the 433rd, Eros, frequently passed within only 19 million km of Earth, the pursuit got even hotter.
Cambridge has been an asteroid-and-comet-hunting hub for most of that time. In 1839, Harvard set up an observatory there. In 1850, the first daguerreotype (an early photograph) of a star (Vega) was taken with the Harvard College Observatory’s world-renowned 15-inch refractor telescope. This paved the way for photography and other imaging techniques to revolutionize astronomy as the decades passed. In fact, everything we know about a near-Earth objectan asteroid or comet that passes through Earth’s neighborhoodbegins with a set of black-and-white images of it against a starry background.
In 1882, an astronomer in South Africa borrowed a reporter’s camera, tied it to a telescope, and took the first modern-process photograph of a celestial object. It was of the comet 1882II, which is visible to the naked eye. Unlike daguerreotyping, this technique used negatives. It also allowed for long exposures to light streaming from stars, asteroids, and other space objects too faint to see even through a telescope eyepiece. As a result, the photographs revealed 1882II flanked by a background of never-before-seen stars. The pictures became a public sensation. Harvard College Observatory began aggressively photo-documenting the sky that year. By the time it stopped in 1989, the observatory had amassed half a million glass photographic plates of the entire available heavens, imaged many times over.
“Technically, each Harvard plate could contain at least one asteroid, if one were to look for it,” says Marsden. Since asteroids reflect sunlight, they look like starry points among the real stars. (The word “asteroid” actually means “starlike.”) “When astronomers look for asteroids, they point the telescope at a patch of sky,” says Tim Spahr, technical near-Earth asteroid specialist for the Minor Planet Center. “They take a long-exposure image, and repeat. Then they’ll ‘flip’ through the pictures in the set. The stars appear stationary, but anything close by, like an asteroid or comet, will move.”
As rocky relics from the formation of our Solar System, most asteroids reside in the main asteroid belt. This ring of dust, asteroids, and meteoroids (smaller asteroids) circles the Sun at a distance of around 400 million km, sandwiched between the orbits of Mars and Jupiter. (Earth orbits the Sun at 150 million km away.) Sometimes, Jupiter’s gravity or other factors will “kick” an asteroid out of the main belt into a new orbit. Some of these ejected objects have a chance of intersecting Earth’s orbitor Earth itself someday.
Stars are orders of magnitude more distant than asteroids. The nearest, after our Sun, is about 40 million million kilometers away. Stars aren’t actually stationary, but because of their great distance from us, they appear at a standstill when photographed over time. The motion of asteroids, comparatively, is obvious.
A handful of photographs from one or two nights of observation reveals only a few details about a newly discovered asteroid. Astronomers can calculate its velocity (speed and direction) and its two-dimensional position in space relative to our Earth vantage point. But its true position in 3-D spaceits distance from Earthis trickier. “Imagine looking at a plane overhead. Is it 30,000 feet away? 50,000 feet? At this point, in all honesty, we make an educated guess,” says Spahr. Only by making more observations of an asteroid’s motion, then applying the physical laws of how objects orbit the Sun, can astronomers calculate its exact position and path through space.
Astronomers can get a preliminary idea of an asteroid’s composition, size, and shape at this stage by analyzing its reflected light. For example, measuring the light’s spectra, or its component wavelengths, can determine if the object is made of dull stone or brighter iron, nickel, and other metals, or some combination. Huge numbers of early star and asteroid spectra were taken at the Harvard Observatory, by placing a prism over the telescope lens.
All Automated, All The Time
Such asteroid analysis, which was hand-worked a century ago, is now fully computerized. “These days, astronomical observation is almost never done by a person looking through a telescope,” says astrophysicist Grant Stokes. Still, it wasn’t until the late 80’sthe 19 80’swhen photography’s successor came about: the CCD, or charge-coupled device.
Stokes is the head of the NASA-funded Lincoln Near Earth Asteroid Research (LINEAR) program. LINEAR’s two telescopes, located near Socorro, New Mexico, hunt asteroids in fulfillment of a mandate Congress issued to NASA in 1998 to find 90 percent of near-Earth objects a kilometer in diameter and larger by 2008. “A kilometer is near the threshold for global environmental damage if the object indeed hits Earth.” says Stokes.
Every clear night, automated digital cameras on LINEAR’s telescopes image large sections of the sky accessible from the New Mexico site. Five frames of each section are collected over a two-hour period. A 6 cm x 5 cm CCD chip in each camera acts as an electronic photographic plate, but is a hundred times more sensitive, so long exposures are no longer needed. When photons of light hit a grid of sensors on the CCD, the asteroid’s brightness can be captured, quantified, and analyzed much more precisely than with a photograph. Also, because the CCD output is digital, LINEAR’s computers can find asteroids in it immediately.
LINEAR takes upwards of 6,000 frames per night and can cover all the available sky in a month. Objects moving with unusual speeds or directions are flagged as “interesting”: potential near-Earth objects. The nightly lot of positional data is then handed over to the MinorPlanetCenter. So far, LINEAR and the several other large sky surveys have found 700 km-and-up asteroidsan estimated 60 percent of the objects in the Congressional mandate.
Each day, the Minor Planet Center computes orbits for every single object reported from observatories worldwide. “It takes probably 6 to 10 images over several nights to determine an orbit,” says Spahr. “And to make a prediction of how close that could come to Earth in the future, we need more observationsover several weeks or months.”
Minor Planet Center posts the initial orbit solutions of a few dozen of the new “interesting” objects online per month. That way, astronomers at universities and amateur observatoriesoften one person sitting in their den, checking their backyard telescope via computercan follow up with more observations. If LINEAR is the bulldozer in a gold mine, these smaller outfits are the pan-sifters, pointing their smaller-field-of-view telescopes at the right patches of sky to track intriguing objects.
The longer the baseline of observations, the more accurate the orbit calculations get. “We can, of course, observe a new object in the future,” says Marsden. But for near-Earth objects, “astronomers are often impatient to find out whether there really is a danger to Earth. So it helps a lot if we can find images of them on old photographic plates.”
“Over geologic time there's a hundred percent chance of a very large asteroid impact on Earth,” says Stokes. “But will it happen in the next hundred years, in the next couple of hundred years? It's highly unlikely.” Still, the risk is clearly worth the government funding projects like LINEAR and the Minor Planet Center. If an object is found to be Earth-bound for certain, the save-the-world idea is that we’d somehow nudge it off course before it has a chance to arrive.
“Suppose that $3 million does ensure us against a really large object that would put an end to the human race,” suggests Marsden. “Well, the world’s gross product is about $30 trillion, so here we're spending $3 million to save $30 trillion. That's a pretty good insurance rate.”
European Space Agency: Near Earth Objects
Clear and comprehensive explanations of everything you wanted to know about the study of near-Earth objects. With great animations, too.
Minor Planet Center
The Lincoln Near Earth Asteroid Research project
More About This Resource...
Supplement a study of astronomy with a classroom activity drawn from this Science Bulletin essay.
- Have students read the essay (either online or a printed copy).
- Working individually or in small groups, have students visit the Near Earth Objects section of the European Space Agency's site ( http://www.esa.int/SPECIALS/NEO/) to learn more about asteroids and comets and report their findings to the class.
SubtopicTools and Methods