Impact! Tracking Near-Earth Asteroids
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.
HARVARD'S FIRST COMPUTERS WERE FEMALE
Eros, the first near-Earth asteroid, was discovered in 1898 in Berlin. Its orbit was determined only after prediscovery images of it were found on three series of Harvard Observatory photographic plates taken between 1893 and 1896. One of the observatory’s keenest “computers” handled the laborious calculations. Her name was Anna Winlock, and she was among the observatory’s first recruits for its unique team of female astronomers.
In the late 19th century, the female physique was considered too fragile to endure long night watches in frigid observatory domes. But Edward Pickering, director of Harvard Observatory from 1877 to 1919, actively sought women to do mathematic analyses of the stars and asteroids on thousands of photographic plates. Some of the women, who earned from 25 cents to 35 cents an hour for 6-day workweeks, went on to become members of the Royal Astronomical Society, emeritae to Harvard University, and directors of other university observatories.
Harvard College Observatory
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 object—an asteroid or comet that passes through Earth’s neighborhood—begins with a set of black-and-white images of it against a starry background.
Astronomical Paparazzi
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 orbit—or 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 space—its distance from Earth—is 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.
