Passage IV

NATURAL SCIENCE: This passage is adapted from the article "A Flash From the Past: New Evidence Supports Moon Blast" by Henry Fountain (©2003 by The New York Times Company).

Humans have gazed at the Moon in wonder since ancient times, but what Dr. Leon Stuart observed one night in 1953 was more wonderful than what anyone had seen before or since.

Looking through his eight-inch telescope at his home near Tulsa, Okla., Dr. Stuart, a radiologist by profession but an astronomer by avocation, saw and photographed a bright flash on the Moon's surface.

Dr. Stuart was certain that he had witnessed a small asteroid hitting the Moon, the flash being the fireball from the event. An amateur astronomy journal published his photograph and report, and it has remained a curiosity over the years. While some scientists thought his explanation plausible, others were convinced that he saw an optical aberration or a much closer object, like a meteorite in Earth's atmosphere (or, embarrassingly, an airplane passing overhead).

Now new research shows that Dr. Stuart's flash on the Moon was no flash in the pan. An astronomer at NASA's Jet Propulsion Laboratory, poring over high-resolution lunar photographs, has found a fresh crater in the precise area where Dr. Stuart saw his flash.

"I think it's a very good candidate," Dr. Bonnie J. Buratti, an astronomer, said of the crater, which is about 250 to 800 yards in diameter. Dr. Buratti worked on the project with Lane Johnson, then a student at Pomona College; the two published their findings in the astronomical journal Icarus.

At the Jet Propulsion Laboratory, Dr. Buratti served on the science team for the Clementine spacecraft, which thoroughly photographed the Moon in 1994. As part of that work, she and others had looked for evidence of transient phenomena like asteroid impacts and had found none. Just as they were preparing a paper on their research, a colleague mentioned what is known as Stuart's event.

"I had never heard of it," Dr. Buratti said. But her curiosity was piqued, so she found a 1956 copy of the amateur journal The Strolling Astronomer and looked at the photo.

She and Johnson were able to determine the approximate location of the flash, a circular area with a radius of about 20 miles. From. the brightness of the event, they estimated the force of the impact to be about half a megaton, equal to a small hydrogen bomb. Their best guess as to the size of a feature created by such an impact, including a crater and its ejecta blanket, the material thrown out around the sides, was 1.2 miles or less-too small to see from ground-based photographs.

So first they looked at photographs taken by the lunar orbiters in the 1960s, which mapped areas of the Moon to prepare for the Apollo landings. These photos were inconclusive, so they turned to the huge database of two million images from Clementine. Many of these photographs were taken with color filters, which can help in determining the age of a surface feature.

On the Moon, material that is freshly exposed has a slight bluish tinge. Over time, because of the constant bombardment of cosmic rays, other high-energy particles and micrometeorites, the structure of the material changes and iron particles tend to predominate, making the material slightly red.

In the Clementine photos, Dr. Buratti and Johnson found one small crater that was “very, very blue and fresh appearing," Dr. Buratti said. It also happened to be in the exact center of the area they were looking. And it was the proper size-slightly less than a mile across, including the ejecta blanket. Dr. Buratti estimated the size of the asteroid at 20 yards in diameter.

She said that although there was a good deal of uncertainty in their study, she was "about 90 percent" confident that the crater was the one created by the fireball Dr. Stuart observed. "There's no other object that stands out as a candidate," she said.

Dr. Stuart, who died in the 1960s, was not one to make wild claims. "He was very careful to eliminate all the other possibilities," she said. "At the time, scientists didn't even agree that craters were caused by impacts. So he was very conservative."

If Dr. Stuart observed an asteroid impact, he saw something that was extremely rare: a rock of that size hits the Moon only once or twice a century, according to best estimates.

But Dr. Buratti said she wasn't surprised there was a witness. "I would contend that at any given time, some amateur or professional astronomer is watching the Moon," she said. "With a blast of this sort, someone would be likely to see it."