How Big Are Those Killer Asteroids? A Critic Says NASA Doesn’t Know.
More than 14,000 known asteroids zip through Earth’s neighborhood. They will all miss Earth in the coming decades.
But hundreds of thousands more have not yet been discovered, and whether any of those are on course to slam into our planet, no one knows. So finding and tracking all the asteroids that could cross Earth’s path would allow officials to issue warnings and potentially provide time to deflect dangerous ones.
The community of scientists contemplating such doomsday possibilities is small and usually cordial — at least it was until Nathan P. Myhrvold barged in. Once the chief technologist at Microsoft, Myhrvold moved on to other endeavors like a six-volume, 2,438-page compendium of cooking knowledge that has been celebrated by chefs. (A sequel, about baking, is in the works.)
深入考虑出现这种危急时刻的可能性的科学家圈子人数很有限，彼此之间的交往相当友好——至少在内森·P·迈尔沃尔德(Nathan P. Myhrvold)闯入之前是如此。迈尔沃尔德曾经担任微软的首席技术专家，之后转向其他事业，比如撰写六卷本、2438页的烹饪知识百科全书，这套书在厨师中已颇具名气。（有关烘焙的那本正在撰写之中。）
He has also become a statistics scold of scientists. His latest target is NASA, in a squabble over data from the Wide-field Infrared Survey Explorer spacecraft.
他也成为以数据质疑科学家的人。此人最近的标靶是美国航天局(NASA)，并已就该机构的广域红外线巡天探测卫星（Wide-field Infrared Survey Explorer，简称WISE）获取的数据与之陷入争执。
WISE, launched in 2009, snapped images of three-quarters of a billion stars, galaxies and other celestial objects, including the heat emissions of asteroids. An offshoot called Neowise used the heat data to calculate the size and reflectivity of 158,000 asteroids.
Myhrvold contends that the Neowise analysis is deeply flawed. “The bad news is it’s all basically wrong,” he said. “Unfortunately for a lot of it, it’s never going to be as accurate as they had hoped.”
He submitted his own analysis of the Neowise results to the journal Icarus.
Myhrvold is not arguing that NASA has overlooked dangers from the known asteroids. But he does question whether scientists know as much as they think they do.
He has also zeroed in on a proposed space-based telescope with a price tag of more than $500 million, the Near-Earth Object Camera, or Neocam, a project headed by some of the same scientists whose work he is second-guessing.
他也把枪口瞄准了一个计划中的、造价超过5亿美元的天基望远镜，即近地天体望远镜（Near-Earth Object Camera，简称Neocam）。领导该项目的一些科学家，与他正在质疑的、获得了相关成果的科学家基本是同一拨人。
Most of the millions of asteroids are found between the orbits of Mars and Jupiter, but some dip closer to the sun. There is no doubt that some will hit Earth someday.
The bigger the asteroid, the greater the potential cataclysm. The reflectivity of the surface — what astronomers call albedo — tells how easily it can be detected.
“From the practical perspective of finding asteroids,” Myhrvold said, “it’s really important that we know the distribution of diameters and the distribution of albedos.”
According to NASA’s scientists, the estimates of asteroid diameters made by Neowise are often within 10 percent of the actual size. But Myhrvold says the uncertainties are much greater, more than 100 percent in many cases.
Space agency officials disagree. “He’s a very smart man,” said Lindley Johnson, who oversees NASA’s efforts to protect the planet from space rocks. “But that doesn’t make him an expert in everything.”
Other scientists say that Myhrvold’s criticisms have merit.
“I do think he’s performed a really very useful service,” said Alan W. Harris, a senior research scientist at the Space Science Institute, “to do the error analysis more carefully and alert people that you shouldn’t just take some of the data out of the WISE table and just assume they’re gospel.”
“我真的认为他做了一项非常有益的工作，”太空科学研究所(Space Science Institute)的资深科学家阿兰·W·哈里斯(Alan W. Harris)说道，“更仔细进行误差分析，然后警告人们你不该只是从WISE的数据表中拿出一些数据，然后就对它们深信不疑。”
But even if Myhrvold is correct, Harris said the Neowise data “has good enough validity to be useful for most purposes.”
Myhrvold was drawn into asteroids research when the B612 Foundation, a nonprofit organization that advocates planetary defense efforts, asked him to contribute money for Sentinel, a privately financed $450-million, asteroid-finding spacecraft.
“What they didn’t know is that I had been interested in killer asteroids for a long time,” Myhrvold said. After all, an asteroid slamming into Earth is believed to have brought the age of dinosaurs to an end 66 million years ago.
But Myhrvold learned of several other efforts, including NASA’s Neocam, which would cost about $500 million (plus the rocket to launch it), and the Large Synoptic Survey Telescope, a ground-based observatory already under construction in Chile.
不过，迈尔沃尔德了解到，还存在好几个类似的项目，包括NASA预计耗资5亿美元（外加用来发射它的火箭的费用）的Neocam项目和一个正在智利建设的地基天文台——大型综合巡天望远镜(Large Synoptic Survey Telescope)。
Each group offered computer simulations of how many unseen asteroids would be spotted, making different assumptions.
In a paper published in March in The Publications of the Astronomical Society of the Pacific, Myhrvold took a simpler approach to provide an apples-to-apples comparison. He looked at the volume of space that each telescope could observe, then calculated what fraction of unseen asteroids would pass through that space.
在今年3月发表于《太平洋天文学会汇刊》(The Publications of the Astronomical Society of the Pacific)上的一篇论文中，迈尔沃尔德采取一种更简单的策略，提供了一种具有可比性的对照。他查看了每座望远镜所能观测到的太空面积，然后计算出有多大比例的不可见小行星将会穿越这片太空。
“I thought it all looked quite reasonable,” said Steven R. Chesley, a senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Nathan definitely brings a fresh approach.”
“我认为这一切看上去很合理，”在加州帕萨迪纳市的美国宇航局喷气推进实验室(Jet Propulsion Laboratory)担任高级研究员的史蒂芬·R·切斯利说。“内森绝对给出了一种新办法。”
In writing the paper published in March, Myhrvold talked with people working on the different projects, including Amy Mainzer, the principal investigator for Neocam and Neowise.
Myhrvold said that Mainzer argued that a ground-based telescope could not detect asteroids within Earth’s orbit because they would be too close to the horizon.
That seemed strange to Myhrvold, because even amateurs can easily spot celestial objects far inside Earth’s orbit like Venus, 26 million miles closer to the sun than Earth.
Myhrvold said Mainzer responded that large telescopes were not designed to point that close to the ground. But the one in Chile is, he noted.
Chesley of NASA is coordinating a more detailed analysis of the telescope’s asteroid-finding prowess, with preliminary findings expected in late summer.
In recent presentations, NASA officials have portrayed Neocam more as a complement than a competitor. “It’s a team sport,” Johnson said. “There isn’t any one system that is going to provide us all the data we need.”
NASA declined to make Mainzer available for an interview. Responding to written questions, she said the Neowise team stood behind the findings and that the results had been validated by independent observations and other researchers.
The Principle of Hibachi Grills
It struck Myhrvold that the albedo calculations of the Neocam team violated a basic tenet of physics known as Kirchhoff’s law of thermal radiation, which says shinier objects radiate less heat.
A simple demonstration of that, Myhrvold said, is the shiny chrome cooking surface of a restaurant hibachi grill. A dark grill surface would bathe diners in uncomfortable waves of heat. The Neocam models failed to take into account the effects of reflected sunlight, he said.
He then took a closer look at Neowise, too. One aspect Myhrvold found curious was that in more than 100 instances, the Neowise team reported asteroid diameters that matched exactly what had previously been determined by other methods like radar measurements and spacecraft flybys. “I think it’s a pretty strong smoking gun that something is wrong,” he said.
For her part, Mainzer said the asteroids with exact measurement matches were used for calibration and thus the values were set to those that had been previously observed.