NEO News (5/12/01) Spaceguard & Extinctions
Dear Friends & Students of NEOs:
This edition of NEO News discusses two different subjects. First is a
brief essay on the meaning of the often-quoted NASA Spaceguard Goal
to find 90% of NEAs larger than 1 km diameter by 2008. The second is
a press release and two journalist comments on recent work by Peter
Ward and colleagues suggesting that the Triassic-Jurassic mass
extinction of 200 million years ago was sudden and catastrophic.
There is no direct evidence of an impact, but the new work is
apparently consistent with an impact-caused global catastrophe.
Welcome to 97 new subscribers to NEO News!
David Morrison
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ORIGIN AND MEANING OF THE NASA SPACEGUARD GOAL
David Morrison (with inputs from Alan Harris and Clark Chapman)
One often sees references to the “NASA Spaceguard Goal” of detecting
90% of NEAs larger than 1 km diameter within a decade. Following are
some clarifications and historical context for this goal.
* Why NEAs (near-Earth asteroids) and not NEOs (all near-Earth
objects)? There are two reasons. First, only NEAs (including the
short-period comets) come past the Earth frequently enough to be
detected in a sky survey like Spaceguard, which aims to find any
hazardous objects decades before they pose a direct threat to the
Earth. Second, we have no way of knowing the number of comets that
may eventually pass near the Earth (become NEOs). There may be
billions of them out in the distant Oort Cloud, but with individual
periods of millions of years, they actually pose a smaller threat
than the nearby NEAs. What really matters is not how many there are,
but how many cross the Earth’s orbit every year. So NEAs are the only
population we can effectively survey, and they also account for most
of the impact risk.
* Why 1 km diameter? Because objects 1 km or larger represent the
greatest hazard. Studies carried out at the time of the original NASA
Spaceguard Survey Working Group in 1992 identified a threshold at
energies near 1 million megatons where an impact had global, not just
local or regional, effects. As first discussed in Chapman & Morrison
(“Impacts on the Earth by asteroids and comets: Assessing the hazard”
Nature 367:33-40, 1994), the individual risk from impacts (the
numerical hazard) jumps by roughly an order of magnitude for energies
at and just above this threshold. More recent work suggests that this
threshold for civilization-threatening impacts is probably nearer 2
km rather than 1 km diameter, but the Spaceguard objective of
detecting 1-km NEAs seems like a reasonable and rather conservative
figure.
* Why don’t we measure diameter directly? Instead we measure
brightness, expressed by an absolute magnitude called H. The
practical objective is to find the NEAs brighter than H=18, which is
approximately equivalent to 1 km diameter for an average asteroid. Of
course, not all asteroids will have this average reflectance, but it
would not be cost-effective to try to measure diameters of most NEAs,
since this would require substantial effort with very large
telescopes. If we want to be sure to get nearly all the darker 1-km
asteroids, it is easiest just to extend the survey to fainter
magnitudes, perhaps to H=18.5.
* Why only 90%? Because that is a reasonable metric. We can’t ever
get 100% (or at least we can’t be sure of having found them all).
Also, 90% is enough to reduce the hazard from the undiscovered NEAs
below the risk from long period comets.
* When is the ten year deadline reached? The specific 10-year
timescale was mentioned in Congressional language in 1994, when the
US House of Representatives asked NASA for a program plan to carry
out the Spaceguard Survey. In Congressional hearings in 1998, NASA
officials adopted the Spaceguard Goal. Measuring from that time, the
90% goal should be reached by 2008.
This is a brief summary of how the goal was developed. It came
initially from the 1992 NASA Spaceguard Survey Report, the timescale
was articulated by the US Congress in 1994, and the goal was formally
accepted by NASA in 1998.
There are two additional important points to be made.
1) We should all understand that the Spaceguard Survey is not limited
to NEAs larger than 1 km diameter. NASA has been accused from time to
time of “ignoring smaller NEAs”, but this is not the case. Unlike
fishing, where you “throw the small ones back,” we collect and follow
up everything within the capabilities of the observing systems
without regard to size (or brightness). Today we are finding about
twice as many NEAs smaller than 1 km as we do those larger than this
value. It is a fact of geometry that the big ones are easier to find
than the smaller ones, so we can expect to reach completion (or 90%
completion) of larger objects sooner than smaller ones. Adding more
or bigger telescopes does not change the fact that we will find all
the big objects sooner than we will find all the smaller ones,
however long that time may be.
2) The “Spaceguard Goal” can be regarded as a metric for tracking
progress of the survey, irrespective of arguments over the
cost-benefit of other levels of surveying. It is not a goal in the
sense that we should stop surveying when we reach 90% completion of
NEAs of H brighter than 18. In order to make a quantitative statement
of progress, we must define specific parameters, namely some
brightness limit to count and some completeness level versus time for
that size. We choose H = 18, 90% completeness, and a target of
achieving that in ten years. We could as well choose H = 19.5, which
for average asteroids corresponds to 500 m diameter, and measure
progress with that metric. Assigning priority to “larger” or
“smaller” objects is a moot point. Many would agree there are valid
reasons that we should eventually take care of the horrific
tsunami-makers, which means going down to 500 m or even smaller. But
there is no practical way to discover only larger or only smaller
NEAs. Larger telescopes do the job faster, and this is increasingly
important as we go to smaller NEAs. But you get what you get, and
various survey strategies make little difference in the makeup of the
catch, just the total numbers.
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PRESS RELEASE ON TRIASSIC-JURASSIC MASS EXTINCTION
University Of Washington
May 10, 2001
Collapse of simple life forms linked to mass extinction 200 million years ago
A mass extinction about 200 million years ago, which destroyed at
least half of the
species on Earth, happened very quickly and is demonstrated in the
fossil record
by the collapse of one-celled organisms called protists, according to new
research led by a University of Washington paleontologist.
“Something suddenly killed off more than 50 percent of all species on Earth,
and that led to the age of dinosaurs,” said Peter Ward, a UW Earth and space
sciences professor.
Evidence indicates the massive die-off was linked with an abrupt drop in
productivity, the rate at which inorganic carbon is turned into organic
carbon through processes such as photosynthesis. The waning productivity
coincided with a sharp decline in radiolaria (included among protists),
which was the focus of the new research. One example of productivity, Ward
explained, occurs in the spring when fertilizer washes into waterways and
triggers large algae blooms. The processes at work in that scenario were
reversed 200 million years ago, he said.
There is no definitive evidence yet on what caused the demise of so many
species, Ward said. However, the suddenness of the event is similar to two
better-known mass extinctions – one 250 million years ago at the end of the
Permian period that killed some 90 percent of all species, the other 65
million years ago at the end of the Cretaceous period that sent the
dinosaurs into oblivion.
The extinction 200 million years ago, at the boundary between the Triassic
and Jurassic periods, killed the last of the mammal-like reptiles that once
roamed the Earth and left mainly dinosaurs, Ward said. That extinction
happened in less than 10,000 years, in the blink of an eye, geologically
speaking.
Ward is the lead author on a paper detailing the evidence, published in the
May 11 edition of the journal Science. Others participating in the research
are James Haggart and Howard Tipper of the Geological Survey of Canada in
Vancouver, British Columbia; Elizabeth Carter, a researcher at Oregon’s
Portland State University; David Wilbur, a UW oceanography research
scientist; and Tom Evans, a UW junior in chemistry and Earth and space
sciences.
The evidence from the extinction was gathered at two sites in the Queen
Charlotte Islands, off Canada’s British Columbia coast.
“These sites are among the most remote places in the world,” Ward said.
“There are no roads anywhere close by. The forests are virgin old growth,
and the wave action is such that you can’t get there by boat.”
Samples from a spot called Kennecott Point, in the northern Queen
Charlottes, and from Kunga Island, about 100 miles to the southeast, showed
a sharp decline in the presence of organic carbon, even at places where
levels of inorganic carbon rose. The organic carbon decline correlated with
the decline of radiolarians, one-celled organisms that serve as a food
source for a number of marine species.
“These provide the best record of how nasty the extinction was at this
boundary,” Ward said.
The mass extinction 200 million years ago occurred just before the breakup
of Pangea, which contained all the land on Earth in one supercontinent. At
the time, the Queen Charlotte Islands – which now lie between 52 and 54
degrees north latitude – were probably on the equator or in the southern
hemisphere, Ward said.
“These are tropical fossils. There are many kinds of fossils in these
rocks,” he said.
And they tell a story of a calamity that came on with stunning swiftness.
“This is the first time ever that we can see how sudden this event was,” he
said. “It was very quick, not a long protracted episode.”
Ward now has done research on the last three of the Earth’s mass extinctions
(scientists know of five) and has found that each happened quite quickly.
Bolstered by a recent astrobiology grant from the National Aeronautics and
Space Administration, he plans to lead researchers back to the Queen
Charlottes this summer to look for more clues in the Triassic-Jurassic
extinction, including potential causes.
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For more information, contact Ward at argo@u.washington.edu
