In the second half of the 20th century, the United States enjoyed stature and prosperity at levels seldom achieved in recorded history. The country's status included predominance in most fields of science and technology (S&T), as well as a phenomenal breadth and pace of innovation. We are now experiencing a global shift to a more level playing field among nations; demographics, economics, and political forces are the driving forces behind this shift. The impact of this shift on U.S. S&T will be significant. By the middle of the 21st century, it is likely that a number of nations will be similarly prosperous and technologically productive. No single nation or group will dominate as the United States did in the latter half of the 1900s. The U.S. share of the global S&T enterprise will decrease, and only a small fraction of U.S. scientists and engineers (S&E) will work on national security problems. This change poses challenges to the roles and conduct of Department of Defense (DOD) S&T. In particular, DOD's ability to maintain an authoritative awareness of S&T developments around the world will become increasingly problematic. Most attempts to quantify these challenges utilize simple linear or exponential extrapolations. Although such approaches are helpful for short-term predictions, they tend to produce unrealistically pessimistic predictions for the timescales considered in this paper. The present work establishes an empirical relationship between an economy's gross domestic product (GDP) per capita and its ability to gene rate S&T knowledge. This paper then employs the results of a full economic analysis for the period 2005-2050 to estimate the S&T knowledge production for each of the world's 17 largest economies. The estimate indicates that U.S. share of S&T productivity will decline from about 26 percent in 2005 to 18 percent in 2050. This decline, while problematic, is not unmanageable. At least through 2050, the United States will remain one of the world's most significant contributors to scientific knowledge. As a result, the U.S. S&T workforce should be large enough, relative to the world S&T workforce, to remain cognizant of S&T developments around the world-although the means of doing so may change. This ability to remain cognizant is important because by 2050, countries other than the United States will produce most scientific knowledge. Maintaining an authoritative awareness of S&T around the world will be essential if the United States is to remain economically and militarily competitive. This awareness includes the ability of the U.S. S&T workforce to authoritatively interpret trends in global S&T. The required awareness can be maintained only if the U.S. S&T workforce is a participant in the global S&T community. This is true for the DOD S&T workforce as well. For DOD to succeed, it will be necessary to find a means to tap the knowledge of the larger U.S. S&T community regarding global S&T. It is only at this level that the United States will have a sufficient number of S&T "brain cells" to actually know what is occurring in the world of global S&T, what is important, and what is not important. Tapping this knowledge will be very challenging for DOD. Nevertheless, we must ensure the global S&T knowledge held by the larger U.S. S&T community is available to the military and that DOD has the internal capability to comprehend and exploit this knowledge through the DOD S&T workforce. The term "DOD S&T workforce" refers to those S&Es who are funded by DOD S&T dollars that fall into the categories of Basic Research (6.1) and Exploratory Development (6.2). This workforce is larger than the DOD federal S&T workforce (often called the in-house workforce), which, of course, has a special role. Some members of the DOD S&T workforce will be employees of DOD; others will be involved through vehicles such as contracts, advisory committees, and cooperative programs with other government agencies.

Terrorist use of radioactive nuclear materials is a serious threat for mass destruction or disruption of civil and military activities. Most worrisome is the use of nuclear devices to cause massive casualties to people and damage to structures. Fortunately, the procurement of adequate material and the engineering design, construction, and transportation and triggering of a nuclear weapon are all difficult problems for terrorist organizations. More likely is a device that combines radioactive materials with conventional explosives to make a radiological dispersion device, commonly called a "dirty bomb." The procurement of nuclear materials for this purpose, the construction of the bomb and its use are all easier than for a nuclear weapon. Fortunately, the effects from the use of a radiological weapon would be much smaller than from a nuclear device, although they could still be very disruptive. Thus, it is important to detect the transport of nuclear weapons and radiological dispersion devices and the materials for their construction. These materials emit gamma rays or neutrons, which can be detected to show the presence and amounts of such materials.

This study is motivated by the observation that the state of health of the United States S&T enterprise seems to be simultaneously characterized by opposite assessments. On the one hand the enterprise is described as being especially vibrant, showing remarkable progress, a high level of innovation and confronted with great opportunities. At the same time the enterprise is described as showing disturbing trends in its workforce, rate of knowledge generation, rate of innovation and international standing.

The most likely means of delivering a nuclear bomb on a major city is through a successful smuggling effort by a terrorist organization. The catastrophic damage it would cause demands cooperative action by all responsible governments. Several U.S. Government programs are in place to deal with this threat.

A strong science and technology (S&T) program has been vitally important to American national security since World War II and has to date given the United States a strategic advantage over competitors. During World War II and throughout the C old War, highly specific and large-scale technology needs led to the concentration of national security S&T (NSST) programs in a few agencies, with little cross-agency coordination. Since the end of the Cold War, circumstances have changed greatly. Meeting new and e merging threats to national security-from global climate change to the proliferation of weapons of mass destruction and global terrorism -requires an effective mechanism for direction, funding, and integration of the highly fragmented and very wide range of Federally supported S&T.

As emerging technologies develop and mature, it is vital that the Department of Defense (DOD) be able to recognize relevant breakthroughs as they emerge and provide the advocacy needed to exploit them quickly. Meeting this responsibility will become increasingly difficult because of the great complexity of the topics involved and the emergence of scientific and technological disciplines that are not extant in the current DOD science and technology (S&T) enterprise, public or private.