first Orbiting Solar Observatory, OSO-I. In summary, the progress to date from an analysis of the OSO-I data is as follows:

Solar ultraviolet and X-radiation observed for more than a year beginning in spring 1962.

Far ultraviolet analyzed in detail and continuously.

Activity centers observed directly in radiations which govern Earth's atmosphere.

Comparative measurements of X-ray brightness of the quiet Sun, sunspot groups, and solar flares.

Time and brightness sequences shown to exist in series of small X-ray flares.

Solar X-ray output showed marked increases in fractions of a second.

Lyman-alpha hydrogen ultraviolet line brightening first seen from flares.

Quiet Sun fluxes of low energy neutrons and Bremsstrahlung X-rays shown to be vanishingly small.

By way of illustration of the importance of these results, let us discuss in some detail the data obtained by the major solar radiation research instrument carried by OSO-I. The Goddard scanning spectrometer analyzed minutely the time variations of the Sun's far ultraviolet or soft X-ray spectrum. In the wavelength range studied, 50 to 400 Angstroms, the light is emitted as many discrete spectral lines. These are identifiable as coming from specific elements in various highly ionized states. Such emissions originate in the Sun's corona, and are directly responsible for much of the varying conditions of the Earth's upper atmosphere. The presence of such radiations reveals temperatures of the order of a few million degrees Centigrade. This is anomalous because the Sun's white light indicates a temperature of only 5,000 degrees Centigrade. Probing the nature and causes of the high temperature of the Sun's corona was the principal objective of OSO-I.

Solar centers of activity, such as sunspot groups, produce coronal heating which directly increases the ionization of the E and F layers of the Earth's ionosphere at 60 and several hundred miles height respectively. OSO-I discovered that this heating lags about a month behind the visible light development of an active center, and persists longer. The magnitude of the hea ing is not great, only 100,000° C. above the "quiet time" coronal temperature of 1,750,000° C. However, the brightnesses of individual coronal emission lines. were observed to increase markedly up to 400 percent for the highest temperature ions. The total ultraviolet flux in these wavelengths changed by about 50 percent. These observational data may be compared directly to Earth-based measurements of solar radio emission, and to the traditional indices of solar photospheric and chromospheric activity. Anomalous differences exist, which prove the importance of this class of spacecraft observations to an effective study of Sun-Earth relations. For 2 months the radio and visible light indices followed closely the variation of the soft X-radiation. Then an important discrepancy occurred, when the indices remained low while the X-ray flux increased to a high peak. Ionization in the Earth's atmosphere responded to the X-rays, of course. Only by studying the ionizing radiation directly can we acquire the under

standing necessary to improve our radio propagation forecasting techniques, or to develop the ability to predict solar proton events.

In addition, the planned measurements of the space environment, set forth earlier, will contribute to advancing our knowledge of SunEarth relations.

Orbiting Solar Observatory (OSO) status-The first Orbiting Solar Observatory, OSO-I (fig. 119), was launched successfully from the Atlantic Missile Range on March 7, 1962. This spacecraft which is still in orbit contains 13 scientific experiments which are studying X-ray and ultraviolet radiations from the Sun. More than 2,000 hours of data have been obtained from this satellite and these data are currently being analyzed. It is planned that seven more spacecraft similar to the one now in orbit will be launched between now and 1968. Figure 120 depicts the current OSO status. The second of the OSO series, OSO-B, is expected to be launched early in 1964. Final environmental testing on this spacecraft is now in progress. OSO-B will study X-rays, gamma rays, and ultraviolet radiation from the Sun, continuing the work of OSO-I with more sophisticated instruments. In addition, OSO-B will have the capability of scanning the solar disc as opposed to merely pointing at the center of the disc. OSO-C which is due to be launched in late 1964 is currently in a test phase at the contractor's plant. The flight experiments for this spacecraft are being produced by the individual experimenters and will be sent to the contractor for integration into the spacecraft early in the summer.

A contract has been signed for preliminary work on the follow-on OSO's D through H. Fabrication of the OSO-D spacecraft and ex

periments is now underway. Fabrication of OSO-E, for which a payload of experiments has been selected, will begin in about 6 months. We are currently planning to launch these spacecraft following OSO-D at 9-month intervals to take full advantage of the spacecraft life and to thoroughly study the solar cycle with planned overlap.

Advanced Orbiting Solar Observatory (AOSO) status. During the past 2 years we have examined carefully the future requirements, beyond the present OSO, for the instrumentation and spacecraft that will be needed to provide the detailed information on the environment of particular regions on the Sun. This examination resulted in the establishment of performance specifications of an Advanced Orbiting Solar Observatory to provide the increased pointing, telemetry, volume, and weight capability necessary for the study of the environment of active areas of the Sun. Spacecraft preliminary design studies were made by three contractors selected on a competitive basis. In the fall of 1963, one of the three study contractors, Republic Aviation Corp., was selected to proceed with a Phase I development contract. The characteristics of the Advanced Orbiting Solar Observatory as determined in the preliminary design studies, are shown on figure 121. The basic objective of Phase I is to establish a detailed observatory design configuration and development plan which will provide the best compromise between performance, reliability, cost, and development schedule. Figure 122 illustrates the scope of the Phase I contract. At the present time detailed design studies of the various subsystems of the spacecraft are underway. The experiments and experimenters for the first spacecraft are being selected. Bread

boards of the spacecraft subsystems are being constructed and the structural model testing is underway.

Future

The Phase I portion of the program is scheduled for completion the latter half of this year, at which time we will be ready to proceed with the Phase II development program in order to launch the first AOSO at the next period of maximum solar activity.

Objectives

GEODESY

The broad objective of satellite geodesy is to obtain data on the figure and structure of the Earth that are not obtainable by other techniques. More specifically this may be stated as follows:

Dynamical geodesy to determine the physical distribution of mass within the Earth; and

Geometrical geodesy to connect the coordinate systems of various islands and continents. The first of these objectives is particularly important as regards trajectory planning for satellites, probes, and missiles. In addition, it will also provide knowledge of the overall structure of the Earth. The second of these objectives is necessary for accurate mapping and will provide basic information needed to study intercontinental drift.

Progress

Several advances were made in satellite geodesy during the past year:

Pear shape of Earth reconfirmed.

Higher harmonics of Earth's gravitational potential were determined. Ten zonal and 19 tesseral harmonics have now been determined.

Areas of excess and deficiency in Earth's mass distribution determined.

Strength of Earth's mantle determined.

Earth's equator not symmetric.

By way of illustration, let us discuss briefly the topic of mass deficits and excesses which are revealed in the satellite studies. It has been known for some time that the outer regions of the Earth are not uniformly dense. A mountain disturbs the local gravity field, but surprisingly not as much as its mass would indicate. To explain this anomaly, geologists postulated that material under mountains is less dense than material under the sea. Thus, it seemed that this compensating effect implies that a layer, say several hundred miles thick, has approximately constant mass in any column of the same size and, hence, is disturbed if large amounts of it move with respect to other large volumes just as a very thick fluid might move. Satellite observations have shown that this is not the case, strictly speaking.

The data from the satellite orbits which have been analyzed yield a result for the figure of the Earth (fig. 123) in which there is a positive anomaly, or a bump (about 160 feet), in the region of the Western Pacific near Indonesia and the Philippines. Other smaller positive anomalies occur elsewhere, and a large negative anomaly, or dimple about the same magnitude as the positive anomaly, occurs in the