INNER SOLAR SYSTEM EXPLORATION

Both American and Russian spacecraft have progressed inward toward the Sun as far as the planet Venus. The Mariner 2 and Mariner 5 spacecraft, the series of Russian Venera probes, and the measurements from Earth-based radar telescopes have revealed Venus to be unique in having a very slow and retrograde rotation and a carbon dioxide atmosphere nearly 100 times as heavy as our own. We think that the cloud layers are so thick as to preclude any chance of seeing the surface from a flyby spacecraft, but we do not really know this for certain. The answer will come in 1973 when a Mariner spacecraft will pass Venus on its way to the unexplored planet Mercury.

Mariner Venus/Mercury 1973

The Mariner Venus/Mercury 1973 mission was approved by the Congress in Fiscal Year 1970 to take advantage of one of the highly efficient but infrequently occurring opportunities to reach the remote planet Mercury using a gravitational assist from Venus. With this maneuver we will be able to reach Mercury using an Atlas Centaur launch vehicle instead of the larger Titan. The single spacecraft to be launched in 1973 is a modification of the Mariner 1969 spacecraft design, utilizing many available Mariner subsystems in order to minimize cost and maximize reliability (Chart SL71-2533). One necessary modification required for this mission is to tilt the solar panels away from the Sun to prevent excess heating as the spacecraft moves in closer to the Sun than any prior manmade object.

After launch during October-November 1973, the spacecraft will pass Venus in early February 1974, taking our first closeup television pictures of the planet and making measurements of the Venus atmosphere with ultraviolet, infrared and radio occultation experiments. During the transit from Venus to Mercury, extensive particles and fields measurements of the interplanetary environment will be made in a region of space yet to be explored by spacecraft.

Late in March 1974, after a fast trip of 252 million miles in only 137 days, the Mariner spacecraft will reach Mercury. We know relatively little about this innermost planet. Viewed from Earth, it is never far from the Sun, making telescope measurements very difficult. Much of our recent knowledge of Mercury has come from Earth-based radar measurements. For example, it was believed

for centuries that Mercury always presented the same side toward the Sun, with a rotational period equal to its 88-day period of revolution about the Sun. In the past few years, however, we have been able to reflect radar signals off Mercury and have clearly measured a rotational period of only 59 days. This means that Mercury is locked in a unique way to the Sun, rotating exactly three times in every two revolutions about the Sun. We also know the size of Mercury is somewhat larger than the Moon, and that it is surprisingly dense.

A state of outgassing on Mercury more advanced than for Earth or even Venus is expected and would at least partially explain this high density. We know little else about Mercury, leaving many questions to be answered by our first flyby mission.

The Mariner spacecraft is well equipped to make extensive measurements of the planet, including high-resolution coverage of its lighted surface with television pictures, measurement of the planet's magnetic field and major body characteristics, a search for any atmosphere or ionosphere, and temperature mapping of both the lighted and dark sides of the planet. The use of both S-band and X-band radio frequencies aboard the spacecraft will permit the measurement of plasma attenuation effects in interplanetary space to be made with greatly improved precision, which in turn will substantially improve the tracking accuracy on all future planetary spacecraft. The improved tracking of this Mariner spacecraft can also provide another check on the accuracy of Einstein's general theory of relativity. It is now beginning to appear possible to send the spacecraft around the Sun and back to Mercury for a second encounter nine months after the first, a maneuver which would further increase the value of this promising mission.

The Mariner 1973 Project is managed by the Jet propulsion Laboratory (JPL). During the past year, with the help of a team of scientists especially assembled for this purpose, JPL has completed the preliminary design of both the spacecraft and the mission. The scientific in truments and principal scientific investigators have been selected and the appropriate contracts have been negotiated. Proposals have been received from industry; and the selection of a systems contractor to perform the detailed design, assembly, and test of the flight spacecraft will occur in April 1971. As we are currently less than two and one half years from launch, the project will hereafter move at a very fast pace. By the end of Fiscal Year 1972 the systems contractor will have completed the overall systems design and the detailed design of the spacecraft. Subsystems fabrication and development testing will be well progressed by the end of Fiscal Year 1972. Systems assembly, environmental testing, and fabrication of the Centaur launch vehicle are all scheduled to commence during the second half of Fiscal Year 1972. Progress on the project has been excellent to date, and we expect this first mission to the planet Mercury to be one of the most scientifically productive and economically efficient Mariner projects that we have ever conducted.

Helios

To reach the planet Mercury, the Mariner 1973 spacecraft will have to travel almost 60 percent of the distance in toward the Sun from Earth, where the solar intensity is six times that received at Earth. The Helios spacecraft to be launched in 1974 and 1975 will venture even farther-up to three-quarters the distance to the Sun. To do so, Helios will have to survive a solar intensity 16 times greater than that experienced at Earth. This ambitious solar probe mission is a joint effort between the Federal Republic of Germany and the United States (Chart SL71-2534) with the West Germans developing the spacecraft and the United States providing the Titan Centaur launch vehicle, mission support, and three of the flight experiments. Helios is the largest cooperative international project yet to be attempted by NASA. We have assigned project management responsibility to our Goddard Space Flight Center (GSFC).

In addition to its scientific return, the 1975 Helios launch will provide double benefits to the U.S. by also serving as an additional confidence-building flight of the Titan Centaur system being developed for the Viking missions in 1975. Following Viking, a second launch of the Helios spacecraft is planned for late 1975.

Progress on this cooperative project has been very good to date, and working relationships between the teams in Germany and in the U.S. have been excellent. In Fiscal Year 1972 a thermal model of the spacecraft will be constructed in Germany and shipped to the Jet Propulsion Laboratory in California for test in the JPL space simulator. Engineering models of the three U.S. experiments will be delivered to the German systems contractor, Messerschmitt-Bolkow-Blohm, for integration and test with an engineering model of the complete spacecraft.

OUTER SOLAR SYSTEM EXPLORATION

Out beyond the orbit of Mars lie most of the bodies in our solar system. Known objects of great interest include five planets with at least 29 satellites, hundreds of comets, tens of thousands of asteroids. Out to at least Jupiter's orbit are the regions of interplanetary space dominated by the particle and magnetic fluxes emanating from the Sun, and finally beyond the influence of our Sun is galactic space.

Pioneer F/G

We will initiate the exploration of this vast region of space with the launch of the Pioneer F spacecraft in late February 1972. Pioneer G will be launched at the next opportunity in late March 1973. The scientific objectives of these spacecraft are to conduct exploratory investigations of the interplanetary medium beyond the orbit of Mars, the nature of the belt of thousands of asteroids which lie between the orbits of Mars and Jupiter, and the environmental and atmospheric characteristics of the planet Jupiter.

The Pioneer F and G spacecraft are descendents from the highly successful Pioneer A-E series, four of which are still returning valuable data after up to six years in deep space orbiting the Sun. However, there are also a number of new engineering features incorporated into the Pioneer F/G model. For example, this will be the first NASA spacecraft completely powered by a radioisotope thermoelectric generator (RTG). The RTG enables the Pioneer to continue to operate with full power even at Jupiter's distance from the Sun where the solar energy is reduced to 25th of what we receive on Earth. While this spacecraft is conconsiderably smaller and lighter than current Mariners, we have been able to include 13 experiments in its 53 pound payload.

Two of the experiments aboard will make measurements of the asteroidal particle distribution and of the penetrating power of those particles as the spacecraft traverses the Asteroid Belt.

Other experiments will monitor the particle and field characteristics of the solar wind as the spacecraft passes over 500 million miles out to Jupiter and beyond. However, the mission tends to get dominated by that colossus of the Planets, Jupiter (Chart SL71-2535). This huge planet is 318 times as massive as Earth, outweighing all the other planets combined. Yet its density is so low that

Jupiter must be composed largely of the light elements hydrogen and helium. Telescopic measurements have also detected methane and ammonia in its upper atmosphere. With a diameter of over 80,000 miles and a rotational period two and one-half times as fast as Earth's, Jupiter has an equatorial peripheral velocity of over 25,000 miles per hour, a velocity which contributes to the generation of an intense magnetic field and to strong motions in its thick atmosphere. The ever-changing bands of Jupiter's atmosphere are of many hues, ranging from grays, browns, and blues to pink and yellow. Immersed in one of these bands is an enormous red spot which measures over three times the diameter of the Earth. There is no known explanation for this red spot, although one theory holds that it is a result of a giant lightning discharge which has generated complex organic compounds from the four gaseous constituents of the basic Jovian atmosphere, i.e., hydrogen, helium, methane, and ammonia. A laboratory experiment simulating these conditions has produced reddish organic compounds, leading to speculation that the Jupiter atmosphere is duplicating the early conditions on Earth which led to the formation of living organisms.

The Pioneer F and G spacecraft will begin the direct investigation of Jupiter. A scanning camera aboard the spacecraft will take numerous pictures of Jupiter at resolutions several times better than can be obtained from the best of Earth's telescopes. Other instruments will measure Jupiter's intense magnetic field and radiation belts and will scan the planet to measure the composition of its upper atmosphere. An infrared radiometer will measure the temperature distribution around the planet to shed light on how Jupiter can apparently generate several times more energy internally than it receives from the Sun.

Development of the Pioneer F/G spacecraft (Chart SL71-2536) is on a very tight schedule, but is progressing well. TRW is the Pioneer systems contractor under the direction of NASA's Ames Research Center. A prototype spacecraft has been assembled at TRW and is currently being tested. The flight spacecraft design has been completed, and much of the flight hardware is already available. During Fiscal Year 1972 the Pioneer F flight model spacecraft will be assembled and checked out before shipment to Cape Kennedy for integration with the Atlas Centaur/TE-364 launch vehicle. The launch window for Pioneer F extends from 28 February 1972 to 16 March 1972. After a trip of almost two years, it will approach Jupiter in late December 1973 to begin direct measurements of one of the most interesting of all the planets.