Solar system exploration is approaching a major turning point just as it is surpassing a major milestone. Fifty years ago, on Aug. 27, 1962, NASA launched the first successful planetary probe, Mariner 2, which visited Venus later in that same year. Regarded as both the evening and the morning star, Venus had long enchanted humans, and all the more so since astronomers had realized that it was shrouded in a mysterious cloak of clouds permanently hiding the surface from view. Mariner 2 was a triumph in helping to reveal the closest planet to ours, and a near twin in terms of size, mass and gravitation, to the billions on Earth.

Although Mariner 1 was lost during a launch failure, its twin, Mariner 2, flew by Venus on Dec. 14, 1962, at a distance of 34,800 kilometers. A 204-kilogram spacecraft, it carried six scientific instruments, a two-way radio, a solar-power system and assorted electronic and mechanical devices. Mariner 2 probed the clouds, estimated planetary temperatures and pressures, mea¬sured the charged particle environment and looked for a magnetic field similar to Earth’s magnetosphere (but found none). Most important, it found that the planet’s surface was a fairly uniform 460 degrees Celsius, thereby making unlikely the theory that ¬life existed on Venus.

There had been longstanding speculation that life might be present on Venus. In the first half of the 20th century, a popular conception held that the sun had gradually been cooling for millennia and that as it did so, the terrestrial planets of the solar system had a turn as a haven for life of various types. Although it was now Earth’s turn, this pop culture belief suggested that Mars had once been habitable and that life on Venus was now just beginning to evolve. Beneath the clouds of the planet, the concept offered, was a warm, watery world and the possibility of aquatic and amphibious life. “Mariner: Mission to Venus,” a 1963 NASA publication about the Mariner 2 mission, noted, “It was reasoned that if the oceans of Venus still exist, then the Venusian clouds may be composed of water droplets; if Venus were covered by water, it was suggested that it might be inhabited by Venusian equivalents of Earth’s Cambrian period of 500 million years ago, and the same steamy atmosphere could be a possibility.”

Mariner 2 helped to determine that none of these speculations was true. The second in a series of planetary exploration spacecraft, this was the world’s first fly-by of a planet. Part of a 1961 NASA planetary exploration initiative that took some of its design from the Ranger program, Mariner 2 bore a striking resemblance to the basic framework, solar panels and antennas of its Ranger predecessor. Its controllers, numbering roughly 75, worked from NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Although Mariner 2 did its job well, NASA lost contact with the spacecraft on Jan. 2, 1963, and it is now in orbit around the sun.

This important milestone in planetary exploration signaled by Mariner 2’s mission to Venus is an appropriate cause for celebration. It is also an ideal time to reflect and take stock of U.S. efforts to understand our solar system because the planetary exploration program is stalling through a lack of resources and technology necessary to advance it. Unfortunately, after years of success in planetary exploration, NASA’s efforts are beginning to wane. Since the 1990s NASA has sent an armada of spacecraft to the red planet, of which the Curiosity rover is the most recent, and two outer planetary missions — Galileo and Cassini-Huygens — have opened the jovian planets to study. The New Horizons space probe to Pluto and the Kuiper Belt at the outer reaches of this system promises to do the same in the coming years. Not to be outdone, Messenger has expanded knowledge of Venus and the inner solar system. A range of smaller and less-complex spacecraft have also undertaken significant investigations.

But we are at the point where not much in the way of planetary exploration is currently approved, under development and moving toward launch. The Mars Atmosphere and Volatile Evolution, or Maven, mission is set to launch in 2013, and it promises to capture important data about the red planet’s upper atmosphere, ionosphere and interactions with the sun and solar wind. An outer planet flagship mission — probably either to Saturn’s moon Titan or Jupiter’s Europa — is also in the planning stage but will probably not be launched for many years, if at all. Some smaller missions — for example, the Osiris-Rex probe to take samples from an asteroid — are also under way. Indeed, of the major projects currently under study or in development by the NASA Science Mission Directorate, only five are planetary spacecraft (two of which are lunar probes), while one of these is the just announced InSight Mars lander as a part of the NASA Discovery program. Most of these projects are Earth science (15), astrophysics (eight) and heliophysics (seven) |spacecraft.

Pure numbers of missions are not the key criteria for assessing the current state of the planetary program, of course, but the planetary science budget also took major hits in federal budget projections, down from $1.5 billion in fiscal year 2012 to $1.2 billion in 2013, with no indication of any increase coming in the next five years. While some outer planet missions remain on the books, only concept studies are approved. There has also been a modest investment in technology development to support planetary exploration, such as the development of the Advanced Stirling Radioisotope Generator that is necessary for outer planetary missions and the restarting of the plutonium-238 production line.

Since the 1990s, America has participated in a golden age of scientific discovery about the solar system, made possible by coordinated efforts to create expansive, challenging and engaging programs — such as the “Follow the Water” agenda for Mars — that have benefited everyone from the millions of students jazzed by the latest images from the martian surface to elders who reflect on how we have advanced and altered our understanding of the cosmos in their lifetimes.

In the critically acclaimed television situation comedy “Sports Night,” about a team that produced a nightly cable sports broadcast, one episode in 2001 included a powerful statement that relates directly to NASA’s planetary science program. Actor Robert Guillaume announced on the show, “You put an X anyplace in the solar system, and the engineers at NASA can land a spacecraft on it.” Through 50 years of planetary exploration that began with Mariner 2, NASA scientists and engineers have engaged in the stunning feats — Curiosity’s landing is only the most recent — that Guillaume spoke about. While I hope I am wrong, I am concerned that there may not be many more “X marks the spot” planetary missions in the near future. Will efforts be expansive and engaging, as in the past, or limited and passé, as seems to be the direction we are presently heading. Since the results of decisions taken today may not be felt for several years, what are the prospects in planetary exploration into the 2020s and thereafter? Do we accept those decisions, and if not, what are we to do about it?


Roger D. Launius is a senior curator at the National Air and Space Museum of the Smithsonian Institution in Washington.

Roger D. Launius is a senior curator in the Division of Space History at the Smithsonian Institution's National Air and Space Museum in Washington