In December, we discussed the history of the Stratospheric Observatory for Infrared Astronomy, or SOFIA, to highlight the tension in science between an ideal of perseverance in the face of obstacles and the reality of competing scientific interests and limited resources. In the case of SOFIA, the costs proved too high to justify continuing to operate the telescope. This week, we turn our attention to NASA’s Parker Solar Probe, a story of the successful perseverance of an idea across generations, until technology, resources, and other factors aligned in its favor.

The origins of the solar probe concept

On April 5, 1950, physicist James Van Allen hosted a dinner at his home in Maryland for geophysicist Sydney Chapman, who had stopped over on his way from Britain to a meeting at Caltech on upper-atmosphere research. Over dinner, the attendees—Van Allen and his wife, Chapman, Lloyd Berkner, J. Wallace Joyce, Ernest Vestine, and Fred Singer—came up with the idea for a global program of research on topics related to the Earth and its environs.

During the discussion, Chapman suggested that 1957-58 would be an ideal time for the program, as it would coincide with the period of maximum activity in the solar cycle. Chapman took this idea to the meeting at Caltech, and from there it percolated up all the way to the International Council of Scientific Unions, where the project was officially dubbed the International Geophysical Year, and organizing meetings began in 1953.

The emphasis on solar activity as a cornerstone of the IGY that Chapman articulated over that first dinner persisted. The United States National Committee for the IGY released an outline of the research programs in 1955, which noted, “One of the controlling fields in this research program is solar activity, for the sun dominates activities on our planet and is the major source of energy for the earth and all life. Some solar effects strongly influence the upper atmosphere, the weather, and radio communications.” Other topics of special interest mentioned in the outline included ionospheric phenomena, geomagnetism, and the aurora, all strongly affected by solar activity.

As part of the preparation for the IGY, the national committee created the Space Science Board of the National Academy of Sciences to consider the scientific opportunities of the human exploration of space, as well as to provide advice on IGY projects and, later, to advise NASA, the Department of Defense, and the National Science Foundation on matters related to human spaceflight and space research.

In 1958, the board set up a committee on the “Physics of Fields and Particles in Space,” chaired by John Simpson and co-chaired by Van Allen. It soon released a report that included as “long-range plans” seemingly far-fetched missions, including a lunar satellite and base, nuclear detonations in space, and a probe that could approach the Sun itself.

While the proposal for a Moon base never came to be, the military conducted space-based nuclear detonations in Operation Fishbowl in 1962. Meanwhile, interest in a solar probe remained strong but progress was slow, with scientists at NASA and the Jet Propulsion Laboratory looking at both the feasibility of the mission and the kinds of investigations that close proximity to the Sun would allow.

The prospective mission presented unique challenges, particularly the intense heat and radiation of the near-solar environment. By 1978, work had progressed to the point that researchers could start looking beyond feasibility studies and begin fleshing out how the mission could work and the kinds of scientific investigations it would allow.

Meetings held at JPL that year identified a number of avenues for inquiry using the probe, including the distribution of mass within the Sun, solar angular momentum, the fine structure of the solar surface and corona, the acceleration of the solar wind and energetic particles, and the evolution of interplanetary dust, as well as, potentially, high-accuracy tests of general relativity and the search for cosmic gravitational radiation.

A new century and new momentum

Budgetary constraints and shifts in priorities at NASA kept the ideas put forward in the 1978 reports on the back burner. In the mid-1990s, the solar probe idea was integrated into a joint program with Russia called “Fire and Ice” that would explore the Sun and Pluto. But it was in 1998 that plans for the solar probe project began to move forward in earnest when NASA bundled it with a mission to orbit Jupiter’s moon Europa and a mission to Pluto and the Kuiper Belt in what was called the Outer Planets / Solar Probe Project (OPSP). Within this framework, the goal was to launch the probe in February 2007.

In 2001, the OPSP concept was dissolved and the solar probe shifted into a new program called “Living With a Star,” which by the end of the decade was integrated into a new Heliophysics Division that continues to be the focal point for NASA’s work on the Sun and its interactions with the Earth and the rest of the solar system. NASA also commissioned a new concept study for the mission from the Johns Hopkins University Applied Physics Laboratory, which became responsible for the spacecraft’s development.

The probe received a boost in 2003, when the first-ever National Research Council decadal survey for solar and space physics listed it as a top-priority mission and recommended it as the only large-scale mission within a broad portfolio of missions. Because of its anticipated expense, the survey also noted that a near-term launch would only be possible with an “augmentation” of expected budgets in its research area.

Given the probe’s expense, in 2007 NASA instituted new restrictions, requiring the total mission cost to come in under $750 million, with a mission duration of no longer than ten years. Moreover, the spacecraft would have to use solar panels rather than a scarce radioisotope power source.

This concept, known as “Solar Probe Lite,” presented challenges related to the fact that the energy cost to escape Earth’s orbit to move toward the Sun is far higher than breaking away. The proposed solution had been to send the probe outward and to use a gravity assist from Jupiter to swing it inward. However, at Jupiter, solar panels are inefficient, meaning a radioisotope source would be required. To preserve the scientific mission under the new parameters, the project scientists devised a new trajectory that would use multiple gravity assists from Venus instead.

This new concept had drawbacks in that it would not allow the probe to approach as close to the Sun as planned, nor would it allow observations of the Sun’s polar regions. Fortuitously, though, it would allow the probe to spend more time in the Sun’s vicinity, improving many of the mission’s scientific capabilities. The design was given the more attractive name “Solar Probe Plus,” and a launch date was planned for 2018.

The formulation and development costs of the probe ultimately totaled to around $1.3 billion, but the mission launched on time on August 12, 2018. In 2017, the spacecraft was renamed after solar physicist Eugene Parker, who published foundational work in the 1950s predicting the existence of the solar wind. The honor was an unusual one in that Parker was still alive at that time, and he was the first person to witness the launch of a spacecraft bearing their name.

On April 28, 2021, the Parker Solar Probe’s trajectory brought it into the solar atmosphere, an event NASA celebrated by announcing, “For the first time in history, a spacecraft has touched the Sun.” On subsequent passes, it repeatedly broke its own records as the fastest-ever spacecraft. It completed its 26th pass of the Sun in December 2025. While its prime mission officially ended in June 2025, the spacecraft is functioning well and the extended phase of its mission is currently under review and will possibly allow it to gather data over the course of the entire 11-year solar activity cycle.

Jon Phillips
American Institute of Physics
jphillips@aip.org

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