In response to ESA's Call for Mission Proposals for the next Medium Size Project (M2), an international team of scientists, led by Everitt and Worden, proposed the STEP (Satellite Test of the Equivalence Principle) experiment in November 1989. After an assessment phase, ESA and NASA conducted a full phase-A study of the STEP project as a joint mission between NASA and ESA. As proposed for M2, STEP would have contained five different experiments, to be launched in 2002 for a total cost of nearly $500 million. The experiments included two versions of the Equivalence Principle experiment, a measurement of the gravitational constant, a test of the inverse square law of gravity, an experiment to search for spin-coupling forces, a geodesy experiment, and an aeronomy experiment. The mission was summarized in the STEP Report on the Phase A Study (red report). The competition for the M2 mission concluded in a very close race between STEP and a competing project, INTEGRAL, which won the mission.

STEP was reproposed for the ESA M3 mission as an all-European program. During the Phase A study for M2 there were concerns about both the technical and managerial complexity of STEP as it grew from a small to medium-sized mission into a large mission. The complexity came from extra organizational and technical requirements for a mission with multiple goals, funded independently in several nations. The question was raised, whether the basic mission could be done for a much lower cost, as originally estimated prior to the M2 selection? In fact, it might be possible to do all of the science goals of STEP for a lower total cost, by doing separate missions in smaller, tightly focused programs without the organizational and technical overhead that comes with a single, large, multipurpose mission. The QuickSTEP concept, proposed by the team at Stanford to NASA, explored whether the Equivalence Principle measurement could be done more quickly and cheaply. The French Agency ONERA at about the same time proposed a similar low-cost mission, based on a rather different technology, called GEOSTEP; and an Italian mission, GG (for Galileo Galilei), somewhat similar to Chapman's original concept, was also proposed.

The QuickSTEP instrument operated on the same principles as the STEP instrument described for M2. It was simplified by deleting some excellent co-experiments not directly related to the basic Equivalence Principle measurement, but which accounted for more than half of the M2 mission's complexity. A joint Stanford-JPL study showed that the cost of QuickSTEP could be reduced to less than $130 million. The other small mission concepts were of a similar size. Unfortunately in an era of shrinking budgets even this major reduction was not enough for NASA, and a further descoping, which reduced the basic scientific return, was required. This resulted in the MiniSTEP concept, which minimizes the total cost of the mission rather than optimizing the scientific return per dollar.

The European study of STEP for M3 also drastically descoped the mission, concluding with a design similar to QuickSTEP but retaining the spin-coupling experiment. Predictably this simplification led to an increase in cost of the mission and consequently STEP was not selected for M3.

MiniSTEP was the name given to a very low-cost version of the STEP mission with reduced scientific goals, which was based on lightsat technology. Its baseline experiment package operated the accelerometers sequentially rather than in parallel as in previous versions of the mission. The name STEP was revived upon returning to the original concept.

The present incarnation of STEP is in response to a NASA Code S AO 99-OSS-05, and proposes for a small explorer mission (SMEX). All of the previous studies had many benefits for this proposal. In producing several Phase A Reports, they gave an excellent degree of program definition and costing information for STEP, which we have further refined to fit within the SMEX guidelines. STEP will be a NASA-led collaborative mission with additional support from the European Space Agency and European national space agencies.

The accelerometer package is contained in a Lockheed-designed superfluid helium dewar to which a European-provided lightsat spacecraft and a payload electronics module are bolted. The entire satellite is made drag-free by reference to one or more of the accelerometer's proof masses, using the boiloff gas from the helium as reaction mass. The gas is vented through proportional thrusters already developed for NASA under the Gravity Probe B program. Extensive borrowing of developed technology ( including thrusters, SQUID controllers and components of the electrostatic suspension and charge control system ), and equally importantly experience and personnel, from GP-B helps to reduce the overall cost of STEP. As currently conceived, STEP would fly in a near-polar sun-synchronous orbit at an altitude of about 550 km, in August 2005.