I’ve been digging into NASA’s Small Spacecraft Strategic Plan out of continuing interest in missions that take advantage of miniaturization to do things once consigned to large-scale craft. And I was intrigued to learn about the small spacecraft deployed on Apollo 15 and 16, two units developed by TRW in a series called Particles and Fields Subsatellites. Each weighed 35 kilograms and was powered by six solar panels and rechargeable batteries. The midget satellites were deployed from the Apollo Command and Service Module via a spring-loaded container giving the units a four foot-per-second velocity. Apollo 15’s operated for six months before an electronics failure ended the venture. The Apollo 16 subsatellite crashed on the lunar surface 34 days into its mission after completing 424 orbits.

Here I thought I knew Apollo history backwards and forwards and I had never run into anything about these craft. It turns out that smallsats – usually cited as spacecraft with weight up to 180 kilograms – have an evocative history in support of larger missions, and current planning includes support for missions with deep space applications. Consider Pandora, which is designed to complement operations of the James Webb Space Telescope, extending our knowledge of exoplanet atmospheres with a different observational strategy.

JWST puts transmission spectroscopy to work, analyzing light from the host star as a transiting planet moves across the disk. A planet’s spectral signature can thus be derived and compared to the spectrum taken when the planet is out of transit and only the star is visible. This is helpful indeed, but despite JWST’s obvious successes, detecting the atmosphere of planets as small as Earth is a challenge. The chief culprit is magnetic activity on the star itself, contaminating the spectral data. The Pandora mission, a partnership between NASA and Lawrence Livermore National Laboratory, mitigates the problem by collecting long-duration observations at simultaneous visible and infrared wavelengths.

Image: A transmission spectrum made from a single observation using Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) reveals atmospheric characteristics of the hot gas giant exoplanet WASP-96 b. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves across the star, to the unfiltered starlight detected when the planet is beside the star. Each of the 141 data points (white circles) on this graph represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its atmosphere. In this observation, the wavelengths detected by NIRISS range from 0.6 microns (red) to 2.8 microns (in the near-infrared). The amount of starlight blocked ranges from about 13,600 parts per million (1.36 percent) to 14,700 parts per million (1.47 percent). Credit: European Space Agency.

Stellar contamination produces spectral noise that mimics features in a planetary atmosphere, or else obscures them, a problem that has long frustrated scientists. Collecting data at shorter wavelengths than JWST’s shortest wavelengths (0.6 microns) helps get around this problem. Pandora’s visible light channel will track the spot-covering fractions of surface stellar activity while its Near-Infrared channel will simultaneously measure the variation in spectral features as the star rotates. A more fine-grained correction for stellar contamination thus becomes possible, and as the new paper on this work explains, the ultimate objective then becomes “…to robustly identify exoplanets with hydrogen- or water-dominated atmospheres, and determine which planets are likely covered by clouds and hazes.”

Pandora will operate concurrently with JWST, complementing JWST’s deep-dive, high-precision spectroscopy measurements with broad wavelength, long-baseline observations. Pandora’s science objectives are well-suited for a SmallSat platform and illustrate how small missions can be used to truly maximize the science from larger flagship missions.

The plan is for the mission to select 20 primary exoplanet host stars and collect data from a minimum of 10 transits per host star, with each observation lasting about 24 hours, producing 200 days of science observations. The lengthy data acquisition time for each star means an abundance of out-of-transit data can be collected to address the problem of stellar contamination. The primary mission has a lifetime of one year, which allows for a significant range of science operations in addition to the above.

Long-duration measurements like those planned for Pandora contrast with data collection on large missions like JWST, which often focus on one or a small number of transits per target. Such complementarity is a worthy goal, and a reminder of the lower cost and high adaptability of using the smallsat platform in conjunction with a primary mission. In addition, smallsats rely on standardized and commercial parts to reduce risk and avoid solutions specific to any single mission. Cost savings can be substantial.

Image: The Pandora observatory shown with the solar array deployed. Pandora is designed to be launched as a ride-share attached to an ESPA Grande ring [(Evolved Expendable Launch Vehicle) Secondary Payload Adapter ring]. Very little customization was carried out on the major hardware components of the mission such as the telescope and spacecraft bus. This enabled the mission to minimize non-recurring engineering costs. Credit: Barclay et al.

Operating at these scales has clear deep space applications. This is a fast growing, innovative part of spacecraft design that has implications for all kinds of missions, and I’m reminded of the interesting work ongoing at the Jet Propulsion Laboratory in terms of designing a mission to the Sun’s gravity lens. Smallsats and self-assembly enroute may prove to be a game-changer there.

For the technical details on Pandora, see the just released paper. The project completed its Critical Design Review in October of 2023 and is slated for launch into a Sun-synchronous orbit in the Fall of this year. Launch is another smallsat benefit, for many smallsats are being designed to fit into a secondary payload adapter ring on the launch vehicle, allowing them to be ‘rideshare’ missions that launch with other satellites.

The paper is Barclay et al., “The Pandora SmallSat: A Low-Cost, High Impact
Mission to Study Exoplanets and Their Host Stars,” accepted for the IEEE Aerospace Conference 2025. The preprint is here.