One of the components of the BepiColombo mission to Mercury [link] is the Mercury Magnetospheric Orbiter (named Mio). This orbiter, in conjunction (in the everyday sense, not necessarily the astronomical sense) with the main Mercury Planetary Orbiter, will map Mercury's magnetic field. It helps, when mapping a planet's magnetic field, to measure the field at multiple locations simultaneously, to try to separate variations in field strength and direction over space from variations over time.
More than two orbiters is even better—ESA's Swarm [link] uses three, and NASA's MMS [link] and their collaborative Cluster mission [link] use(d) four each. But those were all orbiting Earth, where putting things in orbit is cheap. When launching an orbiter for another planet, the budgets (money, mass, data, tracking, …) are all a lot tighter. This (well, the first two) is probably the main reason BepiColombo only has one such additional orbiter for magnetospheric mapping, and other missions have had none, only using a magnetometer on the main orbiter.
To reduce the cost of such additional magnetospheric orbiters, it might help to standardize them and make them available to missions as off-the-shelf products (or as close as can be gotten in the deep space exploration market—they will still end up getting customized somewhat, but they'll provide a starting point at least). This way, more missions will include them, and some missions will include more than one, leading to more detailed knowledge of other planets' magnetospheres. Adding another standard and useful purpose to these orbiters, that purpose being higher-resolution measurement of planetary atmospheres, will probably also help with adoption and be beneficial to science.
These sub-orbiters could be based on the existing CubeSat standard [link]—I imagine them potentially being as small as 1.5U, so a mission can easily include one or more of them. Being manufactured in greater numbers means their hardware can be more standardized and therefore more reliable, while being included in greater numbers on missions means their reliability requirements are less due to the redundancy; together, these mean that redundancy within the spacecraft is less necessary, enabling miniaturization. They will not be large enough to communicate directly with Earth (due to antenna size and power constraints), so their communications will be relayed by their mission's main orbiter, probably using the existing CCSDS standards [link].
These standard sub-orbiters will carry a typical complement of magnetometers [link], mounted on a typical (though possibly shorter/lighter) deployable mast to keep them away from the interference produced by other hardware in the spacecraft's body. This and the use of multiple orbiters measuring the magnetosphere simultaneously in different locations/orbits are well known, so I will not describe this purpose of the sub-orbiters further other than to say that the magnetometers' specifications may be customized somewhat for different missions.
The other main purpose of these sub-orbiters will be radio and optical occultation experiments [link]. These consist of measuring the amount of electromagnetic radiation of either type that can pass through an atmosphere. This is used to measure the vertical profile of the atmosphere, as to pressure, aerosols, etc. This is (relatively) commonly done by Earth satellites by recording the reception strength of GPS signals (or, I suppose, any GNSS signals, but I've only heard of it being done with GPS) as the measuring satellite orbits asynchronously with a given GPS satellite, causing the radio signal line of sight between them to pass up or down through the atmosphere. However, because other bodies in the Solar System don't have navigation satellite constellations, such measurements of other bodies' atmospheres have been limited to relatively rare opportunities when orbiters or flyby probes have passed behind them as viewed from Earth or the Sun and when those bodies have occulted stars as viewed from Earth, meaning we have collected relatively few such atmospheric profiles for those bodies.
These sub-orbiters will solve that by orbiting asynchronously with their main orbiter and helping it perform occultation measurements like the Earth satellites do. They will carry radio transceivers, because they need to communicate with the main orbiter anyway for magnetospheric data return and command & control purposes, and these can be used for received signal strength measurement. They can also carry laser receivers, and/or radio and/or laser retroreflectors [link] (radio retroreflectors being folded up until the sub-orbiters are deployed from the main orbiter). The laser receiver option is probably the least practical, because it would require additional electronic hardware, and because it would likely require the sub-orbiter to aim its receiver at the main orbiter to receive the signal. In contrast, a radio receiver is already present and should work fine using an omnidirectional antenna, and retroreflectors of either type should work omnidirectionally with low mass and (folded) size. As well, using a retroreflector means the signal passes through the atmosphere twice along the same path, doubling absorption and thereby increasing sensitivity in the thin upper atmosphere. (Doing two passes also means the maximum depth within the atmosphere through which the signal can be received is less, but that's OK because we still have the single-pass measurement using the sub-orbiter's radio transceiver.)
These spacecraft can also be used for Earth missions, in which case they will be equipped with GPS receivers and will communicate directly with the ground (being near enough to use omnidirectional antennas, or being equipped instead with directional ones that point down). Being available as inexpensive standard products will enable more such missions, increasing our knowledge of Earth's atmosphere and magnetosphere, and improving forecasting of weather (both atmospheric and space).
A modular power system bay will allow the sub-orbiter's power system to be easily customized for the mission. Available options will include omnidirectional solar arrays; smaller solar arrays with a cooling system, radiators, reflective/radiative glass [link], and/or a sunshield (for Venus, Mercury, and close-range Sun missions); larger directional solar arrays (for outer planet missions); and a small RTG (also for outer planet missions). A battery can optionally be included for passes through the body's shadow. A small command and data handling subsystem is included, along with suitable satellite navigation and maneuvering equipment (TBD, but could include: star tracker, IMU, reaction wheels, cold gas thrusters, magnetorquers, gravity gradient stabilizer). Additional instruments as desired can be equipped in any remaining space (or in increased space, if these sub-orbiters are CubeSat-based and the host has room). These other instruments could be used, like the magnetometers, to provide measurements of the same locations at a higher frequency, or of locations not covered by the main orbiter's orbit.
N/A [2019-02-27]
(Can we have a Science: Spacecraft: Probe or Science: Space: Planets category please?)-- notexactly, Feb 28 2019 BepiColombo mission to Mercury https://en.wikipedia.org/wiki/BepiColomboMentioned in idea description [notexactly, Feb 28 2019] Swarm by ESA https://en.wikipedi.../Swarm_(spacecraft)Mentioned in idea description [notexactly, Feb 28 2019] Magnetospheric Multiscale Mission (MMS) by NASA https://en.wikipedi..._Multiscale_MissionMentioned in idea description [notexactly, Feb 28 2019] Cluster (well, Cluster II, because the first attempt failed) by ESA + NASA https://en.wikipedi...ter_II_(spacecraft)Mentioned in idea description [notexactly, Feb 28 2019] CubeSat https://en.wikipedia.org/wiki/CubeSatMentioned in idea description [notexactly, Feb 28 2019] CCSDS standards https://en.wikipedi...wiki/Template:CCSDSMentioned in idea description. Most relevant is probably Proximity-1 Space Link Protocol [notexactly, Feb 28 2019] Spacecraft magnetometers https://en.wikipedi...ecraft_magnetometerMentioned in idea description [notexactly, Feb 28 2019] Radio occultation https://en.wikipedi...i/Radio_occultationMentioned in idea description [notexactly, Feb 28 2019] Satellite use of retroreflectors https://en.wikipedi...ector#In_satellitesMentioned in idea description [notexactly, Feb 28 2019] Reflective/radiative glass https://en.wikipedi...cal_solar_reflectorMentioned in idea description. The name "optical solar reflector" is too generic and I can never remember it. Anyway, it reflects sunlight while simultaneously radiating heat, by doing those things at different wavelengths. [notexactly, Feb 28 2019] Okay now do the Cliff's Notes version.-- RayfordSteele, Mar 01 2019 I just realized they could be called "Magnetospheric and Occultation Orbiting Nodes", with the acronym being MOON, but that would get confusing because you'd then have both MOONs and moons orbiting the same planet.-- notexactly, Mar 01 2019 random, halfbakery