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?)