The concept is new enough that there’s not even a consensus on what to
call the spacecraft – SmallSats, micro-satellites, tiny spacecraft.
Traditional spacecraft – let’s call them LargeSats – weigh hundreds of
kilograms and cost hundreds of millions of dollars. At the other end of the spectrum, CubeSats are
based on the electronics that have enabled phones to be respectable computers,
entertainment centers, and cameras. The
smallest form factor for CubeSats is just 10 × 10 × 10 centimeters, weighs
approximately a kilogram, and can cost as little as $60,000 to build. Large CubeSats expand the length to 60
centimeters and the mass to approximately 6 kg.
While over a hundred CubeSats have been placed in Earth orbit, their
small size poses challenges for planetary missions. A CubeSat that would travel to and orbit
Mars, for example, must deal with the harsher radiation environment beyond
Earth’s magnetosphere, carry a propulsion system to make course corrections and
brake into Martian orbit, have a beefed up communications system to return data
from up to 300M kilometers away, and provide solar panels large enough to
collect sufficient sunlight at Mars’ greater distance from the sun. There’s just not enough mass or space to do
all that within the CubeSat form factor
CubeSats are likely to play a role in future planetary missions, but most
frequently as daughter craft carried to their destination by larger
spacecraft. (I’ll explore possible
concepts in a future post.)
An emerging class of spacecraft – I’ll call them SmallSats – would fit
between LargeSats and CubeSats. These
spacecraft make use of the design techniques of CubeSats but scale the form
factor up to a meter or so and the mass up to 50 to 100 kg or so. At this size, the spacecraft could meet the
challenges of interplanetary missions.
I’ll also note that the potential for SmallSats isn’t limited to
planetary missions. At least two
commercial companies are planning spacecraft in this class for high resolution
(3 to 5 m) imaging of the Earth. If
successful, they would compete with current companies such as WorldView that
are currently using traditional LargeSat designs that cost and weigh an order
of magnitude more than their planned SmallSat equivalents. (See articles at The
New York Times and Wired Magazine.)
At the Low Cost Planetary Mission
conference last June, several concepts for SmallSat planetary spacecraft
were presented. I’ll use the concepts
presented to explore the potential and limitations of this class of spacecraft
for future planetary exploration.
Engineers from JPL presented their concept for one class of these
spacecraft, the Micro Surveyor, for missions in the inner solar system. While the first planetary SmallSat may cost around
$40M, their goal is to create a design for spacecraft in the $20M range to
enable total mission costs to eventually
be around $100M (compared to $450 to $500M for NASA’s Discovery missions and
$750M to $1B for New Frontiers missions).
Conceptual design for JPL’s Micro
Surveyor planetary spacecraft that would stand just 0.9 m high. The design is sized to fit within the space
allocated for secondary payloads adapter ring (ESPA) on the launch of larger
spacecraft (insert figure in the upper right).
Propulsion would be provided by an electric propulsion system (EP). Credit: JPL.
The design is sized to be carried as a small secondary payload on the
launch of a larger spacecraft such as a communications satellite to enable low
cost launches. Once in Earth orbit, a
Micro Surveyor would use an electric propulsion system (similar to, but much
smaller than the ion engine being used by the Dawn spacecraft) to gradually
change its orbit to flyby the Earth’s moon.
Encounters with the moon would provide gravity assists that would put
the spacecraft on a trajectory to Venus, Mars, or an asteroid or comet near
Earth. (While not mentioned in the
presentation, the moon itself could be a mission target.)
The flight times to Venus or Mars illustrate one of
the limitations that this approach has.
While LargeSat missions to these planets can arrive in less than a year (thanks to a dedicated launch that puts the
spacecraft on a direct trajectory), a Micro Surveyor spacecraft would take ~16
months to reach Venus and ~36 months to reach Mars. Once the spacecraft reaches its target world,
the electric propulsion system would allow a slow spiral into the science
orbit. Patience may be a virtue, but the
longer flight times mean that mission operation costs will build up during the
longer flights.
The small size of the spacecraft also would limit the
scientific payload. While a LargeSat
Discovery mission might have 50 kilograms or more in instruments, the Micro
Surveyor spacecraft is being designed to carry just 15 kg of instruments. The mission planned for the Micro Surveyor
would also need to fit within the limited power available to the instruments
and the restricted bandwidth of data the tiny spacecraft could return to
Earth. A Micro Surveyor could, for
example, carry a camera to Mars for high resolution imaging (3 to 5 m) of the
surface, but the mission would be severely limited in how many images could be
returned per day.
Also, Venus, the moon, Mars, and near Earth asteroids
have been (or are planned to be) visited by many spacecraft. Much of the low hanging fruit in terms of
science has been done. With an estimated
cost of ~$100M per Micro Surveyor missions, finding a new, compelling
scientific missions that fit within the mass, power, and data limitations of
the spacecraft may prove to be a challenge.
At the conference, the JPL presenter did not suggest
any specific mission concepts for the Micro Surveyor. I’ll suggest three possible mission concepts
that occurred to me that might meet the goals of conducting interesting new
science while fitting into the mass, power, and data constraints. I want to emphasize that this is amateur
armchair mission architecture; none of these concepts may pass technical muster
when examined by real mission architects.
The first concept would target Venus. The clouds of Venus have made observation of
the surface difficult by orbiting spacecraft.
While larger missions can carry heavy and power hungry radar systems to
image the surface, mapping the surface composition has been largely impossible
because the clouds block the view. A
series of spectral windows in the near-infrared (between 0.8 and 1.8 μm),
however, allow observation of the surface.
The VIRTIS instrument on the Venus Express spacecraft has mapped
portions of the planet using these windows to examine surface areas for
different compositions and look for temperature differences suggesting volcanic
activity.
Red-orange colors in
this VIRTIS image of Venus’ Idunn Mons volcanic peak indicate warmer areas
suggesting different surface compositions and younger ages than surrounding
material. Press release describing the
science available here
and image caption here. Credit: ESA.
The VIRTIS instrument, however, was designed for
analyzing the cloud deck, and has low resolution (~150 kilometers) and isn’t
optimized for observations at the wavelengths of the spectral windows. A follow on instrument optimized for surface
mapping (along with further cloud studies) has been proposed for future
European Venus missions. The proposed Venus
emissivity mapper would weigh just five kilograms, easily fitting within the
mass constraints of a Micro Surveyor mission.
(In fact, on a dedicated mission, a more capable instrument might be
flown than has been proposed.) Fitting
within the data constraints of a SmallSat mission, though, might require
creative mixing of low resolution global mapping with targeted higher
resolution mapping for priority targets.
A second
concept would explore the diversity of comets. Visits of several comets by spacecraft have
shown that these relics from the formation of the solar system are widely
varied. The spacecraft that made the
observations to date carried different sets of instruments, making systematic
comparisons difficult. One NASA
Discovery mission, CONTOUR, planned to address this problem by visiting two or
more comets with the same spacecraft and instrument suite. Unfortunately, the CONTOUR mission failed
early in its mission while leaving Earth orbit.
The CONTOUR spacecraft carried a diverse suite of
instruments: a remote imager/spectrograph (12.0 kg), an
aft imager for post encounter images (1.8 kg), a mass spectrometer (9.3 kg),
and a dust analyzer (11 kg). Based on
these masses, a Micro Surveyor comet mission could carry any one of the three
larger instruments plus the equivalent of the aft imager. (Because more than a decade has passed since
CONTOUR’s launch, new versions of these instruments could be developed that
would have less mass. However, the small
budgets foreseen for SmallSats probably would allow for only minimal tweaking
of current instrument designs. The
soon-to-be-launched lunar LADEE mass spectrometer derived from the CONTOUR
instrument, for example, is 11.3 kg.
Budgeting something around 10 kg for a SmallSat-scale mission mass
spectrometer seems reasonable.) The
limited payload available for a SmallSat means that their science goals must be
tightly focused.
A tweak on the comet mission concept would be to send
a spacecraft to pass by one of the main belt asteroids that have been observed
to emit gasses like comets, suggesting that they are either captured comets or
very volatile rich asteroids (at which point, there may not be much of a
difference).
At the same conference, researchers presented a
concept for an outer solar system SmallSat mission. This work was done to investigate the power
requirements for such a mission to determine whether a small version of a
radioisotope power system could meet the electric power requirements. To gauge the spacecraft and power system, the
team investigated a flyby of one or more Centaur objects, which have orbits
within those of the outer planets. The
study team selected just two instruments, a narrow angle camera (2.4 kg) a
hyperspectral infrared spectrometer (4 kg).
As a baseline, the study team looked at two SmallSat-scale spacecraft
encountering the asteroid 2060 Chiron and a second pair encountering a second
asteroid. To reach the outer solar
system, the mission would have a dedicated launch that would send the four
craft two Jupiter where gravity assists would redirect the spacecraft to their
final destinations.
Conceptual design for
an outer solar system SmallSat spacecraft that would be 1.76 m high. Multiple copies of this design could be
launched together (insert) to provide redundancy or allow targeting of multiple
destinations. Credit: NASA, JPL.
The Centaur mission again shows the limitations of
small payload masses. Chiron has active
gas emissions. Being able to include a
mass spectrometer to measure the composition of those gasses could greatly
enrich our understanding of the formation of the outer solar system. This is especially true if Centaurs were once
Kuiper-Belt worlds that were later captured within the outer solar system.
(If I can play armchair mission architect for a bit
longer, the instrument masses for the Centaur concept are low enough that they
could be paired with a mass spectrometer and almost fit within the payload of a
Micro Surveyor. A camera, imaging
spectrometer, and a mass spectrometer would be a very nice payload for a comet,
outgassing asteroid, or Centaur mission.)
While the presentation discussed only the Centaur
mission concept, similar flyby missions could be done to any outer solar system
body. (A favorite of mine would be a
flyby of Neptune’s moon, Triton.)
The Centaur mission presentation brought up
interesting questions on how to think about these missions. Traditional planetary missions have sized a
single spacecraft to carry all the instruments necessary to meet the science
goals. SmallSat-scale spacecraft are
cheap enough each ($10M to$20M was mentioned at the conference) that perhaps
you fly two or more to a single target.
For a comet or Centaur mission, perhaps one craft carries a camera and
imaging spectrometer and a second carries a mass spectrometer.
Traditional planetary spacecraft have also carried
dual copies of most critical systems so that if one fails, the spacecraft can
continue operating on the second. To
keep mass and per spacecraft costs low, the concepts presented at the
conference envisioned a single copy of each system. Would risk management be better preserved by duplicating
key systems within a single spacecraft or by flying two copies of the
spacecraft? For the Centaur mission
concept, the mission architects suggest that two craft could be targeted to the
high priority Chiron object and the other two craft could be targeted to
separate objects. If all craft succeed, three
objects are visited. If half fail, you
would likely get Chiron and/or flybys of two different Centaur objects.
The Centaur mission presentation had one slide that
suggests that building planetary spacecraft on tiny budgets only addresses part
of the costs of a planetary mission. The
team estimated that the total mission cost would be $511M, only 20% of which would
be for four copies of the spacecraft hardware.
(The design exercise was trying to hit the mission costs for a
Discovery-class mission (~$500M in the future), rather than hitting a lower
cost target.) The rest of the costs were
for design, testing, launch, mission operations, and budget reserves. At the conference, presenters suggested that
total mission costs for an inner solar system SmallSat-scale mission might be
approximately $100M using a standard spacecraft designs such as the Micro
Surveyor. If that target can be hit, the
cost of the spacecraft hardware would represent 20% to 40% of the mission
costs.
At the moment, engineers and scientists are beginning
to explore what might be accomplished by SmallSat-scale missions using today’s
technology. As technology progresses,
the potential for these spacecraft will grow.
Another JPL team presented a concept for a near-Earth object hunter-seeker
mission. This mission would use a next
generation micro-electro-fluidic-spray propulsion technology. The spacecraft would first maneuver itself
into an orbit just inside Earth’s orbit to begin a search for asteroids as
small as a few meters across. As
interesting objects are found, the spacecraft would rendezvous with five to six
of them for close up examination by a camera and imaging spectrometer and with surface
contact to probe the asteroids structural cohesion. Total mass for the spacecraft with fuel is
estimated to be just 50 kilograms within a body (not counting solar panels)
just 60 centimeters on a side.
Conceptual design for a
next generation SmallSat spacecraft for the near-Earth asteroid Hunter-Seeker
mission. Credit: JPL.
Editorial Thoughts:
SmallSats for planetary exploration are an exciting concept, especially when
NASA’s planetary program faces tight budgets for new missions. As I’ve tried to emphasize, though, they
would complement, not replace larger missions.
The goals for many planetary missions need multiple instruments that
each would approach or break the limits for a SmallSat. The Mars MAVEN Discovery mission, for
example, requires the simultaneous operation of a suite of instruments that
together have a mass several times what a Micro Surveyer spacecraft could carry
to understand the composition and processes in the upper atmosphere. The HiRise camera on board the Mars
Reconnaissance Orbiter has a mass of 64 kg, approaching the mass for an entire
Micro Surveyor spacecraft (~75 kg).
However, SmallSats are promising to carry out highly
focused investigations with smaller instruments or to visit objects that
wouldn’t make the cut for $500M to $1B missions.
Planeatary SmallSat missions could also provide
opportunities for a new generation of scientists and engineers to take lead
positions on smaller missions and gain the experience necessary to manager
larger projects. Currently, the pool of
scientists and engineers with flight mission experience who can take lead
positions for future Discovery and New Frontiers missions is shrinking as
flight opportunities have dwindled.
For approximately half the cost of a Discovery
mission with launch, NASA could fly two to three SmallSat-class missions. I hope that NASA pursues the
opportunity. I expect that the planetary
science community would show considerable creativity in finding ways to use
missions in this class.
Resources:
The following links are to the presentations from the
Low Cost Planetary Science 10 conference that discussed planetary SmallSat
designs
