Since the introduction of the CubeSat standard in the early 2000s, there has been a proliferation of nano-/small microsatellites in low Earth orbit, with 100–300 or more launched annually and at
a growing rate (according to reports from SpaceWorks and Euroconsult). CubeSats have reduced entry-level costs for space missions in low Earth orbit (LEO) by more than an order of magnitude.
With more piggyback opportunities, platform capabilities and small payloads have rapidly advanced to a level that are suitable for real operational missions. Niche commercial applications are emerging based on operating multiple CubeSats together in distributed systems, e.g. constellations and swarms.
In recent years, micro- and nano- satellites have also started venturing into deep-space, beyond low Earth orbit, taking advantage of ride-share opportunities. Now microsatelites and cubesats are being developed beyond LEO out to GEO and cis-lunar and deep space. Piggyback opportunities for CubeSats to lunar orbit and interplanetary space are already becoming available, and innovative miniaturized technologies are being developed to overcome the severe technical challenges of deep-space missions.
These missions so far try to answer focused science investigations or test new technologies, in contrast with typical deep-space missions which use high-TRL parts and carry a suite of instruments.
PROCYON (Proximate Object Close flyby with Optical Navigation), the first micro-sat deep-space mission (67 kg launch mass), and the first deep-space mission by a University, was launched in 2014 as piggyback of Hayabusa. It escaped the Earth gravity and returned one year later for a distant flyby. PROCYON validated a fully capable bus, with low, middle and HG antennas, reaction wheels and cold gas jets, electric propulsion systems, telescope and cameras. PROCYON was proposed, developed, and launched in just about 14 months, and most of the mission team consisted of students of the University of Tokyo.
Two Japanese follow-on missions, EQUULEUS and OMOTENASHI, are both 6U CubeSats being developed by JAXA and University of Tokyo. EQUULEUS will use water resistojet thrusters to be the first CubeSat to go to the Lunar Lagrange point. OMOTENASHI will be the smallest Lunar lander. A demonstration mission, EGG, was recently deployed from the ISS to test a deployable aeroshell that might be used in the future for atmospheric entry or orbital insertion.
Performance has reached a level where the first interplanetary nano spacecraft mission (NASA’s
Mars Cube One – MarCO) was launched in May 2018 as part of the InSight mission (Klesh & Krajewski 2018). And, as was the case for LEO, it is expected that there will be an order of magnitude reduction in the entry-level cost of interplanetary missions, thus paving the way to new mission applications and architectures based on distributed systems of deep-space nanospacecraft.
US National Academies report (NASEM, 2016) on CubeSats provides an overview of technologies needed for scientific advancement, along with recent technological developments. In particular, advances in propulsion, communications, sensor miniaturization, radiation-tolerant parts, and sub-arcsecond attitude control were called out, among others.
The IDA report on small satellites (Lal et al., 2017) provides a more recent assessment of technology trends for small satellites in general, ranging from high bandwidth communications and onboard processing to advances in miniaturization to orbital debris surveillance (Lupo et al., 2018; Santoni et al., 2018) and removal technologies.
ESA General Studies Programme on lunar and interplanetary CubeSat mission concepts have involved either mother–daughter system architectures, where the CubeSats are carried to a target
destination such as lunar orbit or to a near-Earth object (NEO) on a larger spacecraft and deployed at the target in order to fulfil their mission, or a completely stand-alone system executing its own mission.
The mother–daughter architecture alleviates the technical challenges of propulsion, long-range communication and deep-space environment survivability, because the host spacecraft provides resources and accommodation during the cruise, as well as communications to Earth ground stations, in conjunction with local inter-satellite links with the deployed CubeSats. For a
stand-alone deep-space CubeSat (i.e. where there is no mothercraft), these challenges
have to be tackled and suitable technology solutions identified and/or developed.
Telecommunications: Currently, deep-space Cubesats are designed for low-data volume measurements which limits scientific observation. Moreover, deep-space missions need X-band or Ka-band on-ground antennas, and therefore the support of large space agencies with their ground station networks. In the past, ground station time has been negotiated for small missions (on an opportunistic basis and with low priority to other missions), but typically for one or two spacecraft at a time. The democratization of deep-space exploration could require supporting dozens or hundreds of nanosats, especially during the launch and early operation phase. For example, most of the 13 cubesats launched with EM-1 will have to perform a critical maneuver within two days of deployment, for which they need downlink and uplink for operations and for precise orbit determination (two-way Doppler, DDOR). Current developments in optical link equipment and X-band transceivers open new perspectives for solutions. An increasing number of worldwide distributed, smaller ground stations will provide continuous coverage in the future, similar to the radio amateur supported UHF ground station networks of the CubeSat community in UHF/VHF.
Power generation: At significant distances from the sun, energy generation requires large solar arrays (as for ROSETTA) or use of alternative energy sources. Nuclear generators (as for Cassini, or Galileo) have flown on interplanetary spacecraft but are currently not available for CubeSats. Thus, only the very limited storage resources of batteries can be used, demanding very careful operations in order to not waste those scarce resources.
Propulsion: Most CubeSat propulsion systems to date have been cold or warm gas systems due, in part, to their relatively low cost and low level of complexity. A broad array of various types of electric propulsion systems for CubeSats and microsatellites are in development by multiple companies at Technical Readiness Levels (TRL) between 5 and 7.34
In LEO, one CubeSat mission, AeroCube 8 has already demonstrated miniature electric propulsion system capabilities. For deep space missions, (total impulse/volume) must be increased, complemented by larger fuel storage capacities. Innovative technologies like solar sailing are being considered and may provide solutions.
Mission design: Orbital mechanics and navigation is especially challenging for small deep-space spacecraft, which have limited orbit control capabilities, yet need to reach similar destinations as larger-class spacecraft. Mission design is as critical and complex for small satellites, as it is for large satellites, and sometimes even more so, relying on expert manpower and advanced tools. For this reason, mission design activities for SmallSats are mostly carried out by space agencies. Support toward the development of open-source (and ITAR-free) mission design tools would reduce the costs of deep-space nanosats and enable the participation of new stakeholders.
Operations: Like mission design, operations for a small satellite can be as complex and expensive as for a large mission. Operations of deep-space missions are mostly carried out with a man-in-the-loop approach. Autonomy would reduce mission costs, but it is currently not implemented to its full potential on expensive missions because of the associated risk. Deep space missions will benefit from automation efforts in the near-Earth environment currently being developed for swarms and formations. Deep-space nanosats, however, must rely on an even higher degree of autonomy because of the limited ground station availability. Support towards the development of autonomous operation and navigation technologies would enable deep-space exploration by small satellites, and eventually reduce the cost of large-class missions as well.
Launch Opportunities: Increased access to space is also needed to increase the cadence of science flight opportunities. As discussed in Section 1.1.3, launch opportunities have improved significantly for near-Earth missions. The promise of small satellite launchers currently being developed (Table 4.1 in IDA report, Lal et al., 2017) will enable CubeSats to go to a larger range of orbits, without the restriction of going where the bus is going. For example, Rocket Labs’ Electron just launched 13 CubeSats into LEO in December.36 However, for deep space missions, rideshares are more limited (Fig. 1.8). In 2015, NOAA’s DSCOVR satellite was launched on a Falcon-9 to the Earth-moon L1 point with unused capacity of 2500 kg, and NASA’s TESS mission was launched into a trans-lunar injection orbit with 3000 kg of lift capacity to spare.37 The opportunity to routinely open up launcher capacity on lunar, lagrange point or interplanetary missions for small spacecraft would be a game changer for deep space CubeSats. Such opportunities may soon become reality: NASA recently committed to flying an ESPA ring with every science mission in order to make the excess capacity available to small spacecraft
Current technology innovation trends addressing some of the limitations of small satellites in low earth orbit include:
● Noise reduction in miniaturized components: Software approaches based on filter technologies reduce the susceptibility to noise.
● Attitude and orbit control capabilities: recent miniature reaction wheel developments improve attitude control at low power consumption and electric propulsion systems provide orbit control. Thus, even for a 1U-CubeSat, improved instrument pointing and formation capabilities are being realized (e.g., OCSD,33 UWE-4 and TOM missions). A fully magnetic attitude control subsystem is presented by Colagrossi and Lavagna (2018).
● Communication link capacity: new developments on optical links promise capacities beyond 100 MB/s at clear sky (e.g., OCSD, QUBE and TOM missions), but also very miniature X-band transceivers are becoming available (e.g., MarCO mission).
● Extending the lifetime: advanced FDIR (fault detection, identification and recovery) methods and redundancy concepts guarantee reasonable lifetime in orbit, even for commercial off the shelf components (e.g., UWE-3 has operated without any interruption for more than 3 years despite encountered SEU and latch-ups).
● Ground segment: several university ground station networks have been initiated (e.g., GENSO, UNISEC) to support frequent transmission of data from small satellites in order to relieve the on-board data storage and processing requirements. Commercial networks (e.g., KSAT lite) are providing global coverage for SmallSats.
Air Force Taps Blue Canyon to Develop, Test Micro-Satellite for Deep Space Missions
Blue Canyon Technologies, a subsidiary of Raytheon Technologies, secured a $14.6 million contract from the U.S. Air Force to develop and demonstrate a small satellite bus for operations beyond geosynchronous Earth orbit, in Nov 2021.
The contractor will build a satellite capable of operating and maneuvering for up to three years in orbits beyond GEO under the Air Force Research Laboratory’s Space Situational Awareness Micro-Satellite Bus program, the Department of Defense said Monday.
The small satellite bus is also intended to provide support for a broad range of payloads throughout its operation. Work will be conducted at the small satellite manufacturer’s facility in Lafayette, Colorado, through Feb. 28, 2023. AFRL obligated $1.6 million in research and development funding for fiscal year 2021 at the time of the award. The contract was awarded following a competitive procurement process under the Air Force’s Small Business Innovation Research program.