CubeSat
A CubeSat is a class of small satellite with a form factor of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf components for their electronics and structure. CubeSats are deployed into orbit from the International Space Station, or launched as secondary payloads on a launch vehicle., more than 2,300 CubeSats have been launched.
In 1999, California Polytechnic State University professor Jordi Puig-Suari and Bob Twiggs, a professor at Stanford University Space Systems Development Laboratory, developed the CubeSat specifications to promote and develop the skills necessary for the design, manufacture, and testing of small satellites intended for low Earth orbit that perform scientific research and explore new space technologies. Academia accounted for the majority of CubeSat launches until 2013, when more than half of launches were for non-academic purposes, and by 2014 most newly deployed CubeSats were for commercial or amateur projects.
Functions typically involve experiments that can be miniaturized or serve purposes such as Earth observation or amateur radio. CubeSats are employed to demonstrate spacecraft technologies intended for small satellites or that present questionable feasibility and are unlikely to justify the cost of a larger satellite. Scientific experiments with unproven underlying theory may also find themselves aboard CubeSats because their low cost can justify higher risks. Biological research payloads have been flown on several missions, with more planned. Several missions to the Moon and beyond are planning to use CubeSats. The first CubeSats in deep space were flown in the MarCO mission, where two CubeSats were launched towards Mars in May 2018 alongside the successful InSight mission.
Some CubeSats have become countries' first-ever satellites, launched either by universities, state-owned, or private companies. The searchable Nanosatellite and CubeSat Database lists over 4,000 CubeSats that have been or are planned to be launched since 1998.
History
Professors Jordi Puig-Suari of California Polytechnic State University and Bob Twiggs of Stanford University proposed the CubeSat reference design in 1999 with the aim of enabling graduate students to design, build, test and operate in space a spacecraft with capabilities similar to that of the first spacecraft, Sputnik. The CubeSat, as initially proposed, did not set out to become a standard; rather, it became a standard over time by a process of emergence. The first CubeSats launched in June 2003 on a Russian Eurockot, and approximately 75 CubeSats had entered orbit by 2012.The need for such a small-factor satellite became apparent in 1998 as a result of work done at Stanford University's Space System Development Laboratory. At SSDL, students had been working on the OPAL microsatellite since 1995. OPAL's mission to deploy daughter-ship "picosatellites" had resulted in the development of a launcher system that was "hopelessly complicated" and could only be made to work "most of the time". With the project's delays mounting, Twiggs sought DARPA funding that resulted in the redesign of the launching mechanism into a simple pusher-plate concept with the satellites held in place by a spring-loaded door.
Desiring to shorten the development cycle experienced on OPAL and inspired by the picosatellites OPAL carried, Twiggs set out to find "how much could you reduce the size and still have a practical satellite". The picosatellites on OPAL were, a size that was not conducive to covering all sides of the spacecraft with solar cells. Inspired by a cubic plastic box used to display Beanie Babies in stores, Twiggs first settled on the larger ten-centimeter cube as a guideline for the new CubeSat concept. A model of a launcher was developed for the new satellite using the same pusher-plate concept that had been used in the modified OPAL launcher. Twiggs presented the idea to Puig-Suari in the summer of 1999 and then at the Japan–U.S. Science, Technology and Space Applications Program conference in November 1999.
The term "CubeSat" was coined to denote nanosatellites that adhere to the standards described in the CubeSat design specification. Cal Poly published the standard in an effort led by aerospace engineering professor Jordi Puig-Suari. Bob Twiggs, of the Department of Aeronautics & Astronautics at Stanford University, and currently a member of the space science faculty at Morehead State University in Kentucky, has contributed to the CubeSat community. His efforts have focused on CubeSats from educational institutions. The specification does not apply to other cube-like nanosatellites such as the NASA "MEPSI" nanosatellite, which is slightly larger than a CubeSat. GeneSat-1 was NASA's first fully automated, self-contained biological spaceflight experiment on a satellite of its size. It was also the first U.S.-launched CubeSat. This work, led by John Hines at NASA Ames Research, became the catalyst for the entire NASA CubeSat program.
In 2017, this standardization effort led to the publication of ISO 17770:2017 by the International Organization for Standardization. This standard defines specifications for CubeSats including their physical, mechanical, electrical, and operational requirements. It also provides a specification for the interface between the CubeSat and its launch vehicle, which lists the capabilities required to survive the environmental conditions during and after launch and describes the standard deployment interface used to release the satellites. The development of standards shared by a large number of spacecraft contributes to a significant reduction in the development time and cost of CubeSat missions.
Design
The CubeSat specification accomplishes several high-level goals. The main reason for miniaturizing satellites is to reduce the cost of deployment: they are often suitable for launch in multiples, using the excess capacity of larger launch vehicles. The CubeSat design specifically minimizes risk to the rest of the launch vehicle and payloads. Encapsulation of the launcher–payload interface takes away the amount of work that would previously be required for mating a piggyback satellite with its launcher. Unification among payloads and launchers enables quick exchanges of payloads and utilization of launch opportunities on short notice.Standard CubeSats are made up of units designed to provide or of useful volume, with each unit weighing no more than. The smallest standard size is 1U, consisting of a single unit, while the most common form factor was the 3U, which comprised over 40% of all nanosatellites launched to date. Larger form factors, such as the 6U and 12U, are composed of 3Us stacked side by side. In 2014, two 6U Perseus-M CubeSats were launched for maritime surveillance, the largest yet at the time. The Mars Cube One mission in 2018 launched two 6U cubesats towards Mars.
Smaller, non-standard form factors also exist; The Aerospace Corporation has constructed and launched two smaller form CubeSats of 0.5U for radiation measurement and technological demonstration, while Swarm Technologies has built and deployed a constellation of over one hundred 0.25U CubeSats for IoT communication services.
Since nearly all CubeSats are they can all be launched and deployed using a common deployment system called a Poly-PicoSatellite Orbital Deployer, developed and built by Cal Poly.
No electronics form factors or communications protocols are specified or required by the CubeSat Design Specification, but COTS hardware has consistently used certain features which many treat as standards in CubeSat electronics. Most COTS and custom designed electronics fit the form of PC/104, which was not designed for CubeSats but presents a profile that allows most of the spacecraft's volume to be occupied. Technically, the PCI-104 form is the variant of PC/104 used and the actual pinout used does not reflect the pinout specified in the PCI-104 standard. Stackthrough connectors on the boards allow for simple assembly and electrical interfacing and most manufacturers of CubeSat electronics hardware hold to the same signal arrangement, but some products do not, so care must be taken to ensure consistent signal and power arrangements to prevent damage.
Care must be taken in electronics selection to ensure the devices can tolerate the radiation present. For very low Earth orbits in which atmospheric reentry would occur in just days or weeks, radiation can largely be ignored and standard consumer grade electronics may be used. Consumer electronic devices can survive LEO radiation for that time as the chance of a single event upset is very low. Spacecraft in a sustained low Earth orbit lasting months or years are at risk and only fly hardware designed for and tested in irradiated environments. Missions beyond low Earth orbit or which would remain in low Earth orbit for many years must use radiation-hardened devices. Further considerations are made for operation in high vacuum due to the effects of sublimation, outgassing, and metal whiskers, which may result in mission failure.
Structure
The number of joined units classifies the size of CubeSats and according to the CubeSat Design Specification are scalable along only one axis to fit the forms of 0.5U, 1U, 1.5U, 2U, or 3U. All the standard sizes of CubeSat have been built and launched, and represent the form factors for nearly all launched CubeSats as of 2015. Materials used in the structure must feature the same coefficient of thermal expansion as the deployer to prevent jamming. Specifically, allowed materials are four aluminum alloys: 7075, 6061, 5005, and 5052. Aluminum used on the structure which contacts the P-POD must be anodized to prevent cold welding, and other materials may be used for the structure if a waiver is obtained. Beyond cold welding, further consideration is put into material selection as not all materials can be used in vacuums. Structures often feature soft dampers at each end, typically made of rubber, to lessen the effects of impacting other CubeSats in the P-POD.Protrusions beyond the maximum dimensions are allowed by the standard specification, to a maximum of beyond each side. Any protrusions may not interfere with the deployment rails and are typically occupied by antennas and solar panels. In Revision 13 of the CubeSat Design Specification an extra available volume was defined for use on 3U projects. The additional volume is made possible by space typically wasted in the P-POD Mk III's spring mechanism. 3U CubeSats which utilize the space are designated 3U+ and may place components in a cylindrical volume centered on one end of the CubeSat. The cylindrical space has a maximum diameter of and a height no greater than while not allowing for any increase in mass beyond the 3U's maximum of. Propulsion systems and antennas are the most common components that might require the additional volume, though the payload sometimes extends into this volume. Deviations from the dimension and mass requirements can be waived following application and negotiation with the launch service provider.
CubeSat structures do not have all the same strength concerns as larger satellites do, as they have the added benefit of the deployer supporting them structurally during launch. Still, some CubeSats will undergo vibration analysis or structural analysis to ensure that components unsupported by the P-POD remain structurally sound throughout the launch. Despite rarely undergoing the analysis that larger satellites do, CubeSats rarely fail due to mechanical issues.