A robotic truss builder that was conceived, designed, and built by Caltech researchers and funded by the Defense Advanced Research Projects Agency (DARPA) has successfully built a structure in space—without the help of astronauts—for the first time. The truss builder, part of DARPA's Novel Orbital and Moon Manufacturing, Materials, and Mass-efficient Design (NOM4D) program, consists of an autonomous manipulator that picks up struts and joints held in nearby dispensers to repeatedly assemble identical sections, or bays, of a perimeter truss.
"This is the first time that a structure has been autonomously constructed in space," says Sergio Pellegrino , the Joyce and Krent Kresa Professor of Aerospace and Civil Engineering, the Lynn Booth and Kent Kresa Department of Aerospace Leadership Chair, the executive officer for aerospace, and a senior research scientist at JPL, which Caltech manages for NASA.
Pellegrino is team leader of Caltech's Autonomous In-Space Tensioned Reflector Assembly (ASTRA) demonstration, a robotic builder that has spent the last six months in space, working to construct a 1.4-meter-diameter reflector antenna from composite fiber tubes, aluminum joints, and a Kapton reflective surface. According to DARPA, the autonomous robotic assembly is akin to piecing together a high-tech K'Nex or Tinkertoy structure. But the process of automating the build in space is nothing like child's play.
Although the team ran into time limitations and was only able to build the initial section of the antenna, the demonstration has shown the feasibility of the proposed assembly concept, which is scalable to very large perimeter trusses holding giant reflective surfaces.
The space demonstration is the third phase of the DARPA project. In the first two phases, the ASTRA team developed the concept and design methodology and then showed that the concept worked under laboratory settings on Earth. The space payload was completed, tested, and delivered to the payload host, Momentus Inc., in less than 12 months.
"Developing a payload of this complexity on such a short timeline was an enormous undertaking for our small team," says Pellegrino, who is also co-director of the Space Solar Power Project, which aims to harvest solar power in space and wirelessly transmit it to Earth's surface. "As we wrap up the mission, we are finally able to start thinking about the new avenues of research opened by this demonstration."
Scientists would love the ability to deploy large-scale structures such as reflector surfaces in space. These could be used as enormous mirrors for powerful space telescopes, allowing astronomers to observe ever more distant objects; alternatively, these structures could form the aperture of large radio frequency antennas.
Assembling Very Large Structures Autonomously in Space
The size of such structures is currently limited by the size of rocket fairings. Since the beginning of space exploration, there have been two methods to get large structures into orbit or beyond: to fold them up for launch and unfurl them in space, or to manually piece them together from sections sent up over a series of launches. There is now a need for a third option to get larger structures in space—building them right there.
That is where the NOM4D program comes in. DARPA started the program in 2022 with a goal to ultimately "ferry raw materials from Earth for in-orbit manufacturing," according to the program's website. Caltech's ASTRA demonstration is one of only two projects that made it through to phase 3 of NOM4D's program: small-scale orbital demonstrations. The other, led by the University of Illinois Urbana-Champaign, will use one of the modules attached to the International Space Station to test a method for hardening carbon tubes in space.
"NOM4D's objective is to enable efficient, scalable construction of very large structures in space," said Andrew Detor, DARPA program manager for NOM4D, in a written statement . "By 'very large,' I mean hundreds of meters or kilometers in scale. Nothing approaching that scale can be achieved today."
Getting to Space
Early in the morning on March 30, 2026, ASTRA hitched a ride from Vandenberg Space Force Base to low-Earth orbit as one of 10 payloads carried by Momentus Inc.'s Vigoride 7 orbital space vehicle, which itself was launched aboard the SpaceX Transporter-16 mission. The ASTRA payload, roughly 1 cubic meter in size, survived the launch. It carried two prebuilt bays of the truss with diagonal supports made from struts and joints, along with an armlike manipulator. The payload also contained all the parts needed to construct the rest of the truss, held within motorized dispensers.
On the ground, ASTRA's small team of engineers cheered on the launch. Jaden Suh, former Caltech postdoctoral scholar on the team who is now an assistant professor at the University of Texas at Austin, had tears in his eyes. "As a child, I was super curious about and interested in space structures. Just like many other engineers working in this field, sending your stuff to space is what I've been dreaming of my entire life," Suh says. "I was extremely lucky to have the opportunity to work on this project, beginning from scratch. A lot of my original ideas are included in this system, and it was just incredible seeing it reach space."
Once ASTRA was in space, the team began to send commands to the onboard computer through the Vigoride vehicle. First, the launch restraints holding its components during launch were released. Next, the two prebuilt bays were pushed out of the truss builder, and then the assembly of the third bay was carried out.
The commands to build the structure were sent by the ASTRA team members—Charles Sommer, a former Caltech systems engineer; Alan Truong, a Caltech researcher engineer; and graduate student Tara Venkatadri (MS '24)—and the outcomes were checked against photos downloaded from space. The operation was repeated to overcome unforeseen problems, of which there were many. "That's basically the reason you do proof-of-concept testing. There are a lot of differences in the space environment compared to down on Earth," Suh explains. "There are always lots of little surprises."
To help troubleshoot those little surprises, the team had built ASTRA's twin, a flight spare, that they could work with in Pellegrino's Caltech lab. "Every time we confirm there is an issue in space, we come to the laboratory and try to mimic the problem on the ground," Sommer says. There are only about seven or eight short windows of about 15 minutes each day when researchers on the ground can communicate with the system in space. So, the team works out a solution by testing alternatives on the flight spare and then relays the workaround to the twin in space.
Nominally, the main manipulator robot moves around, picks up components, moves them into the right position, and constructs each bay on an assembly plate. Once this sequence is complete, the manipulator attaches a netlike perforated film to the truss to form two reflective cable nets that are prestressed against each other by springs. The reflective material ASTRA used was co-invented by the Caltech team and collaborators at the University of Pennsylvania, led by Professor Igor Bargatin. The perforated film offers several advantages over the knitted metallic mesh normally used in furlable reflectors: It requires a smaller amount of prestress and thus allows engineers to design structures that are much lighter.
Once fully prepped, each new bay is pushed out into the space environment, and the whole sequence is repeated for the next bay. The ASTRA truss is complete when 11 bays are completed, tethered to the netlike reflectors, and pushed out.
With ASTRA's mission coming to an end in December, it appears unlikely that the demonstration will be able to complete the full truss assembly. But the team says they have learned sufficient lessons to show that the proposed on-orbit assembly approach is viable and that they need to simplify and refine the design.
"Once you get to the size limit where structures cannot be prebuilt on the ground and unfolded, you have to think about how to build them in space," Pellegrino says. "So, this work could be useful not only for astronomy and astrophysics but also for communications and other areas, such as building very large solar arrays for space solar power."