As more nations and private companies turn their sights toward the Moon, the region between Earth and lunar orbit has become a frontier of opportunity, risk, and strategic competition.
Now, the U.S. Defense Advanced Research Projects Agency (DARPA) is preparing to illuminate this vast expanse with a surveillance system unlike anything built before.
The agency’s new initiative, “Track at Big Distances with Track-Before-Detect” (TBD2), aims to detect and track objects so faint and distant that today’s telescopes can barely register them. To do this, DARPA plans to combine commercial optical sensors with radically more efficient algorithms and a spacecraft located more than 930,000 miles from Earth; a goal that marks one of the most ambitious expansions of American space situational awareness to date.
“The goal of the TBD2 program is to enable continuous space-based detection and tracking of objects in cislunar space on relevant timelines,” a recent DARPA solicitation reads. “This effort will increase the safety of cislunar commercial and civilian traffic, contributing to the peaceful use of space for the benefit of all nations and enabling a sustainable space ecosystem.”
Earth’s geosynchronous region—about 22,300 miles above the planet—has long been the outer boundary for routine surveillance. Cislunar space extends nearly 240,000 miles farther and in a region about 1,200 times larger in volume than geosynchronous earth orbit (GEO).
Objects at these distances are incredibly dim. A one-meter (3.3 feet) object at 2 gigameters (roughly 1.24 million miles) reflects so little light that it blends into the background noise.
Ground-based telescopes help, but they can’t see through clouds, daylight, or atmospheric distortion. Moreover, because they’re positioned on the ground, they can’t continuously monitor the space between Earth and the Moon.
To solve this issue, DARPA wants to put a sensor at the Sun–Earth Lagrange Point 1 (SEL1)—a stable gravitational point nearly 930,000 miles away. From that position, the Sun always remains behind the sensor, providing an unobstructed view of nearly the entire Earth–Moon system.
“Combining signal processing algorithms with onboard processing and commercially available sensors enables SSA at cislunar distances and beyond, allowing detection and tracking of at least 1 m wide RSOs at 2 Gm and revisit times of less than 12 hours,” reads a technical hypothesis for the TBD2 program.
Detecting the Unseen
In deep space, objects are so faint that they barely show up in any single image. Instead of waiting for a clear detection before tracking an object, TBD2 intends to take the opposite approach: track potential motion first and confirm the detection afterward. This method, known as Track-Before-Detect (TBD), is powerful but extremely computationally demanding.
“While current synthetic tracking and track-before-detect algorithms can theoretically reach these sensitivities, they are computationally expensive—requiring around 300 Trillion Floating-Point Operations Per Second (TFLOPs) (FP32) to operate effectively,” DARAP writes.
By contrast, many widely used radiation-hardened space processors, such as BAE Systems’ RAD5500 family, deliver on the order of 0.9 to 3.7 gigaflops (GFLOPs) of floating-point performance.
Newer radiation-tolerant single-board computers like Frontgrade’s SBC-2A72 reach roughly 10 GFLOPs, while specialized GPU or DSP payload boards from Moog are in the 75 to 150 GFLOP range—still thousands to tens of thousands of times below the ~300 TFLOP level implied by TBD2’s algorithmic needs.
That significant disparity is exactly what TBD2 is meant to address. “Latest technological advances in rad-hard computing are not enough to support the computing needs of cislunar SSA, TBD2 is necessary to close the gap,” a DARPA proposer’s day briefing slide reads.
For the program to succeed, teams will have to develop algorithms that are not just clever, but drastically more efficient—cutting computational demands by several orders of magnitude so they can run on realistic, space-qualified processors available aboard deep-space missions.
The technical ambition behind TBD2 is equally staggering. According to solicitation documents, DARPA wants a single asset stationed nearly a million miles from Earth to scan the entire Earth–Moon system in under 12 hours.
By contrast, while ground-based systems offer strong coverage in low and medium Earth orbit, their ability to observe faint objects in the Earth–Moon corridor is far more limited—hampered by weather, atmosphere, daylight, and viewing geometry that make such detections only intermittent at best.
One Giant Leap for Space Tracking
Achieving continuous or even reliably sub-daily updates across cislunar space would represent a major leap forward. If TBD2 succeeds, it would deliver the most responsive deep-space tracking capability ever fielded.
While SEL1 offers a powerful vantage point, DARPA makes clear in its solicitation that a single sensor there still leaves portions of cislunar space unobserved. Certain regions—such as stretches of the Earth–Moon corridor, the fast-moving zones near lunar orbits, and areas hidden behind the Moon—would remain blind spots unless additional sensors are deployed closer in.
“Four possible missions for the placements of a few TBD2 sensors include: monitoring the Earth-Moon corridor; monitoring lunar orbits, including EML1 and EML2; monitoring MEO/GEO orbits; and the small part of cislunar space that has an obstructed view from SEL1,” DARPA notes.
These additional sensors, flying between 124,000 miles and 248,000 miles from Earth, could spot much smaller objects—down to 4–8 inches—at ranges of 125,000 to 250,000 miles. Together, the SEL1 asset and these additional platforms are intended to provide complete, continuous coverage across the entire cislunar volume.
To simplify future deployment, DARPA is also encouraging proposers to build a single sensor and compute architecture that can operate at SEL1 and in these nearer, faster-moving orbits, creating a unified system explicitly designed to eliminate blind spots between Earth and the Moon.
Every aspect of TBD2’s hardware must operate within strict physical and operational limits. One of the clearest examples is the optical aperture. DARPA caps the telescope diameter at just 0.5 meters—roughly 19.7 inches—because anything larger quickly becomes too heavy, too bulky, and too expensive to launch into deep space.
Power use is similarly constrained. The system must operate at no more than 600 watts during high-power tracking modes and approximately 300 watts during standard operations, an electrical budget comparable to that of a household microwave oven. That level of efficiency is remarkable considering the scale of the sensing mission and the distances involved.
Mass is another critical factor. In its briefing materials, DARPA illustrates how TBD2’s approach could reduce payload mass by as much as eightyfold, a shift that makes deep-space missions far more feasible for modern launch vehicles.
DARPA is also imposing stringent performance thresholds that reflect the true demands of cislunar surveillance. According to the solicitation’s metrics, algorithms must achieve a probability of detection greater than 95% while keeping the probability of false alarms below 1%. This level of accuracy is extraordinarily challenging when tracking faint objects millions of miles away against noisy, cluttered backgrounds.
Prototypes on the Horizon
At the end of its 15-month performance period, TBD2 is expected to deliver three major prototypes. The first is a low-complexity object-tracking algorithm capable of identifying extremely faint moving objects against noisy deep-space backgrounds. The second is a complete payload design for deployment at SEL1, including its sensor architecture, computing platform, optical system, and overall spacecraft integration. The third is a variant of that payload tailored for placement in beyond-GEO or other cislunar orbits, optimized to detect faster-moving objects at closer ranges.
“The government’s expectation is to have prototypes of the fully developed signal processing algorithms capable of meeting program metrics and program goals and government-approved payload designs that can be used to proceed with initial system design by a transition partner,” DARPA writes.
If these prototypes perform as intended, the designs could ultimately transition to the U.S. Space Force or other government partners for further development and operational deployment.
Nations are rapidly expanding their footprint throughout the Earth–Moon system, reshaping how the United States thinks about space awareness.
China’s lunar relay satellites, Russia’s proposed cislunar vehicles, and a wave of commercial landers reflect a growing competition to operate—and gain strategic advantage—in the vast region between Earth and the Moon.
DARPA publicly frames TBD2 in peaceful terms, emphasizing its role in increasing the safety of cislunar commercial and civilian traffic. However, the strategic implications are hard to ignore.
A system capable of seeing every object moving through cislunar space would enable the United States to detect suspicious spacecraft approaching high-value assets, track derelict hardware before it becomes a navigational hazard, and monitor the activities of foreign missions as they transit the lunar corridor.
The significance of such a capability has only grown recently as unexpected deep-space visitors have grabbed public and scientific attention.
The detection of 3I/ATLAS—the third known interstellar object to pass through our solar system—demonstrated how quickly a faint, fast-moving object can appear from beyond Earth’s immediate neighborhood.
Meanwhile, the surge of interest in unidentified aerial phenomena (UAP) has fueled broader questions about what types of objects might transit near Earth without reliable tracking at great distances.
Although TBD2 is not designed to study UAPs or interstellar objects directly, a persistent, wide-area monitoring system extending to SEL1 and beyond would reduce the number of surprises entering the Earth–Moon environment.
In a region where invisibility is currently the norm, even modest gains in detection capability could have outsized consequences for both scientific discovery and national security.
If successful, TBD2 would transform this once-hidden volume of space into a clearly monitored domain—one in which sudden arrivals, unexplained trajectories, or unauthorized spacecraft movements become far more difficult to miss.
Ultimately, as the Earth–Moon system becomes the next major arena for exploration, commerce, and competition, cislunar awareness becomes increasingly essential. DARPA’s TBD2 program seeks to push surveillance to a place no system has gone before: more than a million miles from Earth, tracking objects barely larger than a shoebox, all while running on a few hundred watts of onboard power.
If TBD2 succeeds, the United States would gain something that has never existed: a true wide-area traffic picture of deep space, refreshed roughly every 12 hours and covering everything from geosynchronous orbit to the lunar surface and far beyond.
It would turn the sprawling, poorly monitored Earth–Moon region into a domain where objects can no longer slip by unnoticed. That level of persistent awareness would fundamentally transform how nations understand, manage, and secure the space between Earth and the Moon.
“If successful,” DARPA writes, “TBD2 will improve early warning capabilities for defense and civilian agencies who track potential threats and objects of interest originating from or transiting cislunar space, contributing to the safe and peaceful use of space for all nations.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com