How CAPSTONE will bring home the bacon for Nasa

The National Aeronautics and Space Administration (Nasa), purportedly, plans to send one team of astronauts to the moon each year after 2024. Nasa’s moon missions or the Artemis programme have futuristic agendas: setting up a flying station and building habitats on the moon. This time around, people will set foot on our cosmic neighbour’s south pole, where no human has stepped so far.

Evidently, such ambitious ventures are riddled with extreme risks, unknowns, and surprises that deep space can throw at us. However, practice makes perfect appears to be Nasa’s motto to overcome these challenges. Accordingly, the agency is gearing up with several trial runs and mock drills of the different stages of the Artemis programme.

The first task is to set up the orbiting lunar outpost, Gateway. But before it is hauled up, all eyes are on CAPSTONE.

CAPSTONE is Nasa’s chosen candidate to go to the moon ahead of Gateway. It will perform all the necessary space gymnastics that the flying station will in future and ascertain if the path is safe and viable for Gateway to follow.

While there were several delays in launching CAPSTONE, it is likely to be space-borne this month.

What’s all the fuss? We’ve been to the moon before

Yes, but this time it’s by a road untraveled.

Five decades ago, the Apollo mission landed men on the moon. The trips were short, and the route was comparatively straightforward: blast off from the base, approach the moon tangentially, take advantage of the moon’s spin for momentum, braking, and return to earth. Moreover, throughout the figure eight-like pathway, Apollo was under the gaze of ground-based engineers for guidance and critical manoeuvres.

Angelic halo orbit chosen for humankind’s first lunar outpost. Video: European Space Agency

Unlike those round trips, Gateway will park around the moon for 15 years and run autonomously without much interference from earth. So, with such a critical agenda, CAPSTONE has to test the navigation dynamics of Gateway’s designated orbit and validate the feasibility.

Gateway will be circling another celestial body. Hence, unlike our satellites or the International Space Station, it will not have the earth as its reference. Instead, Gateway’s trajectory is bound around the moon. The flying station will also have to tag along with the moon around the earth. Therefore, for Gateway, the moon’s orbit will be the reference point in space.

The moon’s gravity is a prominent influence on the lunar outpost. As the moon is a deep-space entity, complex orbital mechanics come into play; the references are all in three-dimensional cis-lunar space. One can imagine cis-lunar space as the volume enclosed in a sphere that envelopes both the earth and the moon. The diameter of this sphere is equal to the moon’s orbit around the earth.

Usually, satellites are put in circular or oval trajectories. However, the spacecraft tend to veer off their path from time to time due to changing gravitational fields and other spatial parameters, requiring a course correction to set them back on track. Such manoeuvres take up lots of fuel, and hauling the precious resources from the earth to keep Gateway in its orbit is not a viable option.

To overcome this hurdle, Nasa proposes to operate Gateway in a particular type of path called the Near Rectilinear Halo Orbit (NRHO). Never before has a spacecraft been put in NRHO, so it’s CAPSTONE’s prerogative to go to the moon, test and validate the integrity of this orbit for navigation efficiency and ensure Gateway’s safety.

Cruising along

NRHO takes advantage of the Lagrangian points (L). These are regions in space where the gravity fields between any two celestial bodies are neutralized. So effectively, if a spacecraft is in these regions, it cruises without much effort as there is no drag from gravity. Moreover, due to the absence of resisting forces, the spacecraft consumes minimal fuel to maintain its course, switch to another orbit in the region or perform course corrections.

About 60,000 km above the moon’s surface in space are five Lagrange points between the earth and moon. CAPSTONE will go around the moon’s south pole in a seven-day cycle, settling in an NRHO in the L1- L2 region. In the future, Gateway will use the closest approach (1,600 km from the surface) as the window to offload people and landers once every seven days on the south pole’s surface. (Currently, the south pole is the focal point as water-ice reserves have been found there). Moreover, the orbit is so angled that sunlight will constantly fall on the solar panels.

While CAPSTONE will demonstrate the navigation efficiency in the NRHO, it will also establish the smooth transit between orbits.

Meet CAPSTONE

CAPSTONE is short for Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment.

For its mega capabilities, one would imagine a huge robotic probe. However, CAPSTONE weighs a mere 25 kilos and is the size of a microwave oven. CAPSTONE belongs to the CubeSat category and packs a punch with its stack of 12 units carrying innovative technologies.

Nasa funded and coordinated the CAPSTONE project under its Small Business Innovation Research (SBIR) programme. The programme offers opportunities to partner with private organisations to explore small economic missions to the moon. As a result, multiple private organisations joined hands to bring CAPSTONE to fruition.

Advanced Space, a Colorado-based company, designed the cis-lunar navigation module and will also oversee its operation. The module will employ a trajectory known as the ballistic lunar transfer. This highly efficient trajectory uses very little fuel, using the sun’s gravity to reach this unique orbit around the moon.

Tyvak Nano-Satellite Systems, Inc. of Terran Orbital Corporation (California), built the primary CubeSat platform and tested the unit. Stellar Exploration (California) is providing CAPSTONE’s propulsion system, again a novel technology that maintains control of the craft on thrusters alone. The 8 sq inch by 4-inch deep unit has eight tiny, high-performance thrusters fed by hydrazine propellant from an unpressurized tank.

Rocket Lab’s Photon spacecraft will launch CAPSTONE from a facility in New Zealand. Then, it will take the satellite in a series of orbit-raising manoeuvres to put it in a transfer path to the moon. After that, solar panels on CAPSTONE will deploy and be the primary power source for the satellite’s six-month-long assignment.

After a week-long journey, CAPSTONE will spend the next three months in several critical space manoeuvres to enter the halo orbit.

Nasa says CAPSTONE has additional responsibilities: “During its journey along this path, CAPSTONE will test out spacecraft-to-spacecraft navigation and communications systems with NASA’s Lunar Reconnaissance Orbiter, another spacecraft in orbit around the moon. By demonstrating that two orbiting spacecraft can communicate and track their positions independent of earth.”

With these trailblazing demonstrations up its sleeve, the next few months will reveal how CAPSTONE will bring home the bacon for Nasa.