Apex: The Story So Far

After decades of delays and false starts, NASA at last Back to the moon. The world is eagerly awaiting the launch of Artemis 1, the first demonstration flight of the Space Launch System and the Orion multi-role spacecraft, the combination of which will send humans out of low Earth orbit for the first time since 1972 . But it was delayed.

While the first official Artemis mission naturally grabbed everyone’s attention, the space agency plans to do more than just put a new set of boots on the ground—their long-term goals include what will become a rallying point. The “Lunar Gateway” space station continues to explore our nearest celestial neighbor.

But before launching humanity’s first deep space station, NASA wants to make sure that the unique near-rectilinear halo orbit (NRHO) it will operate is as stable as computer models predict. Enter the Cislunar Autonomous Positioning System Technical Operation and Navigation Experiment, or CAPSTONE.

CAPSTONE in a clean room before launch.

Launched on an Electron rocket in June, the large CubeSat is expected to be the first-ever spacecraft to enter NRHO. By positioning itself in such a way that the gravitational forces of the Earth and the Moon affect it equally, maintaining its orbit only requires periodic position corrections. Not only would this ease the maintenance burden of adjusting the orbit of the lunar portal, it would also reduce the space station’s propellant requirements.

CAPSTONE will also test an experimental navigation system that uses the Lunar Reconnaissance Orbiter (LRO) as a reference point instead of a ground station. In the future, if spacecraft are constantly buzzing around the moon, it will be important to have a navigation system that does not rely on input from Earth to operate.

So, despite costing only $30 million and the size of a microwave oven, CAPSTONE is a very important mission for NASA’s grand lunar ambitions. Unfortunately, so far, things haven’t quite gone according to plan. The trouble started a few days after takeoff, and as of this writing, the outcome of the mission is still very dangerous.

tough start

Rocket Lab’s Electron rocket performed flawlessly during its June 28 launch, before the booster’s third “kick” stage began a series of engine burns to gradually increase its orbit. After the engine was fired six times over a multi-day period before the July 4 release of CAPSTONE, a final Trans-Lunar Injection (TLI) burn was performed during the start-up phase. This puts the vehicle on a low-energy ballistic orbit towards the moon, which will be refined through a series of small course correction maneuvers during the four-month journey.

After entering the free flight phase of the mission, CAPSTONE expanded its solar array to begin charging the batteries and stabilizing itself in preparation for the first course correction burn planned for the next day. But shortly after communicating with NASA’s Deep Space Network (DSN) ground station in Madrid, the connection to CAPSTONE was cut off.

Communications were re-established about 24 hours later, and analysis ultimately determined that an erroneous command from a ground operator had left the spacecraft’s radio in an unexpected state, triggering an onboard fault detection routine. The aircraft automatically resets and clears the fault condition, and automatically performs the necessary maneuvers to keep itself on the intended flight path.

While CAPSTONE got rid of the first anomaly unscathed, and ground controllers believed they could prevent the problem from recurring, the window for the first course correction maneuver was long past. This means that new maneuvers must be planned based on the aircraft’s updated position and velocity, a delicate process that requires additional time.

On July 7, CAPSTONE successfully performed a corrected heading correction burn (officially known as TCM-1) and placed itself on a trajectory within 0.75% of its calculated heading.

disturbing tumbling

After the initial communication difficulties were resolved, the mission continued without any issues. A small course correction was made on July 12 and a larger TCM-2 maneuver was made on July 25 without incident. On August 26, CAPSTONE reached an apogee of 1,531,949 kilometers (951,909 miles), the farthest its ballistic course was from Earth.

But on September 8, just as the planned TCM-3 maneuver was coming to an end, the spacecraft’s attitude began to deviate. For unknown reasons, the CAPSTONE’s reaction wheels were unable to resist the vibrations and the vehicle went into an uncontrolled rollover. Since its antenna was no longer pointed at Earth, communication was again interrupted.

That night, mission controllers declared an operational emergency, which allowed them to use other functions of the DSN. Through this, they were finally able to receive a faint telemetry signal from CAPSTONE the next day, but the data looked grim. Due to its rotation, the spacecraft’s solar panels did not generate enough energy to charge the batteries, which caused the spacecraft to frequently reset due to lack of power. To make matters worse, with no energy to run the onboard heaters, the thrusters that eventually need to stop tumbling are now frozen.

But it’s not all bad news. It was determined that the TCM-3 burn had proceeded far enough that CAPSTONE was in its intended orbit—so while the spacecraft may technically be out of control, it is still heading to the moon.

changing circumstances

Currently, the last update we got from the CAPSTONE team was on September 15th. The big news is that even though the vehicle is still spinning, the solar panels are getting enough light and the batteries are charging. There’s even enough power in the budget to run the heaters, although they’re apparently running at a reduced duty cycle. Even so, it’s good enough to dissipate the chill, and it hopes the propulsion system will soon reach hot enough temperatures to assess its capabilities. Assuming they can come back online, firing the thrusters against the direction of rotation should bring the CAPSTONE back under control.

A few more maneuvers are needed before CAPSTONE reaches the Moon.

But we’re not quite there yet. The update makes it clear that mission controllers are still analyzing the data to determine why CAPSTONE went out of control in the first place and how to prevent it from happening again.The original mission schedule showed that some additional burns were planned to put the spacecraft in its intended orbit, and even then this was just start its mission.

Fortunately, CAPSTONE will not need to make another heading correction for the next few weeks, which will give ground engineers more time to assess the situation. Still, the fact that two of the three main actions caused the vehicle to unresponsive was disconcerting, especially when several engine burns were still planned.

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