CAPSTONE: The story so far

After decades of delays and false starts, NASA is long last return to the moon. The world looks forward to the launch of Artemis I, the first demonstration flight of both the space launch system and the Orion Multi-Purpose Crew Vehicle, which together will send humans out of low Earth orbit for the first time since 1972. But he is late.

While the first official Artemis mission is naturally attracting all the attention, the space agency plans to do more than surface a new set of boots: their long-term goals include the “Lunar Gateway” space station which will be the gathering point for the continued exploration of our nearest celestial neighbor.

But before launching humanity’s first deep-space station, NASA wants to make sure that the unique near-straight halo orbit (NRHO) it will operate in is as stable as predicted by computer modeling. Join the Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment, or CAPSTONE.

CAPSTONE in the clean room before launch.

Launched aboard an Electron rocket in June, the large CubeSat is hoped to become the first spacecraft to enter an NRHO. By positioning itself in such a way that the gravity of the Earth and the Moon affects it equally, maintaining its orbit should only require periodic position corrections. This would not only reduce the maintenance burden to regulate the orbit of the Lunar Gateway, but would also reduce the station’s need for propellant.

CAPSTONE is also set to test an experimental navigation system that uses the Lunar Reconnaissance Orbiter (LRO) as a reference point rather than ground stations. In a future where spacecraft regularly buzz around the moon, it will be important to establish a navigation system that does not rely on terrestrial input to function.

So despite a relatively small cost of $ 30 million and only the size of a microwave oven, CAPSTONE is a very important mission for NASA’s great lunar aspirations. Unfortunately, things haven’t quite gone according to plan so far. The problems began a few days after takeoff and, as of this writing, the outcome of the mission is still very much in jeopardy.

A rocky start

Rocket Lab’s Electron rocket performed flawlessly during launch on June 28, after which the booster’s third “kick” stage began a series of engine burns to gradually increase its orbit. After running the engine six times in as many days, the kick phase performed the final translunar injection (TLI) burn before releasing CAPSTONE on July 4th. This put the craft on a low-energy ballistic trajectory towards the Moon, which would be refined with a series of small course-correcting maneuvers over the course of the four-month voyage.

After entering the free flight phase of the mission, CAPSTONE extended its solar panels to begin charging the batteries and stabilized in preparation for the first course correction burn scheduled for the next day. But shortly after communicating with NASA’s Deep Space Network (DSN) ground station in Madrid, contact with CAPSTONE was lost.

Communications were re-established approximately 24 hours later, and the analysis ultimately determined that a malformed command from ground operators had put the spacecraft’s radio in an unexpected state, which in turn triggered the detection routines of the spacecraft. breakdowns on board. The vehicle resets itself automatically and clears the fault condition, just as it has autonomously performed the necessary maneuvers to stay on the intended flight path.

While CAPSTONE emerged unscathed from this first anomaly and ground controllers felt they could prevent the problem from recurring, the window for the first course correction maneuver had long since passed. This meant that a new maneuver had to be planned given the vessel’s updated position and speed, a delicate process that took longer.

On July 7, CAPSTONE successfully performed the revised course correction burn (officially named TCM-1) and positioned itself on a trajectory within 0.75% of the calculated course.

A worrying fall

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

But on September 8, just as the planned TCM-3 maneuver was about to end, the spacecraft’s attitude began to deviate. For reasons still unknown, CAPSTONE’s reaction wheels were unable to counteract the wobble and the vehicle entered an uncontrolled fall. With its antenna no longer pointed at Earth, communications were again lost.

Mission controllers declared an operational emergency that evening, giving them access to additional DSN capabilities. This way they were able to receive a weak telemetry signal from CAPSTONE the next day, but the data looked gloomy. Due to its rotation, the aircraft’s solar panels were not producing enough power to charge the batteries, which was causing the spacecraft to reset frequently due to lack of power. Worse still, without the energy to run the onboard heaters, the thrusters that would eventually be needed to stop the fall were now frozen.

But it wasn’t all bad news. It was determined that the TCM-3 burn had progressed enough for the CAPSTONE to be on its intended orbital trajectory, so while the spacecraft may be technically out of control, it was still heading for the Moon.

An evolving situation

Currently, the last update we have from the CAPSTONE team was made on September 15th. The big news is that even though the boat is still spinning, the solar panels are getting enough light to charge the batteries. There is also enough energy in the budget to run the heaters, even though they apparently run on a reduced duty cycle. Still, it is enough to calm the cold and hopefully the propulsion system will soon reach a temperature high enough for its functionality to be assessed. Assuming they can be brought back into line, firing the thrusters against the direction of rotation should bring CAPSTONE back under control.

Many more maneuvers need to be done before CAPSTONE reaches the Moon.

But we’re not quite there yet. The update clarifies 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 timeline of the original mission shows that a series of further burns were planned to place the spacecraft in its intended orbit, and even then, that was just the Start or is it mission.

Fortunately, CAPSTONE shouldn’t need to make another course correction for a couple of weeks, which will give the engineers more time to assess the situation. However, the fact that two of the three main maneuvers caused the vehicle to not respond is worrying, especially when many more engine burns are still to be expected.

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