After the fire and fury of liftoff, when a spacecraft is sailing silently through space, you could be forgiven for thinking the hard part of the mission is over. After all, riding what’s essentially a domesticated explosion up and out of Earth’s gravity well very nearly pushes physics and current material science to the breaking point.
But in reality, getting into space is just the first on a long list of nearly impossible things that need to go right for a successful mission. While scientific experiments performed aboard the International Space Station and other crewed vehicles have the benefit of human supervision, the vast majority of satellites, probes, and rovers must be able to operate in total isolation. With nobody nearby to flick the power switch off and on again, such craft need to be designed with multiple layers of redundant systems and safe modes if they’re to have any hope of surviving even the most mundane system failure.
That said, nobody can predict the future. Despite the best efforts of everyone involved, there will always be edge cases or abnormal scenarios that don’t get accounted for. With proper planning and a pinch of luck, the majority of missions are able to skirt these scenarios and complete their missions without serious incident.
Unfortunately, Lunar Trailblazer isn’t one of those missions. Things started well enough — the February 26th launch of the SpaceX Falcon 9 went perfectly, and the rocket’s second stage gave the vehicle the push it needed to reach the Moon. The small 210 kg (460 lb) lunar probe then separated from the booster and transmitted an initial status message that was received by the Caltech mission controllers in Pasadena, California which indicated it was free-flying and powering up its systems.
But since then, nothing has gone to plan.
Spotty Communications
According to NASA’s blog for Lunar Trailblazer, Caltech first heard from the spacecraft about 12 minutes after it separated from the second stage of the Falcon 9. At this point the spacecraft was at an altitude of approximately 1,800 kilometers (1118 miles) and had been accelerated by the booster to a velocity of more than 33,000 km/h (20,500 mph). The craft was now committed to a course that would take it away from Earth, although further course correction maneuvers would be required to put it into its intended orbit around the Moon.
The team on the ground started to receive the expected engineering telemetry data from the vehicle, but noted that there were some signals that indicated intermittent issues with the power supply. Around ten hours later, the Lunar Trailblazer spacecraft went completely silent for a short period of time before reactivating its transmitter.
At this point, it was obvious that something was wrong, and ground controllers started requesting more diagnostic information from the spacecraft to try and determine what was going on. But communication with the craft remained unreliable, at best. Even with access to NASA’s powerful Deep Space Network, the controllers could not maintain consistent contact with the vehicle.
Tumbling and Off-Course
On March 2nd, ground-based radars were able to get a lock on Lunar Trailblazer. The good news was that the radar data confirmed that the spacecraft was still intact. The bad news is that the team at Caltech now had a pretty good idea as to why they were only getting sporadic communications from the vehicle — it was spinning in space.
This might not seem like a problem at first, indeed some spacecraft use a slight spin to help keep them stabilized. But in the case of Lunar Trailblazer, it meant the vehicle’s solar arrays were not properly orientated in relation to the sun. The occasional glimpses of sunlight the panels would get as the craft tumbled explained the sporadic nature of its transmissions, as sometimes it would collect just enough power to chirp out a signal before going dead again.
But there was a now a new dimension to the problem. By March 4th, the the spacecraft was supposed to have made the first of several trajectory correction maneuvers (TCMs) to refine its course towards the Moon. As those TCMs never happened, Lunar Trailblazer was now off-course, and getting farther away from its intended trajectory every day.
By now, ground controllers knew it was unlikely that Lunar Trailblazer would be able to complete all of the mission’s science goals. Even if they could reestablish communication, the vehicle wasn’t where it was supposed to be. While it was still theoretically possible to compute a new course and bring the vehicle into lunar orbit, it wouldn’t be the one that the mission’s parameters called for.
A Data-Driven Recovery Attempt
The mission was in a bad place, but the controllers at Caltech still had a few things going in their favor. For one, they knew exactly what was keeping them from communicating with the spacecraft. Thanks to the ongoing radar observations, they also had highly-accurate data on the velocity, position, and rotation rate of the craft. Essentially, they knew what all the variables were, they just needed to figure out the equation that would provide them with a solution.
Over the next couple of months, the data from the radar observations was fed into a computer model that allowed ground controllers to estimate how much sunlight would hit Lunar Trailblazer’s solar array at a given time. Engineers worked with a replica of the spacecraft’s hardware to better understand not only how it operated while in a low-power state, but how it would respond when it got a sudden jolt of power.
The goal was to find out exactly how long it would take for the spacecraft to come back to a workable state when the solar array was lit, and then use the model to find when the vehicle and the sun would align for long enough to make it happen.
It was originally believed that they only had until June for this celestial alignment to work in their favor, but refined data allowed NASA and Caltech to extend that timetable into the middle of July. With that revised deadline fast approaching, we’re eager to hear an update from the space agency about the fate of this particularly tenacious lunar probe.