Sensors Take Down Spacecraft

29 November 2016

The European Space Agency’s (ESA) Schiaparelli spacecraft was set to touch down on Mars soil on October 19. The lander went dark approximately a minute before touchdown, and the craft hurled to its death, dropping approximately 2.5 miles at more than 186 mph. NASA's Mars Reconnaissance Orbiter (MRO) photos of a 50’ × 130’ patch created by impact and possibly even explosion as propellant tanks were still full, confirm Schiaparelli’s violent demise. The spacecraft fell short of its landing site by more than three miles, but was well within the 62-mile by nine-mile planned landing ellipse.

Figure 1. The crater caused by the crash of the Schiaparelli Mars lander on Oct. 19, 2016. Photo: NASA's Mars Reconnaissance Orbiter, Nov. 1, 2016. Credit: NASA/JPL-Caltech/Univ. of Arizona  Figure 1. The crater caused by the crash of the Schiaparelli Mars lander on Oct. 19, 2016. Photo: NASA's Mars Reconnaissance Orbiter, Nov. 1, 2016. Credit: NASA/JPL-Caltech/Univ. of Arizona

Schiaparelli launched in March with another spacecraft, the Trace Gas Orbiter (TGO), part of a two-phase ExoMars project led by ESA with Russia’s space agency, Roscosmos, as the primary partner. The second phase of this project will be to put a life-hunting rover on the ground in 2021. The first phase tested the technologies necessary to accomplish phase two.

The TGO spacecraft is successfully circling Mars every 4.2 days, calibrating its science instruments, and is on a highly elliptical path, said the ESA. The TGO should achieve its intended final science orbit in March 2018 and begin its official two-year science mission—researching the origin of methane and other low-abundance Mars atmospheric gases. The craft will be a communications relay for the ExoMars rover and for existing Opportunity and Curiosity rovers, and it is expected to “live” until late in 2022.

Initially it was thought a computer glitch caused the vehicle to think it was near the ground when it clearly was not. Most of the effort up until the crash went as planned, until the vehicle’s thrusters ignited for three seconds instead of 30 seconds. All pointed to a software error in the processing of data received from sensors.

On November 23, the ESA indicated that it was an error in its inertial measurement unit that infected its navigation system and caused the on-board computer to act as though it had already landed. In fact, it was still 3.7 kilometers above the Mars surface. The agency claimed that there was an “unexplained saturation of its inertial measurement unit (IMU), which delivered bad data to the lander’s computer and forced a premature release of its parachute.”

These IMUs represent a combination of accelerometer and ring laser gyroscope sensors. The accelerometer measures changes in speed indicating, for example, when it has fired its rocket engines for long enough. The gyroscope measures how rapidly the spacecraft is turning. The gyroscope also estimates the spacecraft's orientation for brief periods. Each IMU has three gyroscopes and three accelerometers representing each axis of the spacecraft. Spacecraft typically have two IMUs onboard, with one serving as a backup.

Based on the IMU data, the lander thought it had already landed or was just about to land. The parachute system was released, the braking thrusters were fired only briefly, and the on-ground systems were activated. There are expectations that more information will be forthcoming on the IMU failure, as available.

Concerns now involve the ability of the ESA governments to raise the $330 million necessary to complete the second phase of the 2020 mission with Russia. So far, Italy has agreed to continue its investment in the program, and Russia has as well.

Sensors and sensing units have wreaked havoc before. For example, a few years ago, sensors also brought down a Russian spacecraft directly after launch. In July of 2013, Roscosmos confirmed that angular velocity sensors that were installed upside-down caused the Proton-M to nosedive.

Figure 2. The nosedive of the Proton-M caused by three DUS angular velocity sensors that were installed upside-down. Source: Roscosmos Figure 2. The nosedive of the Proton-M caused by three DUS angular velocity sensors that were installed upside-down. Source: Roscosmos

At that time, Roscosmos indicated that the installation of the sensors was both complicated and awkward, and that it could have been a failure of the worker that installed the devices or of the engineer that created the blueprints.

A commission found that installing the sensors upside-down was more difficult than installing them correctly and required special tools in order to facilitate alignment, which would have created noticeable damage on the sensor plate. That damage was actually found post-crash. Arrows on the DUS did indicate an upward direction, but a corresponding arrow on the mounting plate for reference did not exist. Color-coded cables were installed, but that installation would have looked the same regardless of the orientation of the sensors and would not have triggered any alarm.

In this case, navigation satellites and $1.3 billion of equipment was destroyed when 17 seconds after takeoff, the rocket veered off course, tried to correct, plummeted towards Earth and exploded on contact near the launch site.

Spacecraft sensors

Sensors on spacecraft include health-related sensors, such as temperature, strain gauges, gyroscopes and accelerometer sensors that monitor the spacecraft. Payload sensors, in comparison, involve radar imaging systems and IR cameras.

Given the outrageous number of sensors on each spacecraft, occasional failure is predictable. This is especially true as the craft continue to evolve well past the “Houston-we-have-a-problem” days, where ground control played a more critical function than it does today. Autonomy today is a result of insufficient ground coverage, noise interference between earth and the craft, and growing security concerns. This autonomy will not only grow, but also it will continue to take an expanding performance role.

In the meantime, there is no doubt that the ESA will figure out exactly what happened with the IMU. And there is also no doubt that Russia has, during the past three years, managed to place a corresponding arrow on devices wherever it can.

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