Monday, 31 October 2016

Anatomy of a Martian Crash

Schiaparelli’s Crash Site
Crash site of the Schiaparelli lander as viewed by  the 
High Resolution Imaging Science Experiment on the
Mars Reconnaissance Orbiter,
imaged on October 25th, 2016.

Image Credit: NASA/JPL/University of Arizona

The planet Mars has seen its share of mechanical disasters. An interesting look at this phenomenon is presented in a Universe Today article written just this past April. But never before have we had the capability to scrutinize a distant crash in such detail.

The Schiaparelli Descent and Landing Demonstrator Module (EMD) crashed into the Martian surface on October 19th at approximately 3 pm (Greenwich Mean Time). The crash was viewed by no less than three instruments. Data from the lander’s descent was recorded by the ExoMars Trace Gas Orbiter (TGO), which had travelled to Mars with the Schiaparelli lander. Collection and transfer of data was also assisted by the ESA Mars Express orbiter, which has been parked at Mars since December of 2003, when it too lost its accompanying lander the Beagle 2. In addition, measurements were made from Earth, using the Giant Metrewave Radio Telescope (GMRT), an experimental telescope array located near Pune, India. Scientists and engineers are still poring over this glut of data to determine what happened. However, several details have already become clear.
Farewell Schiaparelli
Image Credit: ESA – B. Bethge

Schiaparelli’s 6-minute descent through the martian atmosphere was intended to proceed in three stages. The first stage was to be free-fall under the protection of two heat shields, at front and back of the module. In stage two, a parachute (12 meters in diameter) was to be deployed, reducing the lander’s descent speed. In the third and final stage, the parachute and back shield were to be released, so that a set of nine thrusters could fire, further slowing the lander down and bringing it to a soft landing at Meridiani Planum near the martian equator.

Preliminary analysis of the collected data indicates that, for some reason, the thrusters switched off prematurely at a height of 2-4 km above the martian surface. This resulted in a long free-fall for the lander module, much longer than had been planned, and the lander crashed into the surface at speeds of more than 300 km/h. The hydrazine propellant tanks, which fueled the thrusters, are expected to have still been fairly full when the thrusters switched off, so they may have exploded on impact.  The descent data is still being analyzed, so more accurate details may become available in the future.

But monitoring the lander’s descent was not the only observation that was planned for Schiaparelli. Two cameras on NASA’s Mars Reconnaissance Orbiter (MRO) spacecraft were scheduled to image the predicted landing ellipse, a 100 km by 15 km area where the lander was expected to touch down. This allowed the location of the crash to be identified and the aftermath to be studied.

Finding Schiaparelli
Crash site of the Schiaparelli lander as viewed by the Context Camera on the NASA Mars Reconnaissance Orbiter. Image on the left shows the full view, while the image on the right shows a zoomed-in view. Both images toggle between data collected on Oct. 20th, just after the crash, and data collected on May 29th, five months before the crash. The images are located at approximately 2 degrees south latitude and 354 degrees east longitude, are oriented with north towards the top, and have a resolution of 6 m/pixel.
To learn more about this image, go to this ESA website.

Image Credit: NASA/JPL-Caltech/MSSS
MRO’s low-resolution Context Camera (CT) was scheduled to cover the anticipated landing site on Oct. 20th, and so was the first to image the suspected crash site. Scientists compared these images to previous ones, collected by the same instrument less than 5 months earlier, and found exactly where the lander came down. Despite the fatal difficulties, the module landed very close to its intended landing spot, only 5.4 km west and well within the predicted landing ellipse.

The discovery image shows the locations of two distinct components of the lander module. The dark fuzzy patch in the upper part of the image is where the lander crashed, forming an impact crater. The crater itself is fairly small, while the bulk of the dark patch, which is roughly 15 x 40 meters in size, represents surface material that was disturbed by the impact. Below the lander crash site, a small white dot can be seen. This is the module’s jettisoned parachute.

A Closer Look
Higher resolution images of the Schiaparelli lander as viewed by NASA’s Mars Reconnaissance Orbiter HiRISE camera on Oct. 25th.  Zoomed-in views are provided for each identified impact point resulting from module’s crash.
In the image, north is up; west is to the left, and the resolution is 29.5 cm/pixel. The brightness of the zoomed-in sections have been individually adjusted to best reveal the features in question. In addition, the image is corrected to adjust for the fact that HiRISE acquired them at an oblique angle. As a result, the scale bar in the main image is only indicative.
To learn more about this image, go to this ESA website.
Image Credit: NASA/JPL-Caltech/University of Arizona
On October 25th, MRO’s High Resolution Imaging Science Experiment (HiRISE) took a closer look at the crash site.  Close inspection of the resulting image revealed several new insights. At the bottom of the image, the rear heat shield can be seen, still attached to the parachute. At the top right of the image, a new crater was identified and attributed to impact of the front heat shield. Finally, the crater formed by the main lander impact is resolved in the image centre.

The Schiaparelli lander’s impact site consists of a central dark spot, an asymmetrical deposit around the dark spot, and a long dark arc on the north-east side. The dark spot is about 2.4 m across, which is consistent with a crater made by a 300 kg object impacting at a few hundred km/h. The crater is predicted to be about 50 cm deep and it is hoped that more details will be visible in future images. This will be necessary to help explain the enigmatic asymmetric and arc features, which are currently not well understood.

Asymmetrical deposits are not uncommon around impact craters, but such craters are usually formed by meteors travelling at speeds of 40,000 – 80,000 km/h. In such cases, the presence of asymmetric debris around a crater implies that the meteor came in at a low angle (closer to horizontal), so material was ejected from the crater in a preferred direction, with more ejecta material deposited in the direction of travel. However, the Schiaparelli lander is thought to have been travelling at only 300 km/h and descending almost vertically at the time of impact, so directed ejecta would not be expected. Also, the lander entered the atmosphere travelling from west to east. Thus, even if the impact was not vertical, the preferred direction for ejected material would have been to the east of the crater, not to the west as is seen in the HiRISE imagery. It is possible that the module’s propellant tanks exploded in a westerly direction, producing the asymmetrical ejecta, but more analysis is required to confirm this idea.

The long dark arc to the north-east of the crater is also currently unexplained. One suggestion is that an exploding propellant tank may have created this arc by disturbing the surface soil.

Despite many remaining questions, the abundant data from the Schiaparelli EDM crash is allowing us to study an extra-planetary accident in more detail than has ever been possible before. We can only hope that more data will bring more insights into how to avoid such disasters in the future.

Mars Reconnaissance Orbiter views Schiaparelli Landing Site.
ESA ExoMars News, Oct. 21, 2016

Detailed Images of Schiaparelli and its Descent Hardware on Mars.
ESA ExoMars News, Oct. 27, 2016