Ordinary Meeting, 2005 April 23
Bonneville and Beyond: A Year on Mars with the Spirit Rover
Mr Ellison reported that his title was a little out-of-date: Spirit's exploration of Mars was now in its 463rd sol – Martian days, of length 24 hours and 38 minutes; it had somewhat exceeded its 90-sol design lifetime. He explained that the mission had been shaped by both the successes and the failures before it. In 1996, NASA's successful Pathfinder mission had delivered the rover Sojourner to the Martian surface, where it had operated for nearly three months. Sojourner had served as a "proof-of-concept" for several new technologies: it was the first time that airbags had been employed to soften the blow of touchdown, and the first time that a rover had driven on Mars. However, problems had also been encountered. The Martian surface had been found to be very dusty, and, when studying rocks, it had been discovered that the ability to abrade them to clear away their dusty top surfaces would have been of great benefit. Pathfinder had been the second mission undertaken under the motto "faster, better, cheaper", though the speaker greeted with mixed reaction the use of its success to promote that strategy's use.
However, the twin failure of NASA's Mars Polar Lander and Mars Climate Orbiter spacecraft in 1999 had also shaped the Mars Exploration Rover mission of which Spirit, aka MER-A, and its twin Opportunity, aka MER-B, were two halves. The decision to send two identical rovers, with a short delay between their arrival times, would increase the chances of success. With reference to the "faster, better, cheaper" motto, the speaker thought it only ever possible to achieve two, never all three, and in this case, while the rovers had been designed very quickly, and their longevity was testament to their quality, at $820m, they had not been particularly cheap.
Moving on to discuss the technology employed, Mr Ellison explained that each rover had three communications antennae: two for transmitting directly to the Deep Space Network (DSN) of radio receivers on Earth, and one for communicating with NASA's two functioning satellites in Martian orbit, Mars Global Surveyor (MGS, 1999) and Mars Odyssey (MO, 2001). Communication with the DSN used the rovers' omnidirectional low-gain antennae, and steerable, directional, high-gain antennae, which could achieve higher data rates. Their UHF antennae, for communicating with MGS and MO, were more power efficient, and could achieve a greater data rate of 128 kbit/s, as compared to 3-12 kbit/s for direct communication to Earth. Each satellite was typically visible for 8 minutes each time it passed overhead, in which time 60 Mbit of data could be transferred, subsequently to be relayed on to the Earth. The speaker thought it likely that, in view of the vastly superior power efficiency, future landers would communicate exclusively with satellites.
Mr Ellison went on to describe the rover's cameras, firstly its panoramic camera, "Pancam", consisting of two monochrome 1024 × 2048 CCD arrays, each with 16.6° field, mounted on a 1.5 m high mast with a horizontal separation of 30 cm. Each array had a filter wheel that allowed images to be taken in a range of colours, and, in addition, by combining images from both cameras, three-dimensional stereovision could be achieved. The Pancam Mast Assembly (PMA) on which the cameras were mounted could be rotated through a full 360° to give a full panoramic view, hence the instrument's name. Spirit's other cameras included two navigation cameras, "Navcams", also mounted on the PMA, and four hazard detection cameras, "Hazcams", on the body of the rover, each with a 120° field of view. The PMA also housed a mirror which reflected light down its length into an infrared spectrometer, the Miniature Thermal Emission Spectrometer (Mini-TES), with which the approximate surface composition of rocks could be estimated remotely, enabling mission controllers to make more educated choices of scientifically interesting targets to study at close range.
Spirit's journey had started on 2003 June 10, launched aboard a conventional Boeing Delta II rocket. Its sister had needed additional thrust as a result of taking off later, on July 7, near the close of the launch window, and had been the first customer of Boeing's new Delta II Heavy launch vehicle. After a few small trajectory correction manoeuvres along its comparatively straightforward cruise orbit to Mars, Spirit had entered the Martian atmosphere on 2004 January 3, followed by Opportunity on January 25, each housed in protective aeroshells consisting of a heat shield and parachute. The frictional deceleration upon entry had peaked at 6g around 3 minutes 45 seconds prior to landing, slowing them from 12,000 mph to 1,000 mph.
When this speed had been reached, 1m43 prior to landing, Spirit's parachutes had opened, and its heat shield detached 20 seconds later. Mars' atmosphere was so thin that a parachute alone could not slow it further than 180 mph – still not a safe landing speed – and so a three-second burn by Rocket Assisted Descent (RAD) motors had been required six seconds before touchdown, bringing it finally to a dead halt 10-15 m above the surface. A Radar Altimeter Unit on the lander's underside had served to monitor its altitude and control this process, in addition to a camera pointing vertically downwards, the Descent Image Motion Estimation Subsystem (DIMES), which had monitored its horizontal motion and controlled the Transverse Impulse Rocket System (TIRS) to correct for it. Finally, starting eight seconds prior to touchdown, airbags had inflated around the landing craft to cushion its impact, and, three seconds prior to landing, the payload had been dropped from its aeroshell, bouncing several times before coming to rest. The landing module had then slowly retracted its airbags, and opened to reveal the rover within.
One of the first priorities upon touchdown had been to locate Spirit in satellite images: having an estimated lifetime of only 90 sols, it was necessary to decide upon its scientific targets rapidly. Using all available data, NASA had been able to spot it in detailed MGS images by mid-January; it was found to have touched down 10 km from the centre of its target ellipse – the 82 × 12 km region of the Gusev Crater into which it had been aimed.
Meanwhile, attention had been focussed upon dismounting the rover from the lander – a manoeuvre termed egress. The rover's first images revealed that the airbags had not retracted so far as had been intended, and all attempts over following days to retract them further failed. Consequently, on January 9, it was decided to rotate Spirit around on the lander and egress by a different route – a contingency operation which had been practiced during its design phase. Finally, on January 15 (sol 12), Spirit rolled forth onto the Martian surface.
Mr Ellison went on to describe how NASA, in somewhat sombre mood, had memorialised several of those whose lives had been lost in its journey of human spaceflight, on January 6 naming the landing site the "Columbia Memorial Station", later, a range of three hills to the north the "Apollo 1 Hills", and finally, a set of hills to the east the "Columbia Hills Complex" – one commemorating each of the crewmembers of Columbia's final flight. These names had been submitted to the IAU for approval.
On January 21, to the immense concern of NASA, Spirit had fallen silent whilst en route to its first scientific target, a rock named by mission controllers "Adirondack". On the following day, still no data was returned, though brief transmissions indicated the rover to be in a fault mode. It later became evident that its processor had entered a reset loop, repeatedly rebooting itself after experiencing some fault, running for at most an hour at a time. The culprit appeared to be the rover's static flash memory, or the software which managed it; later it was established that the deletion of now-redundant data from the interplanetary cruise had been overlooked, and the memory was simply full. This data was deleted, and a software patch applied to both rovers to prevent a recurrence. On February 6 (sol 33), Spirit had finally returned to full scientific operations.
Three days were spent analysing Adirondack, first grinding off its dusty surface with the Rock Abrasion Tool (RAT), and then undertaking geological analysis of the newly revealed material beneath with a microscopic imager and spectroscope. Mission controllers had then decided to drive towards a nearby crater they named Bonneville, around 400 m distant to the north-east. Mr Ellison provided superb illustration of this journey, utilising stereo images from the Pancam, combined with red-green glasses provided to the audience, to provide a 3D view of Mars.
On February 10 (sol 37), Spirit drove 21.2 m, shattering the previous record of 7 m in a sol, set by Sojourner, and by February 17 had driven a total of 108 m, surpassing the total distance traversed by its predecessor. Among various scientific targets along its route was Laguna Hollow, a circular depression, where Spirit had spun a small deliberate wheel rut, before spending five sols, 45-49, making a detailed analysis of the soil thrown up. After a few sols dominated by long drives, 30 m setting a new record on sol 51, Spirit had paused to study a rock named Humphrey over sols 55-59, grinding three areas of the surface before assessing which minerals were present within, finding evidence that it had once been soaked in water.
Finally, on March 8 (sol 64), Spirit had begun ascending the 15° incline of Bonneville's rim, on the same day surpassing 300 m on its odometer, one of NASA's minimum success criteria for the mission. On the following sol, the opposite rim had come into view and, on the next, a view of the inside of the crater had been gained. Deciding it too dangerous to descend within Bonneville's steep walls, the remainder of the 90-sol mission was spent admiring the newfound scenery and taking soil samples. On March 13 (sol 68), Spirit had even dabbled in astronomy, observing Deimos transit the Sun.
On sol 87, with Spirit still appearing healthy, it had been decided to journey towards the contrasting landscape of the Columbia Hills. On sol 105 it had passed a crater informally named Missoulla, not deemed a high-priority target as it seemed comparatively old, and on sol 118 another, Lahonten. By sol 125, however, a problem had been becoming apparent with Spirit's front-right wheel: its current draw had increased by a factor of 2-3, and wear on its gearbox lubrication seemed likely. Simple remedies effected no change, and so it was decided that, in order to prolong its life, in case it should be needed later in rough terrain, it should be turned off, and the rover driven in reverse, dragging it behind.
Its journey to the hills had continued, reaching their foot by sol 159, but another issue had started to become apparent by sol 200. Its solar panels were becoming dirty and inefficient, and keeping the rover tilted Sunward had become especially important. The viability of certain routes was affected: a single day spent on a south-faced incline could finish it. A number of rocks had been analysed before powering down for sols 239-262, when solar conjunction had brought an 11-day communication blackout with Earth.
After several months exploring the hills, Spirit's health had recently taken some exciting turns. During the night of sol 419-20 (2005 March 19), its solar panels' energy yield had leapt from an average 350 Watt-hours per sol, 60% of its initial capability, to 600 Wh/sol. Given the weather conditions, it seemed likely that a dust devil had fortuitously swept them clean. In addition, the speaker reported that the rover's defective wheel seemed to be improving, perhaps cured by months of being dragged along. Looking ahead, Spirit seemed as healthy as ever.
To close, the speaker summed up how superbly the rover had surpassed its design lifetime. Over its planned 90-sol mission, it had been expected to drive 600 m, take one full 360° panoramic image, and return 5,000 images. Over the 464 sols that it had been operating so far, it had driven 4,250 m, taken seven panoramic images, and returned 71,706 images. There had barely been time to mention its sister, which had achieved similar, and also continued to operate well. Looking ahead, future missions were not far away: Mars Reconnaissance Orbiter (MRO) was scheduled to lift off aboard a Lockheed Martin Atlas V launch vehicle on 2005 August 10, and to reach Martian orbit in 2006 March. Looking further ahead, the next planned landing mission was Phoenix, scheduled to take off in 2007 August, and to touch down near the icy north pole on 2008 May 25.
Following the applause for Mr Ellison's complete and superbly illustrated account, the President adjourned the meeting until May 25 at the Geological Society.
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Dominic Ford