When Science Goes Wrong (32 page)
Thompson’s self-appointed mission is to educate defence lawyers about DNA testing so that they are in a better position to review tests conducted by police labs and spot the problems that call out for independent testing and for challenging the state’s experts. The lawyers and law students who staff the Innocence Projects are clearly hearing his message, because the existence of DNA evidence is no longer a bar to taking on a case the way it was when Josiah Sutton sought the Houston Innocence Network’s aid.
Have errors in forensic science ever led to the execution of an innocent man? Given the sheer number of executions – more than a thousand since the death penalty was re-instated in the United States in 1976 – it seems likely that they have, but identifying a specific instance has proved difficult. One case that has drawn a lot of attention is that of Cameron Todd Willingham, executed in Texas in 2004 for setting a house fire that killed his three children. Willingham asserted his innocence before, during and after his trial, and he did so again in his final statement before execution. At his trial, investigator Manuel Vasquez reported finding scientific indicators of arson, such as the presence of crazed glass, but subsequently-published forensic guidelines have rejected them as mere superstition. Hot glass is easily crazed by contact with water from fire hoses, for example. ‘Each and everyone of the ‘indicators’ listed by Mr. Vasquez means absolutely nothing,’ reported a commission of nationally-respected arson investigators in 2006. Willingham certainly hasn’t been proven innocent, but the evidence for his guilt has largely evaporated.
Josiah Sutton’s story isn’t over. When the independent lab did the DNA testing that ruled him out as a suspect, it was able to reconstruct a complete DNA profile for one of the actual rapists, as well as a partial profile for the other. The complete profile was used to search a database maintained by the Texas Department of Public Safety, but no match was found. Still, new DNA profiles are being entered into the database all the time, because in Texas all convicted criminals have to give a DNA sample. In 2005, a young black man named Donnie Lamon Young, who was serving time on a drug conviction, gave blood for testing. In May of 2006 the DPS found that Young’s DNA was a match to the complete profile from the Sutton case.
The Houston Police Department was notified, and a new sample was taken from Young, who was by then out of prison. Again there was an exact match. In June, Young was arrested and charged with aggravated sexual assault in the case for which Sutton was wrongly convicted. He was held in the county jail after he failed to post a $150,000 bail bond. The victim was unable to pick Young out of an identification line-up, but in January 2007 he pleaded guilty and was sentenced to 10 years’ imprisonment. He also named his accomplice, a man who had died in prison.
When asked by the
Houston Chronicle
for his reaction to Young’s arrest, Josiah Sutton expressed himself laconically. ‘Let’s just say, if he’s the one who did it, that I don’t think we would be two good people to put in a room together.’
His mother was more philosophical. ‘My son had been pardoned,’ she said, ‘but it still weighed on my heart that no one had been arrested and that some people would not believe in Josiah’s innocence until someone was. Now, justice can be done for the victim, and we can really close the book and say he did not do it.’
District Attorney Rosenthal said, ‘I still don’t know enough to know whether [the victim] was mistaken or not. I intend to look into it, but if Sutton is innocent, I will be the first to say he is.’
To which Bill Thompson commented, ‘Chuck, you’re a little late.’*
SPACE SCIENCE: Off Target
ON SEPTEMBER 23, 1999, after a journey of 419 million miles, the Mars Climate Orbiter spacecraft made its final approach to the Red Planet. At one minute after two in the morning, a tired but excited group of engineers and scientists at NASA’s Jet Propulsion Laboratory (JPL) near Pasadena, California, broke into smiles and applause: a signal had arrived to indicate that the spacecraft’s main engine had begun firing. This event would reduce the spacecraft’s speed enough for it to be captured by Mars’ gravitational field and go into orbit. Shortly thereafter, as expected, signals from the spacecraft ceased as it passed into the radio shadow behind the planet. The JPL group waited impatiently for the spacecraft to re-emerge on the other side of the planet, an event that was predicted to occur 21 minutes later. But the silence stretched to 22, 23 and 24 minutes, and then to hours and days. In fact, no signal was ever received from the spacecraft again. The Mars Climate Orbiter was lost, and the mission was a total failure.
Losing a Mars mission was not exactly a new experience for NASA: three out of the 11 previous US missions had ended the same way. Just six years before the Mars Climate Orbiter mishap, the $800 million Mars Observer spacecraft was lost in rather similar circumstances: radio signals from the spacecraft mysteriously ceased during final approach.
Still, the US Mars programme overall had a good track record, especially in comparison with the Soviet programme. Of the 18 Russian spacecraft sent to Mars before 1999, 15 had been total losses – often failing to reach space at all – and the remaining three were only partial successes. And of the successful earlier American missions, some had been extraordinarily complex. These included the
Viking
mission of 1975 – a fleet of two orbiters and two landers, all of which functioned as planned and for far longer than their nominal design lifetimes. By the time of the Mars Climate Orbiter launch, there was a real confidence that NASA and its industrial partners – Lockheed Martin Astronautics, in this case – knew how to get the job done. It may have been this very confidence that sank the mission.
The root cause of the loss was a scientific blunder as old as science itself: the confusion of units. Such errors can be prevented. Yet, in a larger sense, the loss represented a failure in systems engineering – that is, a failure successfully to integrate thousands of individual technical contributions into a single cohesive whole: a product that would fulfil the objectives of the customer, the US government. In that sense, the Mars Climate Orbiter mishap represented what is probably the most common mode of failure in large and complex scientific enterprises, and one that is extremely difficult to eradicate.
In the early 1990s, NASA administrator Dan Goldin spearheaded a new approach to the design and implementation of space missions, an approach that was encapsulated in the slogan ‘Faster, Better, Cheaper’, or FBC. The FBC philosophy was, in part, a response to fiscal belt-tightening imposed by the US government. It also represented a reaction to some of the earlier missions – huge, long-delayed projects that incorporated every imaginable bell and whistle and that went tens or hundreds of millions of dollars over budget. FBC was a leaner approach that aimed to achieve more with less, employing economical strategies such as the re-use of design elements that had proven successful in earlier missions. Although this ‘heritage’ approach promised great savings in time and money, it also injected risk. How could one be sure that a large piece of software, for example, would function successfully in the different environment of a new spacecraft? And the FBC approach also demanded economies in manpower. This was something that might be acceptable so long as things went according to plan, but it might cause problems when unexpected difficulties needed to be surmounted, as happened with the Mars Climate Orbiter.
Goldin’s strategy had some early successes. The 1996 Mars Pathfinder mission, for example, safely delivered the rover
Sojourner
to the Martian surface using a novel airbag landing system. The rover was able to navigate semi-autonomously around the landing site, and it did some simple geological investigations of nearby rocks. It also caught the imagination of the public, including children, back on Earth: Mattel’s rover action model was America’s best-selling toy of the summer of 1997.
Soon after Pathfinder’s landing, another spacecraft, the Mars Global Surveyor, reached the planet and went into a polar orbit. Over the following 10 years, it took nearly a quarter of a million photographs of the Martian surface, and it also operated as a communications satellite for other missions until it ceased functioning in 2006.
In spite of these successes, there were also hints of problems with the FBC approach. In 1997, for example, an Earth-orbiting satellite called
Lewis
was launched, but once in orbit it went into a spin that prevented its solar panels from facing the sun; this caused the batteries to lose charge. The problem occurred at night while the controllers were off duty; economic considerations had prevented the appointment of sufficient controllers for round-the-clock staffing. By the time the controllers returned to work the next morning, the spacecraft was completely out of electrical power and thus could not be resuscitated: it burned up in the atmosphere a few weeks later.
The Mars Climate Orbiter (MCO) was one element in a two-spacecraft mission named the Mars Surveyor ’98 Programme. The other element was the Mars Polar Lander. The role of the Climate Orbiter was to study the Martian atmosphere with a variety of instruments and also to serve as a communication link for the Lander and for other, future, missions. The Lander was to set down near the planet’s south pole – using retrorockets rather than an airbag – and dig into the soil with the particular aim of finding water.
Lockheed Martin won the $l21-million contract to build both spacecraft (not including the scientific instruments) and the company was expected to complete the job with minimal oversight from NASA, consistent with the Faster, Better, Cheaper philosophy.
The error that doomed the MCO spacecraft occurred during the development of its navigational software. To understand the error, it is necessary to appreciate that once a Mars-bound spacecraft has escaped Earth’s gravitational field, it is essentially coasting in an orbit around the sun – albeit a highly elliptical orbit that is carefully planned to intersect the orbit of Mars at a time when Mars itself has reached that location. Thus, if the spacecraft’s initial direction and speed are well enough known, its future trajectory can be readily calculated from gravitational equations provided by Isaac Newton.
Two nongravitational factors can affect the trajectory, however. One consists of deliberate changes in trajectory induced by firing of the spacecraft’s small rocket engines, or ‘thrusters’. Usually, four or five of these trajectory correction manoeuvres (or TCMs) are performed in the course of the flight. Each time a TCM is performed, its effect on the trajectory has to be determined.
The other main source of non-gravitational effects comes from the pressure of solar radiation on the spacecraft, and from the craft’s efforts to compensate for that pressure. Radiation pressure is exerted mainly on the craft’s solar panels, on account of their large area. Unlike the Mars Global Surveyor, which had solar panels on either side of the spacecraft, the Mars Climate Orbiter had all of its panels on one side of the craft. Because of this asymmetrical design, the main effect of solar radiation was a tendency to spin the spacecraft around on its axis. Such a spin was undesirable because it reduced the amount of sunlight received by the panels. To counteract this effect, a set of small reaction wheels resembling the metal discs in gyroscope toys were automatically spun up by electric motors. These spinning reaction wheels generated an equal and opposite rotational force, so no actual rotation of the spacecraft occurred and it maintained a constant orientation to the sun.
Of course, the reaction wheels could only be spun up to a certain limiting speed, which was about 3,000 rpm. Because of the spacecraft’s asymmetrical design, it took less than 24 hours of flight for the reaction wheels to reach this limit. Then their accumulated angular momentum had to be ‘dumped’. This was done by slowing the wheels, while at the same time compensating for the effect of the slowing by firing some of the thrusters. In this way the reaction wheels could be brought back to a stop while keeping the spacecraft’s attitude constant. Then the cycle began again. These dumps were formally named ‘Angular Momentum Desaturation’ events, or AMDs, and they occurred about 10 times a week during the trip to Mars.
If these thruster firings had only affected the spacecraft’s rotation, they would not have disturbed its trajectory through space and would, therefore, have been irrelevant from a navigational standpoint. Unfortunately, design considerations led to the positioning of the thrusters in such a way that the AMD would also kick the spacecraft sideways by a small amount, and the frequent repetition of these events over the course of the flight would cause the spacecraft to deviate by a significant degree from its planned trajectory – enough to prevent the craft from entering the Mars orbit correctly.
The Lockheed engineers knew about this issue. They also knew that it would be difficult to measure the effect of the AMDs on the spacecraft’s trajectory at the time the AMDs occurred. This was because only limited information would be available about the spacecraft’s position, speed and direction of travel during the flight. In general, it is possible accurately to measure a spacecraft’s distance from Earth (based on the time for a radio signal to travel from Earth to the spacecraft and back) and its speed along the line of sight from Earth (based on Doppler changes in a fixed-frequency signal emitted by the spacecraft). However, it is not possible directly to measure its position or speed in the two dimensions that are perpendicular to the line of sight. These other variables can eventually be determined by making repeated measurements of the spacecraft’s range and line-of-sight velocity as it follows its elliptical trajectory, but these determinations are slow and subject to various forms of error. Unfortunately, the effects of the AMD events were exerted largely in the difficult-to-observe dimensions perpendicular to the line of sight from Earth.
