Tuesday, April 6, 2010




Space exploration missions have been a mixed bag over the years, from shining successes to literal crash-and-burns. What is it that makes a successful mission happen?

Those who work on the projects say that experience, careful testing, mature technology, and building in a little extra over the minimum requirements are key to getting the most out of your mission.

To narrow down the playing field, missions were considered for this article only if they went where humans can't follow and move independently of the Earth and moon -- so Sputnik and the Apollo missions, successful as they were, aren’t in the running.

Missions counted as successful if they completed at least one stage of their primary mission; all of the missions that made this list not only finished their primary mission, but embarked on extension missions as well.

The missions in this article have observed planets, explored the surface of Mars, headed out of the solar system, and smashed into a comet -- on purpose. Here are the top five most successful space exploration missions, and a look into what went on behind the scenes that made them work.


Voyager 1 & 2
Destination: Jupiter and Saturn
Launch: September 5, 1977 (Voyager 1, left), August 20, 1977 (Voyager 2, right)
Arrival: 1979
Original mission duration: 4 years
Current mission duration: 32 years and counting
Image credit: NASA/JPL

The Voyager mission consists of two separate spacecraft, Voyager 1 and Voyager 2, both of which aimed to investigate Jupiter before proceeding on to Saturn.

Voyager 1's short trip past Saturn, only days long, returned "more information than was obtained in the entire previous history of human exploration of this system," wrote B. A. Smith in the 1981 article Encounter with Saturn: Voyager 1 Imaging Science Results, published in Science.

After its encounter with Saturn, Voyager 1 headed out of the solar system in September of 1980. Voyager 2 similarly departed for interstellar space in 1989 after visiting Jupiter, Saturn, Uranus, and Neptune. Today Voyager 1 is the farthest man-made object from Earth; at over 10 billion miles from the sun, it is over 100 times farther from the sun than the Earth is.

A new extension, the Voyager Interstellar Mission, began in 1990. Both Voyager spacecraft are still in contact with Earth and continue to return information, even though it takes more than 12 hours for a signal from Voyager 1 to reach Earth.

Chris P. Jones, assistant director for flight projects and mission success at the Jet Propulsion Laboratory (JPL) in California, was also an engineer on the Voyager mission and later team chief. He attributes much of Voyager’s success to the technological legacy it inherited from the Mariner and Viking programs.

"One of the things that really led to Voyager’s success and its long life was this heritage," said Jones. "Money could be spent on making sure that everything was going to work right rather than trying to develop a new technology."

Though Voyager is still running 32 years after launch, according to Jones the technology was designed with only the original four-year mission in mind.

System elements that might be expected to wear out because they were frequently turned on and off or moved “were qualified for a lifetime comparable to the four-year mission,” said Jones.


Voyager's parts program, Jones explained, produced designs with high reliability and high margins. As far as fuel was concerned, "we ended up filling the tank," Jones said. "We had plenty to last the four years with margin. Nobody was planning on a 30-year mission."

One challenge that explorer spacecraft designers face is uncertainty about the target. Without much data about their destination, it’s difficult to know what to make the spacecraft capable of.


"One of the amazing things," said Jones, is that the technology, including the imaging systems, on board the Voyager spacecraft proved to be equal to recording what they found. "The system that we built was up to the task of discovering these amazing new worlds."


Cassini
Destination: Saturn
Launch: October 15, 1997
Arrival: 2004
Original mission duration: 4 years
Current mission duration: 5 years and counting
Image credit: NASA/JPL, D. Seal

The Cassini-Huygens mission, consisting of the Cassini spacecraft and the Huygens probe, was a collaboration between the Jet Propulsion Lab (JPL), the European Space Agency, and the Italian space agency (Agenzia Spaziale Italiana). Cassini was designed and managed by the JPL.

The goal with Cassini was an intense study of Saturn and its satellites, including Saturn’s moon Titan.

Cassini is a complex design, even by spacecraft standards. The Cassini craft alone carries 12 instruments, and weighs more than both Voyager spacecraft put together.

Over its years in service, the Cassini spacecraft has circled Saturn dozens of times and returned tens of thousands of pictures of the planet and its moons.

Cassini was conceived during a time when space exploration was experiencing a change in design principles. In the 1990s, the concept of faster-better-cheaper surfaced at NASA and its agencies.

"At that time the agency felt that it could tolerate failure now and then if it could launch many more missions," said Jones, who spent seven years as the Cassini spacecraft development manager. "You cut costs, you reduce the amount of testing, you take chances ... all in an effort to make [the missions] cheaper and make the development time shorter."

Faster-better-cheaper proved unsuccessful, according to Jones. Cassini's development and design, culminating in its 1997 launch, embodied a return to the principles that had made Voyager a success 20 years earlier.

"The feeling was, this was too big a mission to treat that way," said Jones.

Even outside of faster-better-cheaper, spacecraft designers still must consider cost as a factor, along with reliability and capability.

One such decision on Cassini revolved around whether to work around a flawed chip in the flight engineering computer or to re-engineer the chip at a cost of about $1 million.

Cassini engineers had to consider not only the direct cost, but also the state of the mission's monetary reserves, Jones explained. Since Cassini's construction had been proceeding smoothly, they spent the money to re-engineer the chip.

The success of Voyager and Cassini affirmed the more careful testing and design process they went through over faster-better-cheaper.

"The great thing about success is that when something does succeed like that, it kind of validates what you did," said Jones. The JPL’s Flight Project Practices and Design Principles, still followed today, "effectively chronicle what was done during the Cassini era to make that system and that mission successful."


Mars Global Surveyor
Destination: Mars
Launch: November 7, 1996
Arrival: 1997
Original mission duration: 2 years
Actual mission duration: 9 years
Image credit: NASA/JPL

Over the course of its two-year primary mission and four additional mission extensions, the Mars Global Surveyor (MGS) returned more than 240,000 pictures of the Martian surface.

One of the key instruments was the Mars Orbiter Camera (MOC).

"The big excitement for the planetary geologist [was] the promise of very high resolution imaging," said Dr. Ray Arvidson, director of the Earth and Planetary Remote Sensing Laboratory at Washington University in St. Louis.

Previous Mars missions had revealed a cratered surface with river channels and lava flows, but their images lacked detail. The MOC was one of the instruments that helped remedy that.

"You could see the fine details," said Arvidson. "We were looking at a surface that was in some places intricately carved."

The original plan called for the MGS to take a small number of high-resolution shots, but the mission extension allowed the MGS to take many more photos.

The larger number of more detailed photos affected mission success for another project: the Mars Exploration Rovers.

Those extra images of more sites “became important for site selection for Spirit and Opportunity,” said Arvidson.

According to Arvidson, having other missions in place -- especially missions that can serve as communications relays -- is important to the success of follow-up missions.

"Extended missions are important not only for science," said Arvidson. "It's critically important to have that infrastructure in place."

The MGS eventually stopped communicating with Earth, and the mission was officially ended in 2007.

"To me it was surprising it lasted so long," Arvidson said. "It was way out of warranty."


Mars Exploration Rovers

Destination: Mars

Launch: June 10, 2003 (Spirit), July 7, 2003 (Opportunity)

Arrival: 2004

Original mission duration: 3 months

Current mission duration: 5 years and counting

Image credit: NASA/JPL, Mars Exploration Rover Mission, Cornell

The Mars Exploration Rovers, individually named Spirit and Opportunity, landed on opposite sides of Mars to begin their 90-day missions in 2004. The rovers are part of the exploration of Martian geology, including the search for water on Mars.

Five years later, the rovers are still running, having lasted longer than any of their creators thought they would. Opportunity is making its way toward a crater called Endeavor, while Spirit is now stationary after becoming mired in the side of a sand dune.

Spirit and Opportunity "require a fair amount of care and feeding," Arvidson said, but they are continuing to make discoveries.

The rovers were designed after the high-profile losses of the Mars Polar Lander and the Mars Climate Orbiter in 1999.

"The feeling was: we have to design very well and test very well," said Arvidson.

The rovers were intended to perform geological observations, and those mission objectives drove the choice of what technology to put on board, Arvidson explained. Different types of cameras for driving and for target identification were put on board, as well as instruments to clean rocks.

The team also wanted to be able to do mineralogy using a spectrometer, which would allow them to identify iron in rocks. Developing a Raman spectrometer to a mature enough level to include on the mission would have cost too much. As a compromise, the rovers were equipped with the more cost-effective and mature Mössbauer spectrometers.

Arvidson reiterated that when the spacecraft designers work, they base their requirements only on the primary mission duration.

"You propose a mission that lasts for some length of time," said Arvidson, "and you build and test for that amount of time plus ... so you've got a pretty good bet that the systems are not going to wear out during the primary missions."

One effect of designing for the primary mission shows up in the rovers' spectrometer function. The spectrometers rely on the radioactive decay of cobalt-57, and the mission has gone on so long that much of the radioactive cobalt has already decayed; it now takes days to make measurements that used to take just hours.

"[The spectrometer] wasn't designed for six years," said Arvidson. "It was designed for a year or so, but it still works."

Deep Impact
Destination: Comet Tempel 1
Launch: January 12, 2005
Arrival: July 4, 2005
Original mission duration: 8 months
Current mission duration: 3 years and counting
Image credit: NASA/JPL, University of Maryland, Pat Rawlings

Deep Impact was launched in 2005 to undertake a more interactive exploration mission than most. The spacecraft carried an impactor, which launched from the spacecraft and collided with the nucleus of Comet Tempel 1. The flyby spacecraft made observations of the resulting crater and spray.

Crashing into the comet to investigate its interior might seem extreme, but according to Dr. Michael A'Hearn, principal investigator on Deep Impact, a less dramatic technique like drilling was not the way to go.

"It wasn't going to get you what you wanted," said A’Hearn. "You can't drill deeply enough."

Information from Deep Impact, it was hoped, would help reveal how comets formed, a process that was largely a mystery.

Deep Impact’s engineers had a challenging task: make sure the impactor hit in a sunlit space where the flyby spacecraft could see it, and make sure the flyby spacecraft was looking in the right place when the impactor hit.

To maximize the potential for a successful mission, the science team tried to make sure that, no matter what happened with the impactor, the spacecraft would collect good data.

That effort, including intensive pre-impact monitoring of the comet, was “part of what enabled us to get a lot of surprising results unrelated to what we had proposed,” said A'Hearn.

After its primary mission was completed in late 2005, Deep Impact was put into hibernation and orbited the sun until December of 2007, when it was dispatched on the Epoxi mission.

Epoxi is a combination two projects: the Extrasolar Planet Observations and Characterization (EPOCh) mission, searching for other planets , and the Deep Impact Extended Investigation (DIXI), looking at another comet.

DIXI is possible, A'Hearn said, because "we consciously put in enough fuel to do an extended mission assuming the prime mission was successful."

The original destination of the extension was Comet Boethin, but in the summer of 2007, the Deep Impact team couldn’t find the comet. The backup target, Comet Hartley 2, required a longer trip, which meant more funding.

"Hartley 2 is a better target than Boethin scientifically," said A’Hearn. Boethin was originally chosen to keep costs down. "Fortunately, NASA's willing to spend the money to operate for two years instead of one year."

The impactor already gone on Tempel 1, DIXI will be restricted to no-contact observation of Hartley 2.

The other half of Epoxi, EPOCh, uses relatively little fuel. Instead, EPOCh's search for extrasolar planets uses on the powerful on-board telescope originally used to look at the impact with Tempel 1 to look for extrasolar planets instead.

Even with its extensions, Deep Impact is a relatively small mission. Its success, though, is "far better than I had originally hoped," said A’Hearn. "I think we have had atremendous scientific return."

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