Like everything they build, engineers at NASA's Jet Propulsion Laboratory (JPL) designed Curiosity's Sky Crane landing system to work. But nothing is guaranteed in spaceflight.
The team wouldn't know for sure whether the mission's entry, descent, and landing (or EDL) was successful until they got confirmation from the rover. The problem was that Curiosity's landing site in Gale Crater would be out of range at touchdown, so the team brought in a communications relay: the Mars Odyssey orbiter.
It was a simple and obvious solution, except that Odyssey experienced its first ever malfunctions weeks before Curiosity's landing.
After launching on April 7, 2001, NASA's Mars Odyssey orbiter spent nearly eight months cruising toward the red planet before arriving on October 21. Using a method known as aerobraking — dipping into the Martian atmosphere a little bit at a time to lose speed — it took Odyssey an additional three months to enter into a proper orbit. Once settled, the spacecraft began its primary mission of making detailed maps of our cosmic neighbour.
Odyssey was designed to study and globally map the elemental composition of the planet's surface: determining mineralogy, abundance of hydrogen in the shallow subsurface areas, provide information on the structure and geological processes that have shaped the Martian surface, and return data on the planet's radiation environment. All these detailed measurements were done in the name of determining what health risks the environment of Mars might pose to future human explorers. The information also helped scientists select the most interesting landing sites for future missions, like Curiosity.
Odyssey As A Relay Point
But imaging Mars isn't all Odyssey can do. The orbiter also serves as a communications relay point between Earth and missions on the Martian surface. It's unique in its ability to relay data instantly as well as store information for later download.
Odyssey ended its primary mission and began its extended mission on August 24, 2004, and it's still going strong. It's been an indispensable communications relay between NASA and the Spirit and Opportunity rovers. Odyssey is responsible for transmitting over 95% of the data from the rovers back to Earth; it's still a vital relay between NASA and the latter rover that's now on its own very extended mission. Odyssey was also a communications relay for the Mars Phoenix Lander that reached Mars in 2008.
With its stellar record as a reliable relay spacecraft, NASA planned on using Odyssey and it's capacity for immediate downlinks to follow Curiosity to the surface. There are other orbiters that would be watching the rover's landing — NASA's Mars Reconnaissance Orbiter and the European Space Agency's Mars Express — but neither could receive a signal from Curiosity and send it straight to Earth. There would be an hours-long wait time to collect data from these spacecraft. Odyssey was the best bet for the Curiosity team to get immediate news on Curiosity's fate — allowing, of course, for the 14-minute communications delay between Mars and Earth.
Communicating directly between Earth and Mars is hard, requiring large amounts of power since a radio signals has to penetrate the Martian atmosphere, traverse millions of miles of deep space, and then reach a ground station through the Earth's atmosphere. Curiosity has limited power and direct signals received on Earth are weak, noisy, and come at a low delivery rate. Using Odyssey as a relay point makes things a lot easier. The orbiter is much closer, so if the rover can see the orbiter and the orbiter can see the Earth, real-time communications — allowing for the light time delay, of course — are possible.
Getting Odyssey into the right orbit so that it would fly over Curiosity's landing site at touchdown wasn't, as you'd imagine, a simple move. It took months and short controlled burns of the spacecraft's thrusters to slowly change its orbital trajectory. But that's only half the battle. Odyssey would have to be in the right orientation or attitude in orbit to receive a signal from Mars and send it to Earth.
To achieve and maintain a specific orientation relative to the Earth, Mars and the Sun, Odyssey uses three reaction wheels, which are positioned at right angles to each other inside the spacecraft. They are spun-up or spun-down to generate a gyroscopic force that affects the spacecraft's body, and spinning the wheels a certain way will turn the spacecraft. The reaction wheels run off electricity harvested from Odyssey's solar panels, meaning the limited amount of fuel on board the orbiter isn't wasted on orientation.
If Odyssey loses its orientation, it can lose its ability to relay data between Mars and Earth if its antennae end up pointing in the wrong direction. The solar panels could turn away from the sun, drawing on the spacecraft's batteries and draining its power quickly. Both of these issues, if unadjusted, could jeopardize the spacecraft.
Not So Safe Mode
Odyssey has performed well on its now 11-year mission. But on June 8, the spacecraft put itself unexpectedly into safe mode. Safe mode is a low activity state Earth facing state, a default position the spacecraft assumes when it detects a problem on board. It basically gets ready to hear from home about to fix the problem.
Scientists found that Odyssey detected an unexpected movement in one of its three reaction wheels; one had stuck for a few minutes. The problem was isolated, limited to that one wheel, so Odyssey didn't reboot its computer entirely. Scientists were also to start addressing the problem right away.
Luckily, Odyssey carries a spare reaction wheel. This fourth wheel is set at a skewed angle to all three primary wheels, allowing it to replace any one of the three should one fail. But the spare had been in non-operational storage for more than 11 years, it hadn't moved since before Odyssey's April 7, 2001 launch. Scientists would have to carefully try to bring that wheel online if they were going to have a real time relay for Curiosity.
There's always some risk is using a spare part, particularly one that's been dormant in a spacecraft for ten years since launch. But the risks associated with not fixing the wheel were greater. The lasting damage to Odyssey would have meant losing it was an orbiter and vital relay point for both rovers Opportunity and Curiosity. Not to mention NASA's next mission to Mars, InSight, will possibly need the orbiter as a relay when it reaches the red planet in 2016.
On Wednesday June 12 NASA cautiously tested the spare wheel, spinning it up to 5,000 rotations per minute forward and backward. It was a way of shaking the metaphorical dust off the spare part. It worked, and by June 19 Odyssey was out of safe mode and back to full service on June 27.
Then on July 13, Odyssey put itself back in safe mode for 21 hours. The glitch came after the shortest burn deigned to trim Odyssey's orbit, just 1.5 seconds long. Apparently, this small manoeuvre put an unexpectedly high demand on one reaction wheel. Again, mission scientists worked the problem, brought the spacecraft back to full health, and continued adjusting its orbit. A six-second burn of Odyssey's thrusters on Tuesday July 24 was the final push to get Odyssey lined up to talk to Curiosity and NASA during the rover's landing.
It was that last burn that made the necessary adjustments to Odyssey's orbit and allowed it to come up over Curiosity's landing site right in time for landing. It might seem like a last minute arrival, but that's the nature of coordinating orbiting vehicles.
A Happy Landing
Last Sunday night in mission control at JPL, the entry descent and landing team huddled over their consoles waiting for word from Odyssey that Curiosity had landed. When the signal came, the room erupted in cheers and the mood at JPL changed from tense to exuberant. Had the Odyssey team been unable to fix the spacecraft and confirm its orbit, the Curiosity team wouldn't have had that moment of celebration after hours of tense waiting knowing Curiosity was on the surface but not in what shape. Odyssey played a big role in a lot of people's celebrations over Curiosity's success.
Amy Shira Teitel is a space historian with a lifelong passion for the space race in all its technological, political and human glory. A space writer and blogger, all her work can be found on her website, www.amyshirateitel.com.