Comment: In space, timing is everything

by Ryan Anderson
Friday, July 21, 2017

Ryan Anderson stands beside a full-scale model of Curiosity. Credit: courtesy of Ryan Anderson.

T-minus five…four…three…two…one…liftoff!

It’s fitting that missions to explore our solar system begin with all eyes on a countdown clock, because timing is everything in space exploration. From idea to launch, a planetary mission is the result of years — or even decades — of planning and work, with many milestones and deadlines along the way. But a mission’s success hinges on key events that happen in fractions of a second.

Sending a robot to visit another planet is no easy feat. The spacecraft itself must be designed, and instruments must be developed, tested and integrated into a working probe, which is then tested further to ensure that it will survive its long journey through space and arrive in working order. Of course, before any of that can begin, there is the lengthy process of getting funding for the mission — often the most difficult part.

After the years-long process of designing, building and testing a spacecraft and its tools, the timing of its launch is key. Going to Jupiter or Mercury isn’t like going to the convenience store down the block: You can’t just decide to go anytime you feel like it. Like racers on an inside track, planets closer to the sun go around it faster than those farther out, meaning the distances between Earth and the other planets (and the amount of fuel required to get to them) are constantly changing. The more fuel you have to burn to get where you are going, the less spacecraft mass you have to work with when you get there, so you want to take the most fuel-efficient path to deliver the largest possible spacecraft. That means waiting for the geometry to be just right so that the rocket gets a boost from Earth’s orbital velocity and rotation. For many missions (basically anywhere other than the moon and Mars), even that boost isn’t enough, so the spacecraft has to take a more convoluted path, slingshotting around other planets to steal some of their momentum (or, in the case of missions to Venus and Mercury, giving away some of the spacecraft’s momentum so that it falls toward the sun and the planet of interest).

The short period of time when a rocket can launch and have a chance of delivering its payload to the destination is called the “launch window.” For missions to Mars, no slingshot maneuvers are used, and the launch window is related to how often Earth laps Mars in their race around the sun. As Earth overtakes Mars, the planets are closer together and Earth’s orbital velocity works in our favor, so it is easier to launch something between them. The timing of the orbits means the launch window for Mars missions opens roughly every 26 months. This is why, when missions like the Curiosity rover or the InSight lander have run into development delays, the teams responsible haven’t been able to simply wait a few months and then launch. Instead, the missions have been grounded for two years until the next launch window opened up.

Launch windows for missions to more distant planets have more complicated cadences because multiple planets are involved in the trajectories. Some of the trajectories become extremely complex in the interest of efficiency. For example, the Messenger mission took more than six years to get into Mercury’s orbit — in the process, the spacecraft flew by Earth once, Venus twice and Mercury three times. The European Rosetta mission took 10 years to reach the comet 67P, flying by Earth, then Mars, then Earth two more times before finally arriving in orbit around the comet. The Voyager missions to the outer solar system had an easier time since they didn’t need to end up in orbit, but they used careful maneuvers to swing by multiple planets.

Precise timing is not just needed for spacecraft launches. Even the simplest trajectories require a spacecraft to fire its rockets at precisely the right times to adjust its course; and for complex multiplanet journeys, the precise amount of thrust has to be applied at precisely the right time again and again over many years.

Missions that land on other bodies — whether planets, moons or asteroids — have a whole other set of critically timed elements. A spacecraft must decelerate from the extremely high velocity that got it to the planetary body to a pace that allows it to safely settle on the surface. This can mean not just firing retro rockets at the right time, but also getting oriented correctly so the spacecraft doesn’t burn up in the atmosphere, as well as deploying parachutes, deploying landing gear or wheels, and activating any other descent mechanisms such as airbags or “sky cranes.” And because of the time it takes for signals to get back to Earth, the precisely choreographed events involved in landing on other planets are programmed to occur automatically. This is why, for the Curiosity rover landing, mission scientists lovingly called the descent process “seven minutes of terror.”

Even the day-to-day operation of spacecraft is all about timing. Orbiters zipping around a planet have to snap pictures at just the right time, and in the case of missions like Cassini, precision use of gravity assists, and judicious rocket firings, are required to do things like skim ridiculously close to the south pole of Enceladus or sample the upper atmosphere of Titan. With the Mars rovers, we must decide whether a picture should be taken at midday — when color observations are most accurate — or in the morning or evening when longer shadows better highlight ground textures. Of course, there’s only so much time in the day or in the orbit, so deciding how the spacecraft spends its time always involves a complex give and take among a variety of considerations.

All of this happens in the context of long-term mission goals. Planetary missions must achieve all their goals before the clock runs out on their primary mission. Beyond the primary mission, there’s no guarantee that the spacecraft will keep working. In most cases, good engineering and careful management of expendable resources mean that missions are extended beyond their designed lifetimes. Eventually, however, all missions run out of time, whether from wear and tear, dust on solar panels, decay of a nuclear power source or a dwindling supply of fuel. But while a spacecraft might stop functioning, the data that it has returned to Earth are archived, preserved and studied for decades: The culmination of all that careful and time-critical work is a timeless resource for the future.

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