Cruising

Curiosity is on its way to Mars. But how do you fly a spaceship at 73,000 mph toward a moving target millions of miles away? It’s no pleasure cruise.

cruise

Mars may be the planet next door, but it’s not nearby. Between Earth’s blue shores and the red dust of Mars yawns an expanse of black nothingness, about 35 million miles wide at the very least. It’s more than a hundred times the distance to the moon. If you tried to cross that span at the speed of a jetliner, you would have to fly at top speed—day and night—for the better part of a decade.

After a thrilling, picture-perfect launch last month, Mars Curiosity is sailing those vast seas of interplanetary space at this very moment. And it’s making the dangerous crossing in a little lifeboat of a ship, barely 14 feet wide.

This part of the mission is known as the cruise phase, but for Curiosity and the teams of people who fly her, it’s no vacation. Here are a few things I’ve learned about the journey.

A Good Ship: The Spacecraft

Curiosity is a rover, of course, not a spaceship. It’s designed to have six wheels planted firmly on solid ground, not navigate in the profound cold and imponderable distances of space. That’s where the mission’s spacecraft comes in. It’s the ship that enables the rover to travel between Earth and Mars.

The Mars Science Laboratory spacecraft consists of:

  • The raindrop-shaped aeroshell, which contains the rover and the descent stage (the ‘sky crane’) nestled inside the heat shield and the backshell køb cialis.
  • The circular cruise stage, which steers the entire spacecraft through space, communicates with Earth, and provides power during the long trip to Mars.
The components of the MSL spacecraft. Credit: NASA/JPL-CalTech

The components of the MSL spacecraft. Credit: NASA/JPL-CalTech

The cruise stage is doughnut-shaped, 14 feet, 9 inches (4.5 meters) wide, weighs 880 pounds (400 kilograms) with 10 heat radiators arranged around the perimeter. The hole of the doughnut sits over a cone holding the parachute on top of the aeroshell. During the roughly eight-month trip, the cruise stage performs the critical tasks of the flight, guided by the computer inside the rover.

The underside of the cruise stage, with its propulsion system for navigation and its radiators for dissipating excess heat. Credit: NASA/JPL-Caltech

The underside of the cruise stage, with its propulsion system for navigation and its radiators for dissipating excess heat. Credit: NASA/JPL-Caltech

The computer constantly monitors the health of the spacecraft and relays the information through a transponder and amplifier in the spacecraft’s descent stage to the cruise stage, which then sends it to Earth via two antennas. The parachute low-gain antenna is on the aeroshell’s parachute cone, which is exposed through the center of the cruise stage. The other, the medium-gain antenna, is mounted on the cruise stage itself. The parachute antenna serves during the early weeks of the cruise to Mars, then again during cruise-stage separation. During most of the voyage, the job switches to the medium-gain antenna, which provides higher data rates but requires more restrictive pointing toward earth. The telecommunications system also provides position and velocity information for navigation, as well carrying data and commands.

The spacecraft does not fire engines to thrust to Mars. Curiosity was flung toward the Red Planet by the power of the Centaur booster in the upper stage of the rocket it rode to space. But the cruise stage does have its own propulsion system. That’s because the flight path will need to be fined tuned during the cruise, and the attitude of the spacecraft relative to earth and the sun affects telecommunications, power and thermal performance. So the cruise propulsion system is used to adjust the spacecraft’s orientation, keep it spin-stabilized at two rotations per minute, and power the trajectory correction maneuvers that will take place along the way. Two clusters of four thruster engines in different orientations enable different directions of thrust. Each of the thrusters can provide about 1.1 pounds (5 newtons) of force. They use hydrazine, which does not require an oxygen source. Two spherical propellant tanks on the cruise stage supply the pressurized propellant.

Workers inspect the Curiosity rover tucked into the spacecraft. Credit: NASA/JPL-Caltech
Engineers inspect the Curiosity rover tucked into the aeroshell section of the spacecraft. The cruise stage is above the aeroshell. Credit: NASA/JPL-Caltech

Space is cold, only a few degrees above absolute zero. So the cruise stage also performs the essential function of controlling the temperature of all spacecraft systems. Fluids circulate through pumps and radiators in the Heat Rejection System and to dissipate the heat generated by solar cells and motors into space. Insulating blankets keep sensitive science instruments warm. Thermostats monitor temperatures and switch heating and cooling systems on or off as needed.

The assembled spacecraft stack: cruise stage at the top, aeroshell with the rover inside, heat shield at the bottom. Credit: NASA/JPL-Caltech
The assembled spacecraft stack, about 9 feet tall: cruise stage at the top, aeroshell with the rover inside, heat shield at the bottom. Credit: NASA/JPL-Caltech

The cruise stage has its own electrical power system, consisting of a solar array on top of the stage. The array has six panels totaling 138 square feet (12.8 square meters) of active photovoltaic area. If operated at full capacity at earth, the array could produce about 2,500 watts. At its farthest from the sun, during the approach to Mars, the array will still produce 1,080 watts or more, even when facing as much as 43 degrees away from the sun. The Radioisotope Thermoelectric Generator on the rover supplements the electricity provided by the cruise solar array.

The MSL spacecraft in flight as of December 3, 2011. Mars still looks like a reddish star to the left. This view was generated by the Eyes on the Solar System site. Credit: NASA/JPL
A computer-generated view of the MSL spacecraft as it appeared in flight as of December 3, 2011. Mars is still far enough away that it just looks like a reddish star to the left. The ship is not facing Mars even though it

A Star to Steer Her By: The Flight

A map of MSL's trajectory, with planned course corrections. Credit: NASA/JPL-Caltech
A schematic map of MSL's trajectory, with planned course corrections. Credit: NASA/JPL-Caltech

The best way to get to Mars is to let Mars come to you. Every two years, the speedy Earth and the slower Red Planet meet up in their orbits, and a window opens for a (relatively) short hop between the two.

Of course, you don’t just point the spacecraft directly at Mars and fire. Mars is a moving target. Instead, Curiosity is actually orbiting the sun during its journey, following a parabolic trajectory that will have it meet up with Mars at the right moment next August. Among other things, this means that the spacecraft will end up crossing not just the direct distance between Earth and Mars at their closest encounter, but more like 352 million miles along a curving flight path.

A video camera on board the upper stage of the rocket captured the moment on November 26 when the spacecraft separated and spun off into space. Click to see video. Credit: NASA TV
A video camera on board the upper stage of the rocket captured the moment on November 26 when the spacecraft separated and spun off into space. Click to see video. Credit: NASA TV

Getting underway to Mars started a few minutes after Curiosity launched aboard an Atlas V rocket. Once in space, the rocket’s upper stage Centaur booster, with the spacecraft attached, fired one last time. It pushed the spacecraft past escape velocity, allowing it to leave Earth forever, moving with enough enough momentum to get all the way to Mars. Soon after, the spacecraft separated from the Centaur. At that point it was intentionally aimed to miss Mars! That’s because the Centaur was itself also moving in the same direction at the same speed. But unlike Curiosity, the booster was not sterilized, and in order to avoid contaminating Mars, the booster needs to miss the Red Planet. So during the cruise the MSL spacecraft will alter its course to aim directly at the planet, while the booster will continue in its original course, drifting off into space.

Just how fast is the spacecraft moving now? Try 73,800 mph (118,700 kph) relative to the sun. That’s more than 30 times the speed of a bullet leaving the muzzle of an M16 rifle. (To compare this to other fast machines, check out this post.)

And how does the ship navigate across such vast distances at such astounding speed? It uses the sun and the stars. The cruise stage monitors the spin rate and the spacecraft’s orientation with a star scanner and one of two sun sensor assemblies. Each of the sun sensor assemblies includes four sun-sensor heads pointing in different directions. Based on star tracking and sun sensing information, the cruise stage uses its thrusters as necessary to maintain the spin rate and attitude.

The position of the MSL spacecraft relative to the Earth/Moon system as of December 3, 2011. This view was generated by the Eyes on the Solar System site. Credit: NASA/JPL
The position of the MSL spacecraft relative to the Earth/Moon system as of December 3, 2011. This view was generated by the Eyes on the Solar System site. Credit: NASA/JPL

Radio helps, too. Mission controllers can measure the distance to the spacecraft by timing precisely how long it takes for a radio signal to travel there and back. The Doppler technique measures the spacecraft’s speed relative to Earth by the amount of shift in the pitch of a radio signal from the craft. A newer method, called delta differential one-way range measurement, adds information about the location of the spacecraft in directions perpendicular to the line of sight. For this method, pairs of antennas on different continents simultaneously receive signals from the spacecraft, and then the same antennas observe natural radio waves from a known celestial reference point, such as a quasar, which serves as a navigation reference point.

So what is the spacecraft, and the team of people on the ground, doing during the 36 weeks this amazing trip will take?

Topping the agenda are:

  • health checks and maintenance of the spacecraft in its cruise configuration
  • monitoring and calibration of the spacecraft and subsystems
  • attitude correction turns (spins to maintain the antenna pointing toward Earth for communications and to keep the solar panels pointed toward the Sun for power)
  • navigation activities, including trajectory correction maneuvers, for determining and correcting the vehicle´s flight path and for training navigators prior to approach
  • preparation for entry, descent, and landing and surface operations, including communication tests used during entry, descent, and landing

If there’s enough power available, one of the mission’s science instruments will actually get a workout during the cruise. The Radiation Assessment Detector will be used to study the radiation environment near Earth and between Earth and Mars.

The position of the MSL spacecraft relative to Mars and the Earth as of December 3, 2011. A long way to go yet. This view was generated by the Eyes on the Solar System site. Credit: NASA/JPL
The position of the MSL spacecraft relative to Mars and the Earth as of December 3, 2011. A long way to go yet--MSL

Thrusters on the cruise stage are fired to adjust the spacecraft’s flight path three times during the cruise phase and additional times as needed during the final approach. The first course correction is planned for 15 days after launch, the second for 120 days after launch. These two are used to remove most of the launch-day trajectory’s intentional offset from Mars. The third trajectory correction maneuver, about two months after the second and 60 days before landing, is the first one to actually target the desired Mars atmospheric entry point. Three additional trajectory maneuvers are scheduled during the approach phase, at eight days before landing, two days before landing and nine hours before landing. The schedule also holds a contingency maneuver if needed at 24 hours before landing.

Coming Up Next: “Seven Minutes of Terror”

The cruise ends when the spacecraft is 45 days from entry into the Martian atmosphere. At that point the approach phase begins, during which managers will be preparing for the most dangerous moments of the entire mission, when the rover has to transition from flying through space at about 13,000 miles per hour to a full, safe stop on the ground—in just seven minutes. They call it the seven minutes of terror.

Of course, the ultimate purpose of the spacecraft is to deliver the Curiosity rover to Mars. When its task is complete, the cruise stage will drop away. Credit: NASA/JPL-Caltech
Of course, the ultimate purpose of the spacecraft is to deliver the Curiosity rover to Mars. When its task is complete, the cruise stage will drop away. Credit: NASA/JPL-Caltech

Arrival at Mars will be Aug. 6, 2012, Universal Time.

In the meantime, a good way to keep track of Curiosity’s position during its journey is to use the Eyes on the Solar System site, where you can follow exactly where the spacecraft is, and see simulated views of what it sees, at any time.

Sources: Much of the information in this post comes directly from the Mars Science Laboratory web site, the mission’s official press kit, and presentations made by mission engineers during the launch Tweetup. See the links in the upper right-hand column on this page to learn more.

Curious About Curiosity?

Pictures and Videos

Pictures taken by attendees of the NASA-sponsored Curiosity launch Tweetup:

Rocket at Sunset

Official launch videos:

Videos

Official mission trailer:

Trailer Video

Video of the Tweetup briefings from top NASA scientists and engineers:

Tweetup video

My unofficial launch teaser video, based on behind-the-scenes NASA footage:

Launch teaser video

Why Go to Mars Anyway?

Other Robotic Missions Now in the Cruise Phase