On December 18, 1999, NASA's Earth Observing System (EOS) program launched its flagship satellite-the Terra spacecraft. Terra's mission objective is to measure climate and environmental changes all over the Earth. Using Terra's data, scientists around the world will study our home planet to better understand the causes and effects of climate change, and to distinguish between natural changes and those that are caused by humans. These data will help us to prevent or lessen the impact of natural disasters (such as floods, droughts, severe storms, or volcanic eruptions). Moreover, scientists plan to use the data to develop predictive computer models that will help forecast climate changes years or even decades before they happen.

But Terra will have to function in harsh environments. From the moment of liftoff to the final deployment at an orbital altitude of 705 km (438 miles), the spacecraft was jostled and severely vibrated. Over the course of its six-year (minimum) lifetime, Terra will experience extreme hot and cold temperature shifts and will be continually bombarded by cosmic radiation. These are only a few of the many possibilities that will degrade the performance of the satellite's onboard remote sensors. Such working conditions are nothing new in the life of a spacecraft; but what is new about Terra are the technologies and tactics NASA's scientists and engineers are using to ensure its sensors remain well calibrated throughout its lifetime. Terra's calibration strategy is critical to the success of the mission. At all times, the Terra Team of scientists must know precisely how well the satellite's sensors are performing, and what their margins of error are, before they can know how accurately it is measuring changes on Earth. The Team must take great care to be sure that any changes Terra's sensors measure are actually changes occurring on the Earth, and not on the spacecraft.

One way to periodically check the performance of Terra's sensors is to point them at targets that are scientifically very well understood. The moon and the blackness of deep space provide the perfect targets-both are very stable (they do not change much over time) and their spectral signatures are well known. ("Spectral signature" refers to the unique way an object reflects and emits radiation at different wavelengths of the electromagnetic spectrum.) The moon reflects sunlight in a relatively predictable way, while the temperature of deep space is relatively constant. Repeated measurements of these targets gives the Terra Team a way to compare its sensors' actual measurements with the known spectral signatures of the moon and deep space, and enables a way of determining any on-orbit degradation of instrument optics. Any errors, however slight, in the sensors' measurements will alert the Terra Team that there is a calibration issue and help them figure out how to resolve the issue. But there remains one final problem: Terra's sensors are on its nadir (Earth-facing) side! How can its sensors view the moon and deep space if they are pointing downward at the Earth? The only option is to perform on-orbit maneuvers (rolling, pitching, or yawing the satellite) in a way that allows its sensors to view the desired targets. This is where you come in. Please help NASA's EOS program plan the on-orbit "calibration attitude maneuvers" that are vital to the success of the Terra mission.

Your task is to visualize for NASA the precise timing and mechanics of Terra's on-orbit calibration attitude maneuvers. Specifically, you must communicate to the Terra Team how you would change the "attitude" (or relative orientation) of the spacecraft so that its sensors can view the moon and deep space. In your communication, you must indicate the maneuver(s) to be performed and the time(s) of the maneuver(s), precisely accounting for the relative positions of the Terra spacecraft, the moon, and the sun during the maneuver(s).

You may communicate to the Terra Team in any manner you feel is most effective. For instance, you may construct a three-dimensional model and then photograph or videotape your recommended maneuver(s); you may draw cartoon pictures with captions to make a storyboard; you may communicate textually; or you may come up with your own method for communicating. But there are some basic guidelines to which you must adhere as they are critical to the success of the Terra mission:

• Each maneuver must be performed on the dark side of the Earth and completed before the spacecraft enters into sunlight.
• The nadir side of the spacecraft (the side with the sensor apertures) must never be exposed to direct sunlight.
• The first maneuver can not take place before 71 days after the launch.
• More than one maneuver is allowed to accomplish the lunar and deep space viewing requirements; however, the number of maneuvers should be minimized. (There is an element of risk, every time the spacecraft maneuvers, that it may not be able to properly orient itself again for viewing the Earth, thus failing in its mission.)
• The Terra maneuver(s) which enable(s) the instruments to view the Moon should be restricted to lunar views in which the lunar phase (i.e. Sun-Moon-Earth angle) is between -4 and -90 degrees or between +4 and +90 degrees (see diagram).

Here is some additional background information that may help you develop your solution:

• Terra is in a sun-synchronous, near-polar orbit, descending southward across the equator at 10:30 a.m. local time on every overpass of Earth's daylight side.
• One complete orbit takes about 99 minutes. The spacecraft is in sunlight for roughly 64 minutes of an orbit, and in darkness for approximately 35 minutes.
• In order for Terra's sensors to continually view the Earth, the spacecraft "pitches" at a constant rate to keep the nadir side facing down toward our planet.
• Each Terra sensor has slightly different maneuver requirements, as follows:
  • ASTER needs to view the moon at nadir;
  • CERES must view deep space for 20 minutes. Neither the moon nor the Earth should be in the instruments' field of view during this time;
  • MISR needs to view the moon in its foremost, aftmost, and nadir cameras;
  • MODIS must view deep space neither the moon nor the Earth should be in the instrument's field of view during this time; and
  • MOPITT has no desire to view either the moon or deep space.

Qualifying Round Rules