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Education: FAQ

If you have a question not listed on the FAQ, please email

Question: How does CloudSat stay in orbit?
Answer: This is most easily answered for a satellite in a near circular orbit, like CloudSat. Basically, the launch vehicle boosts the satellite to the desired altitude and desired velocity, i.e., the circular satellite velocity. Now gravity causes the satellite to fall back toward the center of the earth, but because the satellite's forward velocity is so great, the satellite moves with a combination of motions: one accelerated toward the center of earth and one away from the earth along a straight line. The net result of these two competing effects is that the satellite actually moves along a curved path called the orbit and the orbit takes satellite around the earth reconnecting to itself after a revolution. In some textbooks, it would be said that the centrifugal force just balances the centripetal force to keep the satellite in its circular orbit.

Question: How are satellites launched? Are there different ways for different satellites?
Answer: All man-made satellites must be launched using a rocket. The rocket contains explosive fuel and oxidizer that are burnt to boost the satellite from the ground to a high altitude and to impart to the satellite at burn-out a large horizontal velocity. When the rocket reaches the satellite's desired velocity at the desired altitude, the rocket is shutdown and separates from the satellite. When the altitude and velocity, in particular the circular satellite velocity, are achieved, the satellite will remain in orbit. The rocket then fires little thrusters to back away from the satellite and eventually drops into the earth's atmosphere, where it burns up.

Question: How far above the earth is CloudSat's orbit?
Answer: CloudSat moves in a special orbit called a sun-synchronous orbit which has an equatorial altitude of approximately 705 km. This sun-synchronous orbit is nearly circular and is inclined with respect to the earth's equator at 98.2 degrees.

Question: How do you bring down the satellite?
Answer: Satellites in low earth orbit, like CloudSat, are affected by drag which slowly causes the orbit to decay. Thus, after 20 to 30 years, CloudSat's orbit altitude will be reduced to a point where the satellite will re-enter the dense part of the earth's atmosphere and burn up.

Question: How do the scientists talk to the satellite?
Answer: Commands for the satellite are built on computers. These commands are sent to the satellite using large antennas on the ground, similar to the ones in Canberra, Australia. The satellite receives the commands, executes them, and sends answers (called telemetry) back to the same antennas on the ground. The answers are read on computers by the scientists.

Question: If the satellite becomes broken can it be fixed in space?
Answer: Most satellites cannot be fixed once they have been placed in orbit. Only those satellites that are low enough to be reached by the Space Shuttle can be accessed and fixed by special shuttle mission for just that purpose. CloudSat's orbit is much too high to be reached by the Space Shuttle, so it cannot be fixed if it breaks.

Question: Can a satellite be relaunched?
Answer: Only satellites that are recovered and brought back to earth by the Space Shuttle can be launched again. All others will eventually burn up in the earth's atmosphere when their orbits decay owing to atmospheric drag.

Question: What other countries have scientists on the project?
Answer: We already have scientists from many countries, including the USA, Australia, Canada, Japan, the United Kingdom, Netherlands, France and Germany. Other countries may participate later after we launch.

Question: What are the satellites made of?
Answer: The satellites are primarily made of aluminium sheets with an aluminium honeycomb core.

Question: Who makes the satellites?
Answer: The satellites were or are being built by:

  • CloudSat: JPL, CSA (Comdev and CPI), and Ball Aerospace
  • CALIPSO: CNES (Alcatel Space) and Ball Aerospace
  • Aqua: TRW, NASDA (Mitsubishi Electric Corp), GSFC (Raytheon and Aerojet), JPL (BAE Systems), INPE (Matra Marconi), LaRC (TRW)
  • Parasol, Aura: OCO (Orbiting Carbon Observatory): JPL (Orbital Sciences Corporation and Hamilton Sundstrand)

Question: When and from where will CloudSat be launched? Answer: CloudSat will be launched from the Vandenberg Air Force Base near Lompoc, California. Because of CloudSat orbit inclination being 98.2 degrees, it cannot be launched from Kennedy Space Center in Florida.

Question: What if another member of the A-Train becomes broken?
Answer: If a satellite in the A-train becomes temporarily disabled, the constellation has been designed so that there is plenty of time for the disabled satellite to drift within its control box, even between control boxes, before there is a danger to other satellites. This should be enough time for the ground controllers to fix the disabled satellite. If not, it might be necessary for one of the other A-train satellites to perform an evasive maneuver.

Question: When was the first A-Train satellite launched?
Answer: The first A-Train satellite is called Aqua and it was launched in May of 2003.

Question: When will the last A-Train satellite be launched?
Answer: With the current schedule for launch of satellites, CloudSat and CALIPSO, which are launched together, will be the last of the A-train satellites currently planned. Another satellite called Orbiting-Carbon-Observatory may be launch in 2007 or 2008 and may also join the A-train.

Question: How long is CloudSat supposed to stay in space?
Answer: CloudSat's nominal mission is approximately two years. However, CloudSat has the resources, e.g., propellant and power, to survive for at least another year. After that, CloudSat will execute its end of mission plan which will bring the orbit down and hasten its re-entry into the earth's atmosphere.

Question: Is there a central mission control for A-Train or does each satellite have their own?
Answer: Each of the A-train missions is responsible for controlling its satellite with a mission control center. CloudSat uses an Air Force control facility in Albuquerque, New Mexico. However, there is a central information exchange center, called the Earth Science Mission Operations office, at the Goddard Space Flight Center in Greenbelt, Maryland that acts as a clearing-house for all A-Train coordination and information exchange.

Question: Do A-Train satellites talk to each other?
Answer: No, not directly. Information is exchanged through the Earth Science Mission Operations office at Goddard Space Flight Center.

Question: How long until CloudSat sends data back to earth. Where does the data go when it is sent back to earth?
Answer: When CloudSat images a cloud, it takes on average about 4 hours before the satellite flies over one of the large antennas on the ground mentioned in question 5. The data is collected from the antennas and assembled in Albuqueruqe, New Mexico, USA. Once an entire day's data is assembled, it is sent to Colorado to be turned into computer files which can be read by a scientist. We are doing it this way because the people who run the antennas live in Albuquerque and the people who process the data live in Colorado. Other missions may use antennas and processing centers in different locations.

Question: What kinds of orbits do satellites fly? What kind will CloudSat fly?
Answer: There are three kinds of orbits in which satellites fly. The first kind of orbit is called elliptic where the orbital energy is low enough that the satellite remains captured by earth's gravity. Circular orbits are a special case of elliptical orbits but with a zero eccentricity. The second kind of orbit is the parabolic orbit; a satellite on a parabolic orbit just barely escapes the earth's gravity with just enough energy to do so. The third kind of orbit is called the hyperbolic orbit. The hyperbolic orbit is also an escape orbit, but with more energy than the parabolic orbit. CloudSat moves in an elliptical, but near circular orbit.

Question: Why don't satellites run into each other?
Answer: The simplest answer is because space is BIG! There is a lot of empty space between satellites. But, as the space age advances, more and more satellites are being launched, especially into low earth orbits, where the chances of collision is becoming greater and greater with each passing year.

Question: How will we know when the 16th day is?
Answer: CloudSat's sun-synchronous orbit has the unique property that it repeats its groundtrack every 16-days. This means that after exactly 233 revolutions CloudSat will cross the equator at exactly the same geographic longitude as it did 16-days before. And after crossing the equator, it moves over the same identical groundtrack as it did before. Orbits with repeat groundtracks are useful in that once they are established the revisit the same geographic coordinates time after time.

Question: How long is the fly over time and is it always the same time?
Answer: The maximum overflight time at a ground station, meaning CloudSat flies directly through the zenith at the ground station, is approximately 15 minutes. As CloudSat elevation angle becomes less and less, the overflight time also becomes less and less.

Question: How much does the mission cost?
Answer: Space missions, like CloudSat, usually involve large numbers of people working in many locations: At JPL in California around 40-50 people are working on CloudSat; a similar number are building the spacecraft at Ball Aerospace in Colorado; the Science Team involves more than 40 scientists, including those involved through the Department of Energy's Atmospheric Radiation Measurements Program; on the order of a dozen or so additional people are working on the ground operations system at Kirtland Air Force Base in New Mexico; at Colorado State University there are around 5 people working on the data processing system; in Canada, and there are two industry contractors finishing work on parts of the cloud radar. Missions like this can cost many millions of dollars, but they employ hundreds of people working for many years to make the mission happen.