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Phoenix Mission FAQ

May 22, 2008

Science

What advantages does the Thermal and Evolved Gas Analyzer (TEGA) have over the Viking mission’s Gas Chromatograph in detecting organics? The Phoenix mission has two advantages over the Viking mission with respect to organics. The first is that Phoenix is slowly heating the sample to 1000 C, whereas Viking heated very quickly to 500 C. There are many organics thought to be possibly stable on Mars that vaporize in the 700 C to 800 C range. These types of organics are often call kerogens. The other is the location where Phoenix is landing. The Viking mission showed that water can neutralize the effect of the strong oxidant that is hypothesized to be responsible for destroying organics. It is thought that the ice in the polar regions might also protect the sample. In terms of the ultimate sensitivity, the instruments are comparable. It is the nature of the sample the generates the gas that goes to the mass spectrometer that provides the advantage to Phoenix.

Spacecraft

Why is the Phoenix spacecraft a lander instead of a rover? Despite the success of Mars Pathfinder and the Mars Exploration Rovers, the Phoenix mission will use a lander because it is simply a different type of mission. The rovers were designed to study rocks at different locations, looking for evidence that the liquid water once flowed on the surface of Mars. Unlike the rovers, which were hunting for evidence of water at points along the Martian surface, the Phoenix lander knows exactly where to go to find water. To reach it, however, the spacecraft must dig down below the surface. The Phoenix lander is going to an area of Mars where water is believed to exist in the form of ice just below the surface. This water ice is probably spread fairly uniformly throughout the northern plains so the lander should be able to uncover ice wherever it lands.

How fast will the Phoenix spacecraft travel towards Mars?

The Phoenix spacecraft is traveling at approximately 74,000 mph (120,000 km/h). Another number seen in the media is 14,000 mph (22,500 km/h). The 14,000 mph is the speed Phoenix will be traveling with respect to Mars. As Phoenix approaches Mars next May, Mars will be traveling at about 60,000 mph (96,600 km/h) in its orbit. The resulting difference between the speed of Phoenix and the speed of Mars will be around 14,000 mph (22,500 km/h).

How will the Phoenix spacecraft communicate with engineers on the Earth?

Like all of NASA’s interplanetary missions, Phoenix will rely on the agency’s Deep Space Network to track and communicate with the spacecraft. The network has groups of antennas at three locations: at Goldstone in California’s Mojave Desert; near Madrid, Spain; and near Canberra, Australia. These locations are about one-third of the way around the world from each other so that, whatever time of day it is on Earth, at least one of them will have the spacecraft in view. Each complex is equipped with one antenna 70 meters (230 feet) in diameter, at least two antennas 34 meters (112 feet) in diameter, and smaller antennas. All three complexes communicate directly with the control hub at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Phoenix will communicate directly with Earth using the X-band portion of the radio spectrum (8 to 12 gigahertz) throughout the cruise phase of the mission and for its initial communication after separating from the third stage of the launch vehicle. The cruise stage carries two copies of its communications equipment, providing redundancy in case of a problem with one of them. The mission will use ultra high frequency (UHF) links (300 megahertz to 1,000 megahertz), relayed through Mars orbiters during the entry, descent and landing phase and while operating on the surface of Mars. A UHF antenna on the back shell will transmit for about six minutes between the time the cruise stage is jettisoned and the time the back shell is jettisoned. From then on, a UHF antenna on the lander deck will handle outgoing and incoming communications. The UHF system on Phoenix is compatible with relay capabilities of NASA’s Mars Odyssey and Mars Reconnaissance Orbiter, and with the European Space Agency’s Mars Express. Phoenix communication relays via orbiters will take advantage of the development of an international standard, called the Proximity-1 protocol, for the data transfer. This protocol was developed by the Consultative Committee for Space Data Systems in an international partnership for standardizing techniques used for handling space data. The Phoenix spacecraft’s UHF signal might also be receivable directly via the Green Bank Telescope in West Virginia. Data transmission is most difficult during the critical sequence of entry, descent and landing activities, but communication from the spacecraft is required during this period in order to diagnose any potential problems that may occur. An antenna on the back shell will transmit during entry and descent. Another, on the lander deck, will transmit and receive during the final moments of descent and throughout the surface operations phase of the mission.

What is the actual full size of the Phoenix lander/spacecraft?

The Phoenix lander is about 18 feet (5.5 meters) long with the solar panels deployed. The science deck by itself is about 5 feet (1.5 meters) in diameter. From the ground to the top of the MET mast, the lander measures about 7 feet (2.2 meters) tall.

Do engineers use metric or english system of units to command the Phoenix spacecraft on its trip to Mars?

NASA uses the metric system to design the Trajectory Correction Maneuver (TCM). They specify the desired maneuver in terms of a change in velocity (delta-V) which is in meters/second. The Lockheed Martin spacecraft team is responsible for implementing the maneuver given the performance characteristics of the Phoenix spacecraft. Lockheed Martin also works in the metric system and ensures that all measures and reference frames are consistent. There will always be some error between the ideal maneuver design and the actual implementation of a TCM. Phoenix’s first TCM was on August 10th and the resulting error in total delta-V was less than 1 centimeter/sec for the desired magnitude of approximately 18.5 meters/second. That represents less than 0.054% magnitude error. Five more TCMs are planned to further refine the spacecraft’s trajectory.

What is the total weight of the Phoenix Spacecraft?

The total weight of the Phoenix lander is 772 pounds (350 kg).

What is the fuel type and operation mechanism of the in-flight thrusters and course correctors?

The Phoenix spacecraft uses a mono-propellant hyrdazine system. The hydrazine passes through a catalyst chamber and decomposes exothermically into hyrdogen, nitrogen and ammonia. The propellant is fed to the thrusters by pressure applied above the diaphragms in the tanks.

How big is Phoenix’s parachute?

The parachute on Phoenix measures approximately 39 ft (12 meters) in diameter. In comparison, the Viking landers’ parachutes measured approximately 52.5 ft (16 meters) in diameter.

Landing

Will Phoenix’s descent thrusters alter the composition of its landing site? Altering the chemistry of our landing site due to our thruster exhaust is unavoidable. The Phoenix Lander uses hydrazine, a hypergolic propellant that turns into ammonia during combustion. So essentially, we are spraying the surface with ammonia and a small amount of hydrazine that was not combusted. The way we get around that is by 1) knowing that we are going to be producing ammonia and 2) by designing the wet chemistry cells to carefully quantify the amount of ammonia in the regolith. We then use this information to interpret our other results.

What time will Phoenix land on Mars? What time will the first signal be received from Phoenix?

Phoenix will land at approximately 4:36pm Pacific Daylight Time (7:36pm Eastern Daylight Time). We hope to receive the first signal from the lander approximately 17 minutes later at 4:53pm PDT (7:53pm EDT).

Where will the Phoenix spacecraft land in the Martian arctic?

Phoenix’s landing ellipse is centered at approximately 68 degree N latitude, 233 degrees E longitude. This same location on the Earth is in the Northwest Territories of northern Canada, very close to the Arctic Ocean.

Other How many miles will Phoenix have travelled by the time it gets to Mars?

When Phoenix reaches Mars on May 25, 2008, the spacecraft will have traveled about 423 million miles.




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