The vernal equinox is the time when the sun is directly above the Earth’s equator, moving from the southern to the northern hemisphere.

The mean time between two successive vernal equinoxes is called a tropical year–also known as a solar year–and is about 365.2422 days long.

Using a calendar with 365 days every year would result in a loss of 0.2422 days, or almost six hours per year. After 100 years, this calendar would be more than 24 days ahead of the season (tropical year), which is not desirable or accurate. It is desirable to align the calendar with the seasons and to make any difference as insignificant as possible.

By adding a leap year approximately every fourth year, the difference between the calendar and the seasons can be reduced significantly, and the calendar will align with the seasons much more accurately.

In August of 2007, NASA launched The Phoenix Mission to explore the surface of Mars. When it finally lands on Mars on May 25th 2008, it will have traveled around 423 million miles. The University of Arizona is the first public university to lead a NASA mission to Mars. This mission will answer these questions:

(1) can the Martian arctic support life, (2) what is the history of water at the landing site(s), and (3) how is the Martian climate affected by polar dynamics?

Phoenix in the clean room at Lockheed Martin in Denver, CO.

What I find very interesting is just what is involved in the transmission of data for the mission. Read this about the triangulation of frequencies around the earth…

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.

Very nice layout of the mission timeline