Mro-launch qxp

Mars' shadow and slip behind Mars from Earth's perspective. Until it goes behind

Mars, the spacecraft will be in communication with Earth during the orbit-insertion burn.

The burn will end while the spacecraft is out of communication, and the orbiter will

reappear and re-establish communications after about 30 suspenseful minutes without

a signal.

If orbit insertion goes exactly as planned, the first orbit will take 35 hours to complete,

as the spacecraft swings out as far as 44,500 kilometers (27,700 miles) from the planet

before completing the circuit with an approach to about 300 kilometers (186 miles)

above Mars' surface. The ground crew will assess the orbiter's health and run a final

rehearsal of aerobraking procedures before beginning aerobraking activities.

Aerobraking

Aerobraking uses friction with the planet's upper atmosphere to slow the orbiting

spacecraft and change the shape of its orbit. The shape of the orbit Mars

Reconnaissance Orbiter should fly for making best use of its science instruments has

been carefully designed. The desired orbit is a near-polar, low-altitude orbit whose

most distant altitude at 320 kilometers (199 miles) is about 25 percent farther from the

planet than its closest point at 255 kilometers (158 miles). Initially, the orbit-insertion

engine burn will put the spacecraft into an orbit with a very different shape, with the

most distant point about 15,000 percent farther from the planet than the closest point.

The mission will use hundreds of carefully calculated aerobraking dips into the upper

atmosphere over the course of about six months to achieve that dramatic change in

shape.

The process uses brief burns from the thrusters while the spacecraft is at the far end of

the ellipse --called the "apoasis." The burns are used only to adjust how deeply the

spacecraft dips into the atmosphere at the "periapsis" end of the ellipse, where the

orbit comes closest to the planet. It is the atmospheric drag on the spacecraft during

the periapsis part of the orbit that does most of the work to decrease the apoapsis alti-

tude. Compared to relying only on rocket engines to shape orbits, aerobraking reduces

the amount of fuel that must be launched from Earth by nearly one-half. This is the

motive for aerobraking, a technique NASA has previously used with success for the

Mars Global Surveyor and Mars Odyssey orbiters. 

Generally, the density of a planetary atmosphere decreases with increasing altitude.

However, Mars' upper atmosphere is quite changeable in how dense it is at any given

altitude. Experience gained from earlier orbiters plus monitoring by this mission's own

Atmospheric Structure Investigation will aid calculations of how low to bring the periap-

sis from day to day. This constant monitoring is necessary because efficient aerobrak-

ing requires a balance between getting enough friction to slow the spacecraft but not

too much heating. Too much friction could overheat the spacecraft. Too little could pre-

vent getting the orbit to the right shape in time to lock it into the desired time-of-day

pattern for the mission's science phase. 

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In its configuration for aerobraking, the orbiter presents a frontal area of 37.5 square

meters (404 square feet) including the back surfaces of the solar panels and high-gain

antenna. The more compact and massive main structure of the spacecraft is out in

front of these larger surfaces in the direction of travel. This configuration gives the

spacecraft a self-righting stability comparable to a badminton shuttlecock. 

Mars Reconnaissance Orbiter will begin aerobraking about a week after arriving at

Mars. The process will start with a "walk-in" phase for about 12 days, then switch to a

main phase of aerobraking lasting up to five-and-a-half months, concluding with a

"walk-out" phase of about five days. 

During walk-in, thruster burns when the spacecraft is farthest from the planet will incre-

mentally lower the other end of the orbit enough for the spacecraft to begin encounter-

ing atmospheric drag. Once the periapsis lowers to the altitude where the optimal

amount of atmospheric drag is found, the main phase of aerobraking will begin to use

the drag to lower the apoapsis. As the orbit shrinks in size, it will also lessen in dura-

tion. At the start of the main phase of aerobraking, each orbit will take more than 30

hours to complete. By the end of the main phase, the spacecraft will be completing an

orbit every two hours. The walk-out phase will raise the periapsis back out of the

atmosphere. This will slow the final process and prevent overheating during long drag

passes as the orbit becomes more circular and the speed at periapsis is significantly

reduced.

After aerobraking has been completed, a few transition activities in September and

October 2006 will get the orbiter ready to begin its primary science phase. These

include three or four thruster-firing maneuvers for fine-tuning the shape of the orbit,

deployment of the antenna for the radar, opening the cover on the imaging spectrome-

ter, and final testing and calibration of scientific instruments.

The orbit and the spacecraft will be ready for the start of the mission's main event, the

primary science phase. But just before that phase begins, the geometry of the Sun,

Earth and Mars will affect communications while Mars passes behind the Sun, from

Earth's perspective. This event, called a "solar conjunction," occurs about every 26

months. Radio signals between Mars and Earth during a conjunction pass too close to

the Sun to be received reliably. For mission-planning purposes, the conjunction is con-

sidered to last as long as Mars and the Sun are less than five degrees apart in Earth's

sky, from Oct. 7, 2006, to Nov. 8, 2006. 

Science Phase

During its primary science phase, from November 2006 to December 2008, Mars

Reconnaissance Orbiter will examine parts of the planet in unprecedented detail and

monitor the entire planet daily throughout a full cycle of Martian seasons. Beginning

right after solar conjunction allows slightly more than one Martian year of observations

without interruption by another conjunction. Mars will be about one-third of the way

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