MESSENGER ARRIVES, Mercury Orbit Insertion Options

[Astro0]

Less than 20 hours now until the MESSENGER spacecraft fires its engine to enter orbit at Mercury.

Live Webcast March 17th – broadcast starts at 0030 UTC.

Follow the progress: http://messenger.jhuapl.edu/mer_orbit.html.
There will be commentary on the mission, real-time coverage of the maneuver, animation of what the spacecraft is doing, a view of the carrier’s Doppler as they receive it, and live video from MESSENGER Mission Operations.

Remember that the Canberra DSN is providing two-way communication with MESSENGER
We'll be using both DSS43 (70-metre) and DSS34 (34-metre) antennas.

Good luck to the team at Johns Hopkins University Applied Physics Laboratory (APL).

[Phil Stooke]

Twelve hours period, with a major data downlink every other orbit, would simplify operations by having the same DSN coverage each day.

From the mission design page:

"After MESSENGER arrives in the primary science orbit, small forces, such solar radiation pressure – the force exerted by sunlight - slowly change the spacecraft's orbit. Although these small forces have little effect on MESSENGER's 12-hour orbit period, they can increase the spacecraft's minimum altitude, orbit inclination, and latitude of the surface point below MESSENGER's minimum altitude. Left uncorrected, the increase in the spacecraft's minimum altitude would prevent satisfactory completion of certain science goals.

To keep this minimum altitude below 500 kilometers (310 miles), propulsive maneuvers must occur in pairs once every Mercury year - every 88 days. The first maneuver in each pair increases the orbit period to 12 hours, 15 minutes by speeding up the spacecraft at its closest distance from Mercury. Two-and-a-half orbits later a maneuver at the farthest distance from Mercury slows the spacecraft just enough to adjust the orbit period back to 12 hours and return the minimum altitude to 200 kilometers (124 miles). Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these maneuvers is limited to a few days when Mercury is near the same point in its orbit as it was at Mercury orbit insertion."

So the current period might not be exactly 12 hours but that's immaterial, the intention is to keep adjusting it to stay around 12 hours all the time (and the correct altitude etc.).

Phil