MESSENGER Mission News: Sun Cuts into MESSENGER's Dance Around the Solar System
30 Oct 2007
(Source: Johns Hopkins University Applied Physics Laboratory)
MESSENGER entered solar conjunction on October 26, when the spacecraft's trajectory moved it behind the Sun and out of clear view from Earth for several weeks. The team has just a limited time left before the Sun's interference with the probe's radio transmission severely limits communication with mission operations at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
"We expect we'll be able to communicate with MESSENGER for about another week or two before we completely lose contact," says APL's Andy Calloway, the mission operations manager.
Although this is the longest solar conjunction of the mission - 47 days - it's not the first. The previous solar conjunction period began October 17, 2006 (just before the first Venus flyby on October 24, 2006), and lasted about a month, including about two weeks with no communications at all. "We learned a lot from that first solar conjunction," Calloway says. "Our planning for this solar conjunction is based, in part, on our experience with that event."
Prior to a deep-space maneuver this past October 17, engineers began shutting down the instruments, with the exception of the Gamma-Ray Spectrometer (GRS) sensor on the Gamma-Ray and Neutron Spectrometer (GRNS) instrument. The GRS has been left on but placed in a "sleep mode" that allows it to maintain a safe temperature without help from mission operators on Earth.
Operators also programmed MESSENGER's onboard computer to go 54 days without receiving a command from Earth. Typically, MESSENGER's autonomy system will put the spacecraft into a safe state if it goes a week without successfully receiving a command from home. This week, the team transmitted a series of commands that will carry MESSENGER through its basic operations until mid-December.
The extra-long upload also includes commands to rotate MESSENGER on roughly a daily basis during the solar conjunction. "The guidance and control team developed an effective strategy to rotate the spacecraft each day to keep the spacecraft's momentum from building up too quickly because of the combined effects of all the natural torques a spacecraft experiences," Calloway says.
MESSENGER's navigation team has asked operations to conduct a series of tracking observations called delta differential one-way ranging or DDOR (pronounced "Delta Door"). These measurements have been used effectively on the project since the first Venus encounter, and they improve spacecraft navigation accuracy in a direction not observable with ranging and Doppler observations alone. The technique uses distant celestial objects known as quasars for reference points. The quasars, along with the separation of two DSN complexes and highly accurate clocks, combine to determine the angular position of the spacecraft in the plane of the sky.
"DDOR is a supplement to the Doppler and ranging data that the navigation team normally uses and helps them shrink their error ellipse so that they know much more precisely where MESSENGER is in space," Calloway says. "We would like to execute trajectory correction maneuver 19 soon after coming out of conjunction on December19, so we have included many DDOR observations so that if the Sun remains quiescent we can use those data to plan and execute the maneuver."
The operations team will also use the conjunction period to test the final encounter sequence for the January 14 Mercury flyby on ground simulators and review contingency plans and simulations so the team is prepared for any outcome at the end of the conjunction period. This is important because the team has three opportunities in December and two opportunities in January to execute the next trajectory correction maneuvers as MESSENGER's final approach to Mercury is fine-tuned.