Rosetta’s third Earth swingby on 13 November 2009 provides an important opportunity to obtain calibration measurements for the science instruments and to perform science observations of both the Earth and the Moon from a unique vantage point.
Rosetta is currently on its inbound trajectory towards Earth for the mission’s final gravity assist manoeuvre. On 13 November, at about 7:46 UT, the spacecraft will swing by Earth at an altitude of 2481 km.
Approximately 8 hours later, as Rosetta continues on its outbound trajectory it will pass the Moon at a distance of about 233 000 km (the average distance between the Earth and Moon is 384 400 km).
This swingby is crucial to the mission and therefore spacecraft operations take top priority during the gravity assist manoeuvre.
However, several instruments on the Rosetta spacecraft will be active during the swingby. Between 8 and 18 November, when not conflicting with the swingby mission operations, the spacecraft will regularly perform slews to adjust its attitude and allow pointed observations of the Earth and Moon with the Rosetta instrument suite.
Instruments and objectives
The instruments that are active during the Earth swingby will perform both science observations and calibration measurements.
These measurements are used to characterise the performance of the various instruments and to prepare for the science observations scheduled for later in the mission, specifically those planned during the encounter with asteroid Lutetia and, ultimately, with comet 67P/Churyumov-Gerasimenko.
The calibration measurements consist mainly of obtaining reference images and spectra as well as flat field measurements, both for the science observations made during the swingby and for absolute instrument calibration. In addition, two parts of a boresight campaign are scheduled for 8 and 18 November to determine the relative boresights of the four remote sensing instruments: ALICE, MIRO, OSIRIS and VIRTIS.
The four remote sensing instruments (ALICE, MIRO, OSIRIS and VIRTIS) will make pointed observations of the Earth and the Moon. The RPC suite of instruments requires an attitude within a certain solar aspect angle range for its in-situ measurements during the swingby. This attitude will be fulfilled whenever it does not conflict with the requirements for the remote sensing instruments.
In addition to the scientific instruments, the spacecraft’s standard radiation environment monitor (SREM) will be on during the entire swingby. It will be used to determine the spatial distribution of the Earth’s radiation belts along Rosetta’s trajectory as the spacecraft passes the Earth.
Some observations made during the swingby serve to perform specific instrument tests. These include observations with OSIRIS (Moon tracking as a test for the comet observations), RPC (interference test), and with the navigation camera NAVCAM (test of asteroid flyby mode for the Lutetia flyby in July 2010 – verify NAVCAM’s ability to track Lutetia at its maximum angular diameter).
Regular data transfer from Rosetta
During the Earth swingby operations, several ground stations are used to monitor and track the spacecraft. The regular passes over these stations will also be used to receive the data obtained during the observations made by the Rosetta instruments. The majority of the passes are over the ESA ground station in New Norcia, with additional passes over the ESA ground station in Kourou, and two NASA DSN stations: Goldstone and Madrid.