Rosetta's observations during the third Earth swingby

10 November 2009

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 active instruments and their main science objectives are listed in the table below.

Instruments which are active during the Earth swingby ALICE Ultraviolet imaging spectrometer - Gather Earth aurora data - Measure high energy electrons in magnetosphere - Study lunar atmosphere with stellar occultation MIRO Microwave instrument for detecting continuum emission and obtaining spectra - Look for water in the Moon's spectrum OSIRIS Dual camera instrument for imaging at optical, near infrared and near ultraviolet wavelengths - Gather Earth aurora data - Search for lunar sodium and OH tail, during lunar straylight calibration - Imaging of the Earth and Moon (also for calibration) RPC Set of 5 instruments to study the local plasma environment - Monitor the Earth's plasma environment and perform magnetospheric studies for a period of one week around closest approach VIRTIS Visible and infrared thermal imaging spectrometer - Study Earth's atmosphere (non-LTE emission, and oxygen airglow)

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.

Instrument tests

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).

Timeline of observations

The following table gives the timeline of the main planned imaging and science observations. For a more complete table that includes the observations made for calibration and test purposes, see the related documentation in the right-hand menu.

Observations during Earth swingby Time (UTC) Instrument and observation 9 Nov 20:00 - 16 Nov 13:00 RPC Plasma and magnetospheric studies of the Earth-Moon system for one week around closest approach 11 Nov 22:30 - 12 Nov 22:30 OSIRIS Acquire image sequence of approaching Earth over a 24-hour period 13 Nov

00:45-07:15 OSIRIS Search for aurorae. Earth night side targeted pointing during approach phase (pointings to Atlantic Ocean, New York, Nadir, South Pacific, and Earth's limb) 13 Nov

01:25-07:15 ALICE Earth night side pointing during approach, to gather Earth aurora data 13 Nov

02:00-05:40 VIRTIS Earth night side observations, simultaneously with VIRTIS-M and VIRTIS-H 13 Nov

06:10:00 -06:21:30 VIRTIS Earth night side limb scan for non-LTE emission and oxygen airglow studies 13 Nov

06:35-07:10 VIRTIS Earth day side limb scan for non-LTE emission and oxygen airglow studies 13 Nov

07:15-08:00 ALICE Measurement of high energy electrons while passing through Earth's magnetosphere Earth closest approach at 07:45:40 UTC - 2481 km 13 Nov

08:00-11:10 OSIRIS,

ALICE,

VIRTIS Imaging and calibration, with several Earth pointings (at South Atlantic, Berlin, photometric centre and limb to limb scan) 13 Nov

14:00-14:30 VIRTIS Observations of Earth day side simultaneously by VIRTIS-M and VIRTIS-H 13 Nov

15:25-15:50 ALICE Occultation of ρ Lupi by the Moon: obtain lunar atmosphere absorption spectra Moon closest approach at 15:40 UTC - 233 000 km 13 Nov

15:55-16:25 VIRTIS,

ALICE Moon observations, with VIRTIS boresight on the Moon 13 Nov

17:20-18:20 MIRO Attempt to detect H 2 O on the Moon 13 Nov

18:25-21:56 OSIRIS Search for lunar sodium and OH tail during off-field straylight investigation of the Moon 15 Nov

14:00-18:00 OSIRIS Observation of the Earth-Moon system when both are in the field of view of the WAC

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.