08 Jul 2008

Simulation of the Rosetta fly-by of asteroid Steins. (Click on the image to view a larger animation of the trajectory of Rosetta and the orbit of Steins.)

Launched in March 2004, Rosetta will reach its final destination, Comet 69P/Churyumov-Gerasimenko, in 2014. During its 10-year journey it will pass by two asteroids (2867 Steins and 21 Lutetia) providing opportunities for unique observations of these main-belt asteroids. The first fly-by opportunity arises this year when Rosetta draws near to asteroid Steins. Closest approach will be on 5 September 2008 at 18:37 UT when the asteroid is at a heliocentric distance of 2.14 AU and a geocentric distance of 2.41 AU. At this time, the distance between the spacecraft and the asteroid will be about 800 km.

Asteroid fly-by tests

The fly-by of Steins will be very fast with a relative velocity between the spacecraft and the asteroid of 8.6 km/s. During the fly-by the distance between the spacecraft and the asteroid will change very rapidly – by a factor of ten within 15 minutes before and after the closest approach.

The geometry of the fly-by scenario is such that a flip in the attitude must be performed about 30 minutes before closest approach in order to comply with the thermal constraints of the spacecraft. This complex manoeuvre was successfully tested on 24 March 2008 during a full in-flight simulation of the fly-by. (Further details can be found in Status Report No 96 – Asteroid Fly-by Test.)

Schematic diagram illustrating the manoeuvres during the asteroid fly-by test on 24 March 2008, with time running from left to right and the spacecraft's attitude indicated by its +Z axis (toward representative position of asteroid) and its +X axis (green)

A number of parallel tests were also performed at this time. These allowed the spacecraft controllers to verify the behaviour of the attitude and orbit control system software in asteroid fly-by mode. In addition, the behaviour of the star trackers and the camera with stray light from the Sun, and the behaviour of the lander, under Sun exposure were also examined. During the month of July the spacecraft and payload will undergo a series of tests to ensure that all systems are ready for the fly-by campaign.

Optical navigation campaign

From 4 August to 4 September asteroid Steins will be regularly observed by the Rosetta navigation cameras and the OSIRIS Narrow Angle Camera. These observations, which cover the four weeks immediately preceding closest approach, will provide accurate astrometric measurements which will allow the model of the asteroid orbit to be refined. Currently the orbit is known with an error of ± 100 km; a successful optical navigation campaign will reduce this to ± 2 km perpendicular to the flight direction. This will give Rosetta the possibility to adjust its trajectory and to update its pointing for the fly-by if required.

Observations of Steins during fly-by

Steins is an irregular shaped asteroid, with average diameter of 4.6 km. Visible and infrared observations have classified it as an E-type asteroid. Polarimetric measurements indicate that it has a high albedo (0.45 ± 0.1). Light curves determined from OSIRIS observations in 2006 yielded a rotational period of 6.052 ± 0.007 hrs.

Since Steins is a key scientific target of the Rosetta mission nearly all of the orbiter’s instruments and the ROMAP magnetometer on the lander will be performing scientific measurements. Most scientific observations will be performed in the few hours around closest approach, although some measurements will begin a few days before.

The planned fly-by strategy allows for continuous observation of the asteroid before, during and after closest approach and has been optimised for low closest approach and for a large phase angle coverage (0 to 140 deg). The table gives an overview of some of the planned scientific measurements that will be made during the fly-by.

The journey until now

Since its launch in 2004 Rosetta has been following a trajectory designed to bring it to a rendezvous with comet 69P/ Churyumov-Gerasimenko in 2014. A total of 4 gravity assist manoeuvres are required to get to the comet. Three of these have been completed (see timeline of key events). Following the most recent planetary swing-by on 13 November 2007 Rosetta headed out towards the asteroid belt, located between the orbits of Mars and Jupiter, where it will encounter asteroid Steins.

The key scientific observations of the Rosetta mission take place during relatively short periods (compared to the mission duration). Outside of these observation periods the instruments, and many of the spacecraft subsystems, are switched off or put into a dormant state in order to optimise their lifetime. Checks are performed at regular intervals to ensure that all elements remain in good working condition. A summary of these check is provided in the Rosetta Status Reports (see Status Reports Archive).

Mission timeline - key events

The table below summarises the key events in the 10 year voyage of Rosetta to Comet 69P/Churyumov-Gerasimenko.