ESA Science & Technology - Publication Archive

Context: In 2004 asteroid (2867) Steins has been selected as a flyby target for the Rosetta mission. Determination of its spin period and the orientation of its rotation axis are essential for optimization of the flyby planning.

Aims: Measurement of the rotation period and light curve of asteroid (2867) Steins at a phase angle larger than achievable from ground based observations, providing a high quality data set to contribute to the determination of the orientation of the spin axis and of the pole direction.

Methods: On March 11, 2006, asteroid (2867) Steins was observed continuously for 24 h with the scientific camera system OSIRIS onboard Rosetta. The phase angle was 41.7 degrees, larger than the maximum phase angle of 30 degrees when Steins is observed from Earth. A total of 238 images, covering four rotation periods without interruption, were acquired.

Results: The light curve of (2867) Steins is double peaked with an amplitude of 0.23 mag. The rotation period is 6.052 +- 0.007 h. The continuous observations over four rotation periods exclude the possibility of period ambiguities. There is no indication of deviation from a principal axis rotation state. Assuming a slope parameter of G = 0.15, the absolute visual magnitude of Steins is 13.05 +/- 0.03.

ESA's Rosetta spacecraft was pointed towards the Deep Impact target comet Tempel 1 from 28 June to 14 July 2005. The OSIRIS cameras, the wide angle camera (WAC) and narrow angle camera (NAC) continuously observed the comet with high time resolution (down to less than a minute) around the impact on 4 July. The filter sets of the WAC were designed to observe the gas coma emissions whereas the NAC made broadband and panchromatic observations. The scale of the NAC at the comet was 1500 km per pixel. A strong increase of intensity (by almost a factor seven) was observed within a radius of one pixel centred on the nucleus position, followed by a short levelling, and a slow decrease back to the value before the impact. The WAC observed an increase in OH and CN. From this unique set of observations the total amount of excavated dust and water ice can be deduced and will be discussed.

The nucleus of Comet 81P/Wild 2 is modeled by assuming various smooth triaxial ellipsoidal or irregular body shapes, having different rotational periods, spin axis orientations, and thermophysical properties. For these model nuclei, a large number of surface activity patterns (e.g., maps of active and inactive areas) are studied, and in each case the resulting water production rate and non-gravitational force vector versus time are calculated. By requiring that the model nuclei simultaneously reproduce certain properties of the empirical water production curve and non-gravitational changes of the orbit (focusing on the orbital period and the longitude of perihelion), constraints are placed on several properties of the nucleus. The simulations suggest that the nucleus bulk density of Comet 81P/Wild 2 is low, rho_bulk <= 600 kg/m^3, and that the nucleus rotation is prograde rather than retrograde. The active area fraction is difficult to constrain, but at most 60% of the nucleus is likely to have near-surface ice.

Comets spend most of their life in a low-temperature environment far from the Sun. They are therefore relatively unprocessed and maintain information about the formation conditions of the planetary system, but the structure and composition of their nuclei are poorly understood. Although in situ and remote measurements have derived the global properties of some cometary nuclei, little is known about their interiors. The Deep Impact mission shot a projectile into comet 9P/Tempel 1 in order to investigate its interior. Here we report the water vapour content (1.5x10³² water molecules or 4.5x 10⁶ kg) and the cross-section of the dust (330km² assuming an albedo of 0.1) created by the impact. The corresponding dust/ice mass ratio is probably larger than one, suggesting that comets are 'icy dirtballs' rather than 'dirty snowballs' as commonly believed. High dust velocities (between 110ms^-1 and 300ms^-1) imply acceleration in the comet's coma, probably by water molecules sublimated by solar radiation. We did not find evidence of enhanced activity of 9P/Tempel 1 in the days after the impact, suggesting that in general impacts of meteoroids are not the cause of cometary outbursts.

Rosetta, the first planetary cornerstone mission of the ESA Scientific Programme, was launched on 2 March 2004 on its ten year journey to rendezvous with comet 67P/Churyumov-Gerasimenko. In summer 2014, Rosetta will go into orbit around the comet's nucleus, approaching to within a few kilometres of its surface, will deliver a Lander called 'Philae' onto its surface to make in-situ measurements, and will then accompany the comet on its onward journey for about 1.5 years. The launch and the first 1.5 years of flight operations have been very smooth, with the spacecraft, its payload and the ground segment performing almost perfectly, with no major anomalies and all parameters well within specification. All planned mission activities have gone according to schedule, and additional 'bonus' scientific and technological operations were even added to the intense operations schedule of the first few months. Among the mission events to date were the observations of the NASA Deep Impact probe's encounter in July 2005 with comet 9P/Tempel-1, from a 'privileged' position in space just 80 million kilometres away.

By considering model comet nuclei with a wide range of sizes, prolate ellipsoidal shapes, spin axis orientations, and surface activity patterns, constraints have been placed on the nucleus properties of the primary Rosetta target, Comet 67P/Churyumov-Gerasimenko. This is done by requiring that the model bodies simultaneously reproduce the empirical nucleus rotational lightcurve, the water production rate as function of time, and non-gravitational changes (per apparition) of the orbital period (Delta P), longitude of perihelion (Delta omega tilde), and longitude of the ascending node (Delta Omega). Two different thermophysical models are used in order to calculate the water production rate and non-gravitational force vector due to nucleus outgassing of the model objects. By requiring that the nominal water production rate measurements are reproduced as well as possible, we find that the semi--major axis of the nucleus is close to 2.5 km, the nucleus axis ratio is approximately 1.4, while the spin axis argument is either 60+/-15 or 240+/-15 degrees. The spin axis obliquity can only be preliminary constrained, indicating retrograde rotation for the first argument value, and prograde rotation for the second suggested spin axis argument. A nucleus bulk density in the range 100-370 kg/m^3 is found for the nominal Delta P, while an upper limit of 500 kg/m^3 can be placed if the uncertainty in Delta P is considered. Both considered thermophysical models yield the same spin axis, size, shape, and density estimates. Alternatively, if calculated water production rates within an envelope around the measured data are considered, it is no longer possible to constrain the size, shape, and spin axis orientation of the nucleus, but an upper limit on the nucleus bulk density of 600 kg/m^3 is suggested.

The Solar Wind Anisotropies (SWAN) instrument is a scanning Lyman-alpha imager on board the Solar and Heliospheric Observatory (SOHO) spacecraft. Since becoming operational in January 1996 SWAN has been producing full sky Lyman-alpha maps which are primarily used to study the interaction between solar wind and the interplanetary neutral hydrogen. In addition to that SWAN images can be used to study the hydrogen coma of comets down to about a visual magnitude of 12. After the retargeting decision of the Rosetta mission the SWAN archive was checked for possible occurrences of 67P/Churyumov-Gerasimenko. Five values were obtained for the 1996 apparition but none for the 2002 apparition because of degraded instrument sensitivity and larger observing distance. The observations suggest a perihelion water production rate of about 8x10^27 per second and possible post perihelion increase of activity.

In November 1993, the International Rosetta Mission was approved as a Cornerstone Mission in ESA's Horizons 2000 Science Programme. Since then, scientists and engineers from all over Europe and the United States have been combining their talents to build an orbiter and a lander for this unique expedition to unravel the secrets of a mysterious mini ice world – a comet.

Table of contents:

• Rosetta: Europe's comet chaser
• Why 'Rosetta'
• Life and survival in deep space
• The cosmic billiard ball
• The long trek
• Rendezvous with a comet
• Debris of the Solar System: asteroids
• Debris of the Solar System: Comet 67P/Churyumov-Gerasimenko
• The Rosetta orbiter
• Science from the orbiter
• The Rosetta lander
• Long-distance communication
• Rosetta overview

Note: a more recent Rosetta mission brochure (ESA BR-321) is available here.

Rosetta is an ESA cornerstone science mission to study, in situ, the environment of cometary nuclei and their evolution in the inner solar system. The main scientific objectives of the mission are to investigate the origin of the solar system by studying the origins of comets and to study the relationship between cometary and interstellar material. To enhance the scientific capabilities of the mission, the orbiter spacecraft will carry one probe, a lander which will land on the comet surface of the comet and perform investigations in situ. The Rosetta orbiter spacecraft will be launched in 2003 and, after a 9-year cruise, will begin the cometary close observation phase. By 2012 the in situ investigations will be complete. The lander is being developed by combined effort in Germany, Italy, France, the United Kingdom, Hungary, Finland and Austria.

Direct evidence of the constitution of cometary volatiles is particularly difficult to obtain, as the constituents observable from Earth and even during the flybys of Comet Halley in 1986, result from physico-chemical processes such as sublimation and interactions with solar radiation and the solar wind. What we know today about cometary material from those earlier missions and ground-based observations does, however, demonstrate the low degree of evolution of cometary material and hence its tremendous potential for providing us with unique information about the make up and early evolution of the solar nebula.

Definition of the Rosetta ground segment began in 1996 and it soon became clear that a common checkout and mission-control system would be very beneficial for the mission. The chosen approach for achieving this goal was to develop building blocks for the Central Checkout System that can be re-utilised later in the development of the Flight Control System. The Rosetta prime contractor and AIV contractor fully endorsed this approach and the complete system is currently under development. The first delivery of the database system should take place in November 1998, followed by that of the first Central Checkout System in 1999.

The Rosetta mission is designed to study in-situ a cometary nucleus' environment and its evolution in the inner Solar System. To be launched in January 2003 by an Ariane-5, Rosetta will rendezvous with Comet P/Wirtanen in 2011, after one Mars- and two Earth-gravity assists, and two asteroid fly-bys. The near-comet operations, which are scheduled to last about 1.5 years, will require a minimum return-link telemetry data rate of 5 kbit/s to meet the scientific goals, with about 14 hours of daily coverage.

Few enterprises are more difficult or hazardous than space travel. Yet, even when compared with the achievements of its illustrious predecessors, ESA's Rosetta mission to orbit Comet Wirtanen and deploy a lander on its pristine surface must be regarded as one of the most challenging ventures ever undertaken in more than four decades of space exploration.

The International Rosetta Mission was approved as a Cornerstone mission within ESA's Horizon 2000 Science Programme in November 1993. Even at this early stage, it was envisaged that the ambitious mission would be scheduled for launch in the 2003 timeframe and a number of comet rendezvous opportunities were identified. Although the original target, Comet Schwassman Wachmann 3, has since been superseded by another periodic intruder into the inner Solar System, Comet Wirtanen, there has been little shift in the original launch schedule.

On the night of 12-13 January 2003, one of the most powerful rockets in the world will blast off from Kourou spaceport in French Guiana. On top of the giant Ariane-4, cocooned inside a protective fairing, will be the Rosetta comet chaser, the most ambitious scientific spacecraft ever built in Europe.

In November 1993, the International Rosetta Mission was approved as a Cornerstone Mission within ESA's Horizons 2000 science programme. Since then, scientists and engineers from all over Europe and the United States have been combining their talents to build an orbiter and a lander for this unique expedition to unravel the secrets of a mysterious mini ice world - a comet.

Table of contents:

• Rosetta: Europe's comet chaser
• Life and survival in deep space
• The cosmic billiard ball
• The long trek
• Rendezvous with a comet
• Debris of the Solar System: asteroids Otawara and Siwa
• Debris of the Solar System: comet 46P/Wirtanen
• The Rosetta Orbiter
• Science from the Orbiter
• The Rosetta Lander
• Long-distance communication
• Rosetta overview

Note: a more recent Rosetta mission brochure (ESA BR-321) is available here.

The Rosetta spacecraft will fly-by a few asteroids during its course to the final cometary target. The candidate asteroids presently are 3840 Ministrobel (S-type), 2703 Siwa and 140 (C-type).With the limited data presently available on these bodies we calculated some approximate quantities which may be useful to select the fly-by trajectories of the ROSETTA probe. In particular we derived the zones in which particles could stably orbit by analyzing Hills problem of three hierarchical masses--the sun, the asteroid and the orbiting particle. Then, following the approach of Hamilton and Burns, the effects of solar radiation pressure and of the ellipticity of the orbits were also taken into account. In this way for each asteroid we could calculate not only a classical quantity like the radius of the Hill sphere, but also the critical starting orbital distance (as a function of orbital inclination) within which most orbits remain bound to the asteroid, and outside which most escape as a consequence of perturbations. Moreover we determined the orbital stability zone, defined as the union of all the numerically integrated orbits showing long-term stability, for each of the target asteroids. The particular shape of these zones would suggest to have the spacecrafts close approach out of the orbital plane of the asteroids.

Amorphous silicate dust grains have been produced in the laboratory by means of laser ablation of solid targets in different ambient atmospheres. In this work we show that, if the condensation occurs in the presence of hydrogen, the spectra of silicate grains, together with the characteristic 10 and 20 µm features, exhibit an absorption band around 4.6 µm. Such features, absent in the spectra of the same silicate grains produced in an oxygen atmosphere, may be attributed to a fundamental stretching vibration of -SiH functional groups bound into the grains or on their surface.

Several instruments have been described at the annual meetings of the EGS (European Geophysica Society) and the DPS (Division for Planetary Sciences) The numerous abstracts which were published can be found in:

Bulletin of the American Astronomical Society, Vol 31, No. 4 (DPS)

Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we present a detailed energy budget analysis for the surface of a model comet in the orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effective energy input. This yields lower gas fluxes from the surface. For an 100% active, non-dust covered surface we obtain a water gas flux on the order of about 1.5×10²⁸ molecules s^-1 at perihelion, assuming a radius of 600 m. The calculated gas flux of water is within the order of measured values for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon—not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the radius of comet 46P/Wirtanen may be much larger than the accepted value of 600 m. A radius in the order of 2 km seems more likely to explain the measurements. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effective surface area of sublimation.