BepiColombo flys by Venus

On Thursday the space probe BepiColombo will fly past Venus. Measurements in the planet's magnetosphere are planned.

October 14, 2020

On its way to Mercury, the planet closest to the Sun, the European-Japanese space probe BepiColombo will change course again on Thursday, 15 October. Following the Earth flyby in April this year, the first of two Venus flybys is now imminent. It will take the spacecraft, which was launched into space two years ago, past our neighboring planet at a distance of 10720 kilometers. Some of the scientific instruments on board, to which the Max Planck Institute for Solar System Research (MPS) in Germany contributed, will have the opportunity to collect data in the vicinity of Venus.

This Thursday, BepiColombo will fly past Mercury.

As our neighboring planet, Venus is used to encounters with unmanned space probes from Earth. Since 1961, numerous missions have visited the planet; the European probe Venus Express orbited Venus for more than eight years and the Japanese spacecraft Akatsuki has been there since 2015. "Despite the successful long-time observations of earlier probes, a flyby is not only a critical flight maneuver, but also of scientific interest," explains Dr. Norbert Krupp from MPS, who is part of the BepiColombo science team. Even a snapshot like the upcoming one can provide new insights or at least verify or refine older measurements.
BepiColombo will have the opportunity to do so in the next few days. The smallest distance of 10720 kilometers between the surface of Venus and the probe will be reached on Thursday, October 15, at 5:58 a.m. (CEST). Individual instruments will start their measurements up to two days earlier; they will last up to four days.

Ten of BepiColombo's 16 instruments will collect data during the flyby. These include MPPE (Mercury Plasma Particle Experiment), for which researchers and engineers from MPS developed and built parts of the MSA (Mass Spectrum Analyzer) mass spectrometer, and SERENA (Search for Exospheric Refilling and Emitting Natural Abundances Experiment), for which MPS contributed parts of the PICAM ion camera. BepiColombo’s other six instruments will remain switched off during the flyby. The spacecraft consists of two separate probes, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MIO), as well as a transfer module, which are permanently connected to each other during the journey to Mercury and thus obscure the view of individual instruments.

"BepiColombo will approach Venus from the day side and remain in its ion tail for about eight hours after the flyby," Dr. Markus Fränz of MPS describes the flight path. "For many questions concerning the magnetosphere of Venus, this is extremely favorable," adds the scientist from the MPPE team.

Unlike Earth, Venus does not have a strong magnetic field that is generated deep in its interior and surrounds the planet like a kind of protective shield. However, the solar wind, the continuous stream of particles from the Sun, interacts with the charged particles of the Venusian ionosphere to create a comparatively weak induced magnetic field. The charged particles in the vicinity of Venus are only weakly bound in this field; on the night side they reach far into space in a kind of ion tail.

Among other things, the MPS researchers hope to detect carbon ions there, which are likely to occur in very small quantities at best. "Venus Express was not able identify any carbon ions," explains Fränz, who was also involved in the earlier mission to Venus. The carbon signals were possibly obscured by those of the much more frequent oxygen ions. "BepiColombo's mass spectrometer MSA is able to distinguish more precisely between ions of similar mass. This time it could work," says Fränz.

The quantities of carbon ions in the Venusian magnetosphere are an important piece of information that helps to understand numerous processes in the planet's atmosphere. The last and only detection of these ions was achieved 24 years ago by the instrument CELIAS (Charge, Element and Isotope Analysis System), also with the cooperation of MPS, on board ESA’s and NASA’s space probe SOHO (Solar and Heliospheric Observatory). In a rare event, a very constant solar wind allowed the ion tail of Venus to expand to Earth's orbit, so that a conjunction with Venus made observations with SOHO possible. "We are very curious to see whether we can confirm the carbon measurements of that time," says Fränz.

Nevertheless, the night of the fly-by should be a quiet one for the MPS researchers. "All commands for the instruments have already been programmed; we won't interfere during the flyby", says MPS scientist Dr. Harald Krüger, member of the BepiColombo science team. The data from SERENA and MPPE will not reach Earth until a few days later.

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