Ariane 5 boosts BepiColombo mission enroute to Mercury

by Chris Bergin

Arianespace has conducted the opening part of the BepiColombo mission with the launch of the spacecraft on a flight to study Mercury. The launch was conducted by an Ariane 5 that lifted off at 01:45 UTC on Saturday from the European Spaceport at Kourou, French Guiana.
BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency. It was named in honor of Italian mathematician and engineer Giuseppe “Bepi” Colombo.

The mission has been delayed by about five years, although that is not uncommon for major flagship missions.

At one point it was set to launch within weeks of the James Webb Space Telescope (JWST) – also on an Ariane 5 – before that mission also suffered numerous delays.

BepiColombo consists of two orbiters: Japan’s Mercury Magnetospheric Orbiter (MMO) and ESA’s Mercury Planetary Orbiter (MPO), both of which will be carried together by the Mercury Transport Module (MTM).

All the elements during integration – via Jacques van Oene of provided to NSF

While MPO will go into an approximately 400 x 1500 km mapping orbit around Mercury, MMO will enter a highly elliptical orbit to study the planet’s enigmatically strong magnetic field.

A combination of spacecraft will be launched together, with the Mercury Composite Spacecraft (MCS) consisting of two orbiters: MMO, MPO; as well as two additional elements: MTM, the Magnetospheric Orbiter Sunshield and Interface Structure (MOSIF).

The mission will undertake a seven year cruise to Mercury, using a combination of solar electric propulsion and nine gravity assist flybys at Earth, Venus and Mercury.

The opening element of that journey involved a ride on the Ariane 5 rocket.

This interplanetary mission comes hot on the heels of Ariane 5’s 100th flight, placing two telecommunications satellites into orbit in the process – which are the bread and butter missions for this launch vehicle.

Designated VA245 in Arianespace’s launcher family numbering system, this was the 23rd major scientific mission performed by the company to date.

Launch profile – via ESA

The launch window for BepiColombo opened on 5 October and would have closed on 29 November 2018 – a window designed to ensure the trajectory and multiple planetary flybys and gravity assists needed to eventually insert BepiColombo into the orbit of Mercury can be achieved via planetary alignments of Earth, Venus, and Mercury.

Based on an October 2018 launch, BepiColombo will perform the following flyby / planetary encounter sequence leading to orbit insertion at Mercury on 5 December 2025:

Date Milestone
6 April 2020 First (and only) Earth flyby
12 October 2020 First Venus flyby
11 August 2021 Second (and final) Venus flyby
2 October 2021 First Mercury flyby
23 June 2022 Second Mercury flyby
20 June 2023 Third Mercury flyby
5 September 2024 Fourth Mercury flyby
2 December 2024 Fifth Mercury flyby
9 January 2025 Sixth Mercury flyby
5 December 2025 Mercury orbit insertion

Ariane 5 lofted estimated payload mass of 4,241 kg from ELA-3, with spacecraft separation occurring 27 minutes into the flight.

The carrier spacecraft will use a combination of electric propulsion – which has undergone steering tests on the ground – along with multiple gravity-assists to complete the 7.2 year journey to the Solar System’s mysterious innermost planet.

The 22 cm-diameter T6 thruster was designed for ESA by QinetiQ in the UK, whose expertise in electric propulsion stretches back to the 1960s. The spacecraft built was led by Airbus, but involves numerous companies and countries.

After arriving at Mercury, the modules will separate and from their respective orbits the science orbiters will make complementary measurements of Mercury’s interior, surface, exosphere and magnetosphere, following up on many of the open questions raised by NASA’s Messenger mission.

BepiColombo will investigate properties of the innermost planet of our Solar System that are still mysterious, such as its high density, the fact that it is the only planet with a magnetic field similar to Earth’s, the much higher than expected amount of volatile elements detected by NASA’s Messenger probe and the nature of water ice that may exist in the permanently shadowed areas at the poles.

NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft ended its mission in 2015 with a dramatic, but planned, event – crashing into the surface of the planet that it had been studying for over four years.

The follow on information from BepiColombo will provide more data about the origin and evolution of a planet close to its parent star, providing a better understanding of the overall evolution of our own Solar System. The two orbiters will be providing a huge amount of data to scientists on Earth.

MTM is equipped with three monitoring cameras, which provide black-and-white snapshots in 1024 x 1024 pixel resolution. One of the monitoring cameras is positioned on the MTM with a field of view looking up towards the Mercury Planetary Orbiter (MPO), which sits above.

The MTM’s solar arrays are currently folded for launch, resulting in the presented image, but after their deployment the camera will have a clearer view. In particular, the MPO’s high-gain antenna will be in the field of view of the camera around one day after launch.

The other two cameras are placed on the other side of the module: one will look down the extended solar array of the MTM, the other towards the MPO, capturing glimpses of the medium-gain antenna once deployed and, later, of the magnetometer boom.

MTM cameras, via ESA

The actual deployment of the solar arrays and antenna will be confirmed by telemetry data sent by the spacecraft after launch. Later, the monitoring cameras will be switched on. The first sets of images are planned to be taken around 12 hours and 1.5 days after launch and returned to Earth shortly after.

The monitoring cameras will be used ad-hoc during the cruise phase, notably during the flybys of Earth, Venus and Mercury.

While the MPO is equipped with a high-resolution scientific camera, this can only be operated after separating from the MTM upon arrival at Mercury in late 2025 because, like several of the 11 instrument suites, it is located on the side of the spacecraft fixed to the MTM during the cruise phase.

Nonetheless, it will be possible to operate or partially operate as many as eight of the 11 instruments on the MPO during the flybys, along with three of the five instrument packages onboard JAXA’s Mercury Magnetospheric Orbiter. This will afford some unique data collection opportunities at Venus, for example.

BepiColombo carries enough propellant to potentially be extended for an additional year, resulting in two Earth years of observation around Mercury – or 8.2 Mercurian years.

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