ESA Science & Technology - Mission Description and Operations

The New Gravitational wave Observatory (NGO) mission will employ three spacecraft deployed in a 'V' configuration with an arm length of 10⁶ kilometres, positioned in heliocentric, Earth trailing orbits (HETOs). The spacecraft at the apex of the 'V' is referred to as the 'Mother' spacecraft; those at the ends of the arms are 'Daughter' spacecraft.

Schematic diagram of NGO's science orbit. Credit: ESA

Launch

The three spacecraft will be launched using two Soyuz vehicles. One will carry the two 'Daughter' spacecraft and their propulsion modules to an orbit with an apogee of 20 000 kilometres, from where the propulsion modules will power them to escape. The other will carry the 'Mother' spacecraft and its propulsion module into geostationary transfer orbit, from where the propulsion module will again drive the escape. The propulsion modules will also manoeuvre the spacecraft during their transfer to the operational orbit. Once the operational orbit is reached, the propulsion modules will be separated to ensure that they do not disturb the payload.

The Daughter spacecraft will be launched first; the optimum launch window lasts for six continuous months per year, with year-round launches possible by incurring a ΔV penalty of 100 ms^-1. Optimal launch timing for the Mother spacecraft is six months after the daughters, but it could be launched up to 12 months after the daughters with extra propellant supplies in its propulsion module allowing it to 'catch up' with a ΔV of < 200 ms^-1.

Operational orbit

The selection of the operational orbits for NGO is influenced by a number of drivers:

• the requirement for a benign environment for the payload;
• minimisation of non-gravitational perturbations to allow accurate micro-propulsion control;
• the need for a thermally stable orbit to avoid large thermal gradients or thermal shocks that would affect the scientific measurements;
• the orbit must allow the quasi-static equilateral triangular constellation to be maintained without active control;
• minimization of the propellant required to reach the final orbit;
• limited drift rate, such that the distance from the Earth after six years is less than 65 million kilometres, which is essential for communication reasons

Selecting HETOs represents the best compromise between the various drivers. Slow-drift, Earth-trailing orbits have been selected, with the plane of the constellation inclined at 60° with respect to the ecliptic and the three spacecraft 'cartwheeling' annually around the centre of the constellation - a configuration that has been shown to be stable for at least six years. At the end of the transfer phase, the orbits will be 10° behind Earth, giving a nominal ground-to-constellation distance of 2 × 10⁷ kilometres. After four years, the planned drift will have increased the ground-to-constellation distance to 5 × 10⁷ kilometres; this distance further increases to 6.5 × 10⁷ kilometres after six years. This distance poses a limit to the full bandwidth telemetry download to ground. If the mission were to be extended beyond this time, a reduction in the amount of data downloaded would become necessary.

Mission phases

The operational phase of the NGO mission will be divided into the following phases:

• LEOP (lasting 30 days, during which the spacecraft will be configured for the cruise phase and their health will be checked);
• Transfer (lasting 14 months, during which the spacecraft-propulsion module composites will travel towards their operational orbit, propelled by the chemical propulsion module);
• Commissioning – a three-month period to achieve the science mode configuration required for science operations. Key activities include acquisition, drag-free testing, and instrument calibration;
• Calibration – a three-month period during which the characteristics of the instrument will be established. The activities performed in this phase can be fully or partly repeated during the science operations phase as needed;
• Science Operations – a nominal two-year period during which science data are collected and downloaded to ground every other day in turn by each spacecraft and science data products are generated at the Science Operations Centre;
• Post-operational – covers the period after the science operations phase comes to an end. During this phase, the main activities are in the data centres and the data archive. The duration of this phase will be at least two years;
• Archive – support will be provided to the scientific community for the usage of the NGO data products. It is planned that the NGO archive will continue to be operated beyond the end of the archive phase.

Ground segment

The NGO ground segment will comprise the Mission Operations Centre (MOC), based at ESA's European Spacecraft Operations Centre (ESOC), and the Science Ground Segment (SGS). The latter is composed of the Science Operations Centre (SOC, operating from ESA's European Space Astronomy Centre (ESAC)) – responsible for coordinating the development of the science operations ground segment and its operations to optimise the scientific return of the NGO mission; the Science Data Processing Centre – responsible for generating and providing main science products to the SOC; and the Instrument Operations Teams – responsible for the payload-related activities to be performed during the operations.

Communication between the spacecraft and the ground station during the operational phase will use the ESA 35-metre antenna at Cebreros, Spain.

During the science operations phase, nominal communication is scheduled every second day to one of the three spacecraft. The nominal communication schedule will be superseded by an extended communication schedule in the case of, for example, an upcoming black hole merger event – which can be predicted with sufficient notice.

(For more detailed information about NGO mission operations please consult the NGO assessment study report (Yellow Book) - see link in right-hand menu.)