# ESA Vigil

Vigil, formerly known as Lagrange, is a space weather mission developed by the European Space Agency. The mission will provide the ESA Space Weather Office with instruments able to monitor the Sun, its solar corona and interplanetary medium between the Sun and Earth, to provide early warnings of increased solar activity, to identify and mitigate potential threats to society and ground, airborne and space based infrastructure as well as to allow 4 to 5 days space weather forecasts. To this purpose the Vigil mission will place for the first time a spacecraft at Sun-Earth Lagrange point 5 (L$_5$) from where it would get a 'side' view of the Sun, observing regions of solar activity on the solar surface before they turn and face Earth.
Monitoring space weather includes events such as solar flares, coronal mass ejections, geomagnetic storms, solar proton events, etc. The Sun-Earth L$_5$ location provides opportunities for space weather forecasting by monitoring the Sun beyond the Eastern solar limb not visible from Earth, thus increasing the forecast lead time of potentially hazardous solar phenomena including solar flares, fast solar wind streams.
The Vigil mission will improve the assessment of Coronal Mass Ejection (CME) motion and density, speed/energy, arrival time and impact on Earth to support protection of the critical infrastructure on ground and in space. The mission will also perform in-situ observations of the solar wind bulk velocity, density, and temperature as well as the Interplanetary magnetic field (IMF) at L5, to provide enhanced detection and forecasting of high-speed solar wind streams and corotating interaction regions.

## Status

As part of the Space Situational Awareness Programme (SSA), ESA initiated in 2015 the assessment of two missions to enhance space weather monitoring. These missions were initially meant to utilize the positioning of satellites at the Sun-Earth Lagrangian L$_1$ and L$_5$ points.
Eventually, in the frame of the cooperation on space-based space weather observations between the European Space Agency (ESA) and the United States National Oceanic and Atmospheric Administration (NOAA) National Environmental Satellite Data and Information Service (NESDIS) the following was agreed:
* NOAA/NESDIS will launch a Space Weather Follow On (SWFO) Mission to Lagrange Point L$_1$ for continuity of operational space weather observations and to reduce the risk of a measurement gap in the current coronal mass ejection (CME) imagery and in-situ solar wind measurements.
* ESA will launch a mission to Lagrange Point L$_5$ to provide capability for solar and space environment monitoring away from the Sun-Earth line.
In the scope of this agreement the two agencies will share data and provide each other with instruments to be embarked on the respective platforms.
The space segment of the Vigil mission completed the first part of Preliminary Definition (Phase B1) in June 2022. On 21 November 2022, ESA issued a Request for Quotation to Airbus Defence and Space Ltd. for the design, development and verification (Phase B2, C and D) of the Vigil Space Segment. The formal start of the activities is planned before the end of 2023.
Vigil is scheduled to be launched in 2031, followed by 3 years of cruise to L$_5$. The mission aims to operate nominally for 4.5 years, with a possibility of extension up to 5 additional years.

## Objectives

Vigil mission objectives can be grouped in two main categories:
* Nowcasting with the aim to provide an early warning about solar flares and the onset of a Coronal Mass Ejections (CMEs). Thanks to the side view from SEL5, the Vigil mission will also be able improve the accuracy of the predicted arrival CME arrival time on Earth by 2 to 4 hours compared to the current capabilities; this will be achieved by monitoring the entire space between Sun and Earth allowing mid-course tracking of CME and in general solar wind features as they travel towards Earth.
* Forecasting up to 4 to 5 days of the developing solar activity thanks to the monitoring of active region development beyond the East limb no visible from Earth. In-situ measurements in Sun-Earth L$_5$ will allow monitoring of high-speed solar wind streams and magnetic field several days in advance before they reach the Earth.

## Mission Architecture
### Space Segment
#### Platform

The Platform supplies all service-related functions required to support the proper operation and data collection of the Vigil Payload Suite. The key feature of spacecraft concept for an operational mission like Vigil is a robust avionics architecture able to remain operational during the most extreme space weather events seen in the last hundred years. The Failure Detection Isolation and Recover (FDIR) will be designed to enhance the autonomy of the spacecraft, thus reducing the risk of service interruption requiring ground intervention.
The Mission Data downlink is via X-band at an average data rate of ~1 Mbps (~86 Gbits per day) with 24/7 coverage provided by ESTRACK supplemented by additional commercial stations.
The mass at launch is projected close to 2500 kg. To reach SEL5 the proposed design will rely on a bi-propellant Chemical Propulsion System equipped with a 450 N main engine.

#### Payload Suite

Payload Suite will include:
* 3 remote sensing instruments;
* 2 in-situ instruments;
In the frame of the inter-agency cooperation between ESA and NASA, Vigil will offer the possibility to accommodate an additional instrument NASA instrument of opportunity (NIO).

##### Remote sensing instruments

The remote sensing instruments will allow to estimate size, mass, speed, and direction of CMEs.
* Compact Coronagraph (CCOR): it will image the solar corona and be used to observe Coronal Mass Ejections (CMEs). With CCOR data the size, mass, speed, and direction of CMEs can be derived. The CCOR Instrument will be provided to ESA by NOAA and manufactured by U.S. Naval Research Laboratory (NRL). The design will instrument is based on the heritage of a similar instrument for NOAA's mission SWFO-1 and GOES-U.
* Heliospheric Imager (HI): it will provide wide-angle, white-light images of the region of space between the Sun and the Earth (i.e., the heliosphere). These images are required to enable tracking of Earth-directed CMEs over their propagation path once they have left the field-of-view of the coronagraph instrument.
* Photospheric Magnetic field Imager (PMI): it will scan a selected solar spectrum to generate 3D maps of the magnetic field (field strength, azimuth, inclination) and crucial physical parameters (e.g. distribution of vertical and horizontal magnetic fields, distribution of inclination angles, twist, writhe, helicity, current density, share angles, photospheric magnetic excess energy etc.) for enhanced space weather applications. The instrument will also generate solar white light images as by-products of magnetograph measurements and produced as continuum images observed at an additional wavelength point in the vicinity of the magnetically sensitive spectral line.

##### In-situ instruments

In-situ instruments can be used to monitor the Stream Interaction Regions (SIR) and Co-rotating Interaction Regions (CIR) up to 4–5 days in advance before their arrival at Earth.
* Plasma Analyser (PLA): it will measure Solar wind bulk velocity, solar wind bulk density and solar wind temperature, are required for monitoring of the solar wind that is turning towards the Earth and particularly for detection of high-speed solar wind streams that produce Stream Interaction Regions (SIR) and Co-rotating Interaction Regions (CIR).
* Magnetometer (MAG): it will measurement of the Interplanetary Magnetic Field (IMF) at L$_5$; to minimise the effects of the electromagnetic interferences generated by the Vigil spacecraft itself, the MAG will be placed at the end of a 7m boom.

### Ground Segment

The Ground Segment, consists of:
* Mission Operation Centre (MOC) located in European Space Operations Centre (ESOC) responsible for Satellite commanding, Satellite health monitoring, orbit control and on-board software configuration and maintenance.
* The Payload Data Centre (PDC) responsible for mission data acquisition, processing, archiving and distribution to the customer/users, as well as mission planning;
* Ground Station Network (GSN). The GSN shall be made up of a mix of ESA ESTRACK stations and commercial stations as Vigil has a specific need to maintain a 24/7 downlink capability, including over the Pacific Ocean where there is a gap in ESTRACK coverage, third party stations will be required.

## Launcher

The launcher service is baselined as Ariane 6.2 by Arianespace from the Guiana Space Centre. The launcher will be in dual-launch configuration for injection in GTO. The spacecraft will be launched as secondary passenger with a commercial customer bound for geostationary orbit in a dual-launch with Ariane 6.4. This transfer option makes use of the Sun-Earth L$_1$/L$_2$ connection and the Weak Stability Boundary effects near L$_2$ to reach L$_5$.
After release of the spacecraft into GTO, it will perform a series of 3 Apogee Raising Manoeuvres (ARM) to make its way towards L$_1$ within a period of 14 days, planned to minimise the transitions through the Van Allen belts.
From L$_1$ the spacecraft will be placed on a zero to low-cost transfer trajectory towards L$_2$ from which it will then leave towards SEL5. Deep Space Manoeuvres (DSM), preceded and followed by correction manoeuvres, will be executed as needed.
When the spacecraft reaches L$_5$, a braking manoeuvre to insert the spacecraft into the final orbit will be executed. Different options are investigated, resulting in a split of such manoeuvre in two burns.
The cruise to L$_5$ can take up to 3 years. To increase the use of the Vigil spacecraft, the mission will enter in a pre-operational phase once the halfway through the journey L$_5$.
Alternatives include the use of Ariane 6.2 for direct injection in SEL5, Ariane 6.4 or Falcon 9 provided by SpaceX.