ELSA-d

Astroscale.

Overview

ELSA-d was a two-spacecraft demonstration of prepared debris capture: a servicer of about 175 kg carrying rendezvous sensors and a ferromagnetic capture mechanism, and a demonstration client of about 17 kg carrying a ferromagnetic docking plate with a fiducial pattern [1]. The two launched mated on 22 March 2021 and were deployed into a sun-synchronous orbit at 550 km and 97.5 degrees inclination.

The design point that separates ELSA-d from prior rendezvous and docking missions is the deliberate absence of any communication link between servicer and client during the demonstrations, and the absence of precise knowledge of client location [1]. The client is commandable from the ground but not from the servicer, so the servicer had to acquire, track and close on it using its own sensors. Capture is reversible, unlike the net and harpoon methods flown on RemoveDEBRIS. Astroscale describes the mission as the first commercial demonstration of the core technologies for debris docking and removal [6].

Specifications

Parameter	Servicer	Client	Source
Mass	about 175 kg	about 17 kg	[1]
Body dimensions	about 0.6 by 1.0 m	not published	[5]
Propulsion	8 thrusters, 1 N, high performance green propellant	none	[3], [5]
Attitude control	reaction wheels and reaction control thrusters	3-axis	[1], [5]
Power	deployable solar array with battery	battery and power system	[5]
Attitude and orbit sensors	star trackers, gyros, magnetometers, sun sensors, accelerometers, GPS	GPS	[5]
Rendezvous sensors	two visible cameras, blue light device, laser range finders, low power radio	laser retroreflector, HD camera	[1], [5]
Capture interface	ferromagnetic capture mechanism, extensible, with concentric permanent magnets	ferromagnetic docking plate on a stand-off with fiducial pattern	[1], [5]
Client platform	not applicable	SSTL-42 constellation variant	[5]

Published masses differ: 175 kg servicer and 17 kg client [1] against 180 kg and 20 kg [5].

The sensor suite was sized by a size, weight and power trade against a constrained mass and volume envelope [1]. One visible camera works with the low power radio for far-field relative navigation and long distance ranging; a second visible camera tuned for blue light works with the blue light source to illuminate the docking plate fiducial markers and determine client attitude and tumble rate at close range; multiple laser range finders give distance during close approach. All three planned demonstrations were to be flown with that one suite.

The docking plate is a flat ferromagnetic disc on a stand-off structure, sized to minimize the size, weight and power burden on the client while giving a secure connection for mated maneuvers [1], [6]. The servicer-side mechanism uses concentric permanent magnets [5], so holding force requires no power once mated. The fiducial pattern on it was designed to maximize the servicer’s ability to recover client attitude and rotation rate optically [1].

Mission profile

Event	Date	Source
Launch, mated, on Soyuz-2.1b Fregat-M from Baikonur	2021-03-22	[1], [5]
First release and magnetic recapture of the client	2021-08-25	[2]
Autonomous capture demonstration begun; release and autonomous relative navigation	2022-01-25	[3], [4]
Servicer and client separated to a maximum of about 1,700 km during anomaly investigation	2022-01 onward	[3]
Closest approach after the anomaly, 159 m	2022-04-07	[3]
De-orbit operations completed on four remaining thrusters	2024-01	[4]

Total in-orbit operations exceeded two years [4]. The servicer was left at approximately 500 km with a predicted re-entry in about 3.5 years, and the client with a predicted natural de-orbit within five years. The original plan was for the two to remain mated after the demonstrations and lower their altitude together to re-enter within five years, followed by passivation of the servicer by expelling remaining propellant and draining the batteries [1].

Operations were run by Astroscale from the In-orbit Servicing Control Centre National Facility at the Satellite Applications Catapult, Harwell, using a ground network of 16 sites selected to chain contacts together for extended duration coverage [1]. The launch was procured through GK Launch Services [5], and Astroscale records the mission as complete [6].

Planned demonstrations

Three demonstrations of increasing complexity were defined [1], each exercising the same sensor suite and the same docking plate interface [1], [5].

Demonstration	Content
One, capture of a stable client	Release, servicer holds at a set distance behind the client, navigation checkout and calibration on the rendezvous sensors, client commanded to a fixed orientation, approach through a series of holding points using the fiducial pattern, capture mechanism extended, capture, mated maneuver, re-release [1]
Two, capture of a tumbling client	Client commanded into a natural tumbling motion; servicer images it, images processed on the ground, maneuver commands uplinked, flight dynamics system aligns servicer to client, capture matching the tumble rate [1]
Three, diagnosis, search and capture	Daylight fly-around inspection with an operator go/no-go decision, then a client search using a different sensor set, servicer thrusts away to a recovery point and into a safety ellipse, a client-lost scenario is induced at long range, servicer re-acquires and closes as in demonstration one [1]

Demonstration one was flown, in the reduced form of a close-range release and recapture, in August 2021 [2], [5]. Demonstration two was not flown [3], [4].

What was achieved

The mechanical locking mechanism holding servicer and client together was released and the magnetic capture system alone held them, after which the client was separated for the first time and recaptured, validating the magnetic capture system [2]. That was performed on 25 August 2021.

The autonomous capture demonstration began on 25 January 2022 with release of the client and autonomous relative navigation, holding a distance of 30 m from the client over multiple orbits [3]. Anomalous spacecraft conditions were then detected and the team elected not to proceed to capture [4]. The servicer and client were deliberately separated further so the anomalies could be investigated from a safe distance, and drifted to a maximum separation of about 1,700 km [3].

Four of the eight 1 N high performance green propulsion thrusters were lost [3], [4]. A system issue affected three of them; the root cause of the fourth loss was not established and was under joint investigation by Astroscale and the thruster supplier, Bradford/ECAPS [3].

With four thrusters, the servicer navigated back from about 1,700 km to 159 m from the client on 7 April 2022, executing the handover from absolute navigation to relative navigation using the low power radio sensor [3]. Astroscale described that handover as the most demanding operation of the mission to that point. The tumbling capture of demonstration two was never flown.

The capability list Astroscale published at mission end comprises autonomous guidance, navigation and control algorithms, closed-loop control on onboard sensors, autonomous thruster maneuvering and attitude control, navigation from 1,600 km to within 160 m of the target, the absolute to relative navigation transition, and over two years of in-orbit operations [4].

Autonomy and mission operations

Autonomy was allocated by a trade between onboard capability and ground intervention, constrained by real-time contact time, servicer compute power and mission safety [1]. Relative navigation and collision avoidance ran onboard, together with fault detection, isolation and recovery logic that reacts autonomously to anomalies during autonomous operations. Human-in-the-loop control was required for final approach to capture, for departure and home position acquisition, and for the first execution of any maneuver type; once a maneuver type had been validated it no longer required continuous visibility.

The approach and capture algorithm uses the visible cameras, the blue lighting device, the laser range finder and the low power radio to estimate range and tumble rate, then commands reaction wheels and the reaction control system to match the client tumble rate and align the capture mechanism with the docking plate before extending it [1]. It relies on knowledge of where the docking plate sits on the client, a clear line of sight to it, and the sensor suite, and on no active interface with the client. In flight this loop held the servicer at 30 m from the client across multiple orbits [3].

The ground segment was designed for on-orbit servicing rather than adapted from a conventional satellite operation, seaming passes across multiple stations to achieve 20 to 30 minutes of near-continuous contact [1]. No relay satellite service was used, on grounds of availability, cost and the need for a dedicated transceiver. Operations ran for over two years on that architecture [4].

Propulsion

The reaction control system uses non-toxic green propellant [1], [5]. It maintains and controls the servicer attitude through the docking approach and provides the abort maneuvers that prevent unplanned collision during close approach [1]. Propellant loading at the launch site was simplified relative to conventional propellants. Half the set was lost in flight [3], [4]. The flight units were 1 N high performance green propulsion thrusters from Bradford/ECAPS, eight in total [3].

Thermal, compute and communications

Thermal design figures are not published. Onboard compute power is identified as one of the constraints that shaped the autonomy allocation, but no processor is named [1]. The client uses S-band and carries GPS [5], and also carries a laser retroreflector and an HD camera. The servicer carries GPS alongside star trackers, gyros, magnetometers, sun sensors and accelerometers. No link budget or data rate for the servicer is published in the sources obtained.

Technologies developed

ELSA-d took the core technologies for prepared end-of-life removal to TRL 9 by demonstrating commercial rendezvous with, docking to, and maneuvering of a client with no servicer-to-client communication [1]. The transferable products are the ferromagnetic docking plate and its optimized fiducial pattern as a client-side preparation interface; the extensible ferromagnetic capture mechanism, which is reversible where nets and harpoons are not; a rendezvous sensor suite built from low-cost, low-mass parts within a smallsat envelope; and a servicing ground segment with a 16-station contact chain. Mission-level results carried forward are the autonomous guidance, navigation and control algorithms and the absolute to relative navigation transition flown in April 2022 [3], [4]. Astroscale identified three follow-on directions at the time of the SmallSat paper: an improved docking plate, a reusable servicer able to service multiple clients within its life, baselined as ELSA-M, and rendezvous and docking with an unprepared client, baselined as ADRAS-J [1].