Don Quijote concept
ESA's Don Quijote is an asteroid deflection precursor mission concept, assessed by the General Studies Programme in the Future and Strategic Studies Office. It is designed to demonstrate and validate the technology that one day could be used to deflect an asteroid threatening the Earth.
An internal study to define the baseline specifications and requirements of the Don Quijote mission concept was completed in July 2005 at ESA's Concurrent Design Facility (CDF). This study was carried out in preparation for the industrial work to commence in early 2006. Several feasible cost-effective yet compelling mission scenarios were identified for two selected (non-threatening) targets.
OverviewESA's Don Quijote mission concept consists of two spacecraft which are to be launched in separate interplanetary trajectories:
- An Orbiter spacecraft, called Sancho After arriving to the target asteroid and be inserted into an orbit around it, it will measure with great accuracy its position, shape, mass, and gravity field for several months before and after the impact of the second spacecraft.
In addition, the Orbiter will operate as a backup data relay for transferring all the data collected by the Impactor during approach and image the impact from a safe parking position. It will also investigate the surface composition of the asteroid and, after completion of the primary objective, carry out the ASP-DeX.
- An Impactor spacecraft, named Hidalgo After following a very different route from that of the Orbiter, the spacecraft will Impact an asteroid of approximately 500 m diameter at a relative speed of about 10 km/s. This spacecraft will demonstrate the ability to autonomously hit the target asteroid based on onboard high-resolution camera.
The primary objective of the Don Quijote concept is to impact the target Near-Earth Asteroid (NEA) and to be able to determine the deflection resulting from the impact. To achieve this, it will measure with extreme accuracy the asteroid's position in space before and after impact. A baseline and backup asteroids have been identified for design purposes, both being completely outside Earth's (AMOR) thus non-threatening.
There is also a secondary objective, involving the so-called Autonomous Surface Package Deployment Engineering eXperiment (ASP-DeX). In this experiment a small device, an Autonomous Surface Package or ASP, would be released from the Orbiter spacecraft while it's on orbit about the asteroid. It would then passively free-fall towards the asteroid surface after its release, and touchdown within a certain distance of a target landmark, most likely the crater resulting from the impact of the Hidalgo spacecraft.
In addition, part of the mission secondary goals are to and study the asteroid's surface chemical composition and the characterization of the thermal and mechanical properties of the asteroid surface.
The mission of the Impactor spacecraft is a peculiar one: the spacecraft should remain in a dormant state during most of its lifetime until the last days of asteroid approach where the autonomous guidance takes over and targets it toward the asteroid. During the cruise phase only minimum functions are required but before the impact all the sub-systems have to be up and functional with high level of reliability.
A major system design constraint is also on the spacecraft mass that (contrarily what normally is required) shall be above a certain threshold to achieve the required asteroid orbit deflection and lower than the launch system escape performance. In order to increase the impact mass, the propulsion module is not jettisoned at escape but is kept attached during the whole mission until impact as a ballast.
The major design drivers for the Impactor spacecraft are:
- Advanced on-board computers and high resolution camera to provide the required optical autonomous navigation and target with the required 50m accuracy.
- No moving appendages (solar arrays and antennas), in order to achieve the stringent AOCS pointing accuracies.
- No main propulsion required because of the ballistic trajectory; all the trajectory correction manoeuvres will be performed with the RCS.
- Low-cost approach by re-using an existing design. For the structure of the spacecraft, the Lisa Pathfinder science module was considered.
The mass budget for the Impactor spacecraft is:
|Dry mass||532 kg|
|Payload mass||9 kg|
|Propellant mass||1162 kg|
|Wet mass||1694 kg|
For the design of the Orbiter, a re-use of the SMART-1 bus was considered during the internal mission feasibility studies. Though this approach provides a good reference case to assess mission costs and the maturity of the technologies, there are some limitations, mainly given by the availability of a single PPS-1350 engine, a fixed Xenon tank capacity that limits the propellant mass, and finally a given bus structure.
In order to accomplish the mission, some modifications need to be performed on the SMART-1 bus.
- The input power to the Solar Electric Propulsion system requires an increased solar array surface of one extra panel per wing.
- A different communication subsystem is required, consisting of two-degree-of-freedom steerable 70 cm high gain antenna, medium and low gain antennas and a UHF antenna for the communication with the Impactor during targeting phase and the ASP.
The seven years required mission lifetime for Don Quijote is far longer than the one of SMART-1 (2.5 years by design). However, an analysis showed that the SMART-1 design can in principle be compatible with the extended required lifetime considering the encountered radiation dose levels, the qualification of the equipment and the thrusters lifetime.
The mass budget for the Orbiter spacecraft, with the extended payload set (see below), is:
|Dry mass||395 kg|
|Payload mass||20.6 kg|
|Propellant mass||96 kg|
|Wet mass||491 kg|
The payload of the Orbiter spacecraft plays a major role in the mission. Not only it will allow to charaterise the target and determine its position, shape, mass, and gravity field, but it will also meaure the deflection caused by the Impactor spacecraft to the asteroid.
A basic payload (made up by the navigation camera, the Radio Science Experiment and a LIDAR) addressing the primary objective of the mission would be complemented by a set of scientific instruments dedicated to the secondary mission goal, as shown in the accompanying figure.
Also being part as this secondary objective, the Autonomous Surface Packaged (ASP) would enable to investigate the mechanical properties of the asteroid's surface. This knowledge will be important to determine the feasibility of coupling devices onto the surface of an asteroid, and it would be required for the implementation of mitigation strategies relying on a direct contact with the asteroid.
The ASP would be thus part of the payload of the Orbiter, which would carry and deliver it to the surface at the end of the mission, from an orbit about the asteroid. This approach has been taken to minimise the uncertainties related to the orbiter operations during its deployment. In the 2002 AT4 asteroid scenario, deployment would take place from an orbit of 1 km in radius, a ΔV of 16 cm/s being required for the release and deorbit of the ASP. The duration of the drift towards the asteroid surface would be of around 2 hours and the impact velocity would range between 11 and 21 cm/s. After deployment the ASP should reach autonomously its preferred location, which would most likely be the interior of the impact crater.
Though the ASP has not been investigated in detail in the frame of the internal ESA studies, a minimum functionality has been assumed and a preliminary assessment of its payload has been conducted. The ASP would carry communications equipment enabling data and telemetry to be relayed to the Orbiter. The ASP chassis would most likely include a hopping mechanism in addition to housing all subsystems required for the nominal operation of the device.
In addition to this the ASP would most likely carry a set of scientific instruments, including at least the following instruments:
- a micro-camera, both for scientific investigations and for navigation
- an environmental package made up by a tri-axial accelerometer used to detect touchdown and analyse the mechanical properties of the asteroid surface and a thermal sensor for the characterisation of the thermal conductivity of the surface material
Additional work on SANCHO orbiter
Following the industrial assessment of Don Quijote mission concept, engineers from ESTEC and ESOC participated in a common and interdisciplinary effort to analyse the massive quantity of documentation relating to the possible solutions put forth by industries. As a result, a set of recommendations and key lessons learned were compiled. This precious knowledge inspired and suggested alternative ways to optimize the orbiter spacecraft design that have been further analyzed by ESA's General Studies Programme. For more information: Sancho Study: designing the minimum Earth escape spacecraft.