Near-Earth Exploration Minimum System - NEMS

15 January 2012

The NEMS – Near-Earth Exploration Minimum System internal ESA study was conducted at ESA’s Concurrent Design Facility (CDF) at ESTEC, the Netherlands between March and April 2011. The short study was carried out by an interdisciplinary team participating in eight concurrent design sessions, and it was aimed at the identification of the main trade-offs, system drivers and critical risk areas associated with basic human exploration missions beyond Earth-Moon system.

The objectives of the NEMS study were the following:

  • Investigate a system to transfer a crew of three astronauts from the Earth to an accessible Near-Earth target beyond the Earth-Moon system (e.g. NEA) and return them back to Earth by 2030, using as much as possible European systems and technologies.
  • Identify and report on the main trade-offs, system mass budget drivers and the critical risk areas involved in the design and implementation of such system.
  • Derive recommendations on more specific assessment activities and critical technology development areas in Europe and only from the point of view of technical feasibility (i.e. not considering any programmatic or policy aspects).

Requirements

Several high level requirements are the basis for the mission design:

  • crew of three astronauts,
  • safe return of astronauts to earth by 2030
  • No surface operations are assessed
  • European technologies were possible
  • Two design points
  • Short mission – “Weekend escape” (150 days)
  • Long mission – “Summer holidays” (300 days)

Target Selection

Assumed and Known NEO Numbers over Size (Harris 2011)

Choosing a target for human exploration is a complex task, often involving many requirements that have to be met simultaneously and limited physical data even on the known object population. Requirements to be met by the target often include a small delta-v for transfer, suitable phasing for selected launch date and in case of surface operations monolithic bodies of significant size and limited rotational period are preferred.

During the study, therefore a survey of possible target asteroids was performed helping to understand the range of possible mission durations and associated mission delta-v costs.

Mission design points

As a result of the NEO survey and the requirements, two mission design points were chosen for either the "Summer Holidays" and the "Weekend Escape"
Mission Weekend Escape Summer Holiday
Target 2006 RH120 2000 SG344
delta-v [km/s] 6 4.5
Duration[d] 150 300
Asteroid arrival 2028/08/21 2018/09/05
IM LEO [t] 300 215
ECLSS Open Loop Condensate water &
urine recycling
Stages CM+SM+HM+PS1
+PS2+PS3
CM+SM+HM+PS1
+PS2

Mission analysis

Mission Design - long mission (Summer Holidays)

All mission scenarios involve a simple ballistic transfer. In these trajectories spacecraft-Sun distances are typically within 0.9 – 1.1 AU, while Spacecraft-Earth distances are below 0.2 AU throughout the mission.The geometry of a typical mission is shown in the figure.

Mission architecture

Configuration - Weekend escape

The space system comprises the following elements:

  • Crew Module (CM): provides Earth re-entry capability and other life-support functions to crew.
  • Service Module (SM): unpressurized; provides services to the other modules and in particular to the CM, in particular propulsion, AOCS, telecommunications, OBDH, thermal, and power.
  • Habitation Module(s) (HM): habitation volume & other functions to crew. Even if no surface operations are considered, airlocks and storage (samples etc) spaces have been considered.
  • Propulsive Module(s) (PM or PS): dedicated modules for propulsive capability of the entire composite.

Concerning the Launch Vehicle an analysis resulted in the need of a heavy-lift vehicle able to bring 70-100tons to LEO. Use of a 20t launcher e.g. Ariane 5 ECA would require at least 16 rendezvous and docking operations or 2 years due to the limited launch rate. With a heavy-lift launcher four launches are necessary. The mission sequence is shown in the graph below.

In-Space Propulsion

Weekend Escape - Mission Sequence

Different technologies have been considered for the propulsion stages; in the baseline design, the two selected options are storable bi-propellant (Isp∼325 s) and cryogenic LH2-LOx (Isp∼465 s) propulsion. Cryogenic propulsion has the advantage of a higher Isp, however in-space long-term cryogenic propulsion is technically challenging due to the LH2 boil-off (during assembly phase and during the transfer).Using active cooling e.g. cryocoolers is not practical due to high power demand; passive cooling leads to boil-off (4% per month) which demands for additional propellant in the tanks to compensate for the losses. CH4/LOx could represent a competitive alternative to both conventional bi-propellant and cryogenic propulsion, but engines using such combination are not mature yet.

Habitability aspects

As for the crew aspects, the approach has been to identify the spacecraft environment needs for the crew to remain healthy and safe in all mission conditions and phases.

The ECLSS design assumes an open loop system with grey water recycling for both missions. For the Summer Holidays 300-day scenario, condensate water and urine recycling results in initial system mass savings and therefore have also been included in the design, while for the Weekend Escape case the dry mass of these systems almost totally offsets any saving in consumable water mass.

Mission scenario Weekend Escape Summer Holiday
Duration (d) 150 300
ECLSS (t) 7.2 9
Consumables (t) 2.9 3.2
Structure (t) 9.7 9.7
Thermal Control (t) 2.7 2.7
Other HM mass (t) 2.5 3.4
Total HM mass (t) 25 28

Important factor for interplanetary space travel is also the space environment. The main hazards result from radiation due to the Galactic Cosmic Rays and, in unsettled solar conditions, the Solar Particle Events (SPE). In quiet conditions i.e. in the absence of any SPE, a minimum of 2 g/cm2 shielding (equivalent to 8 mm Al) shall be provided, for the short (“Weekend Escape”) mission scenario. For the longer “Summer Holidays” a minimum of 5 g/cm2 (equivalent to 20 mm Al) shall be ensured. Shielding design is also complicated due to uncertainties of dose limits and the biological effects of Heavy Ions present in Galactic Cosmic Rays.
A storm shelter with a minimum of 20 g/cm2 of shielding (equivalent to 80 mm Al) shall be available in the spacecraft to provide a safe haven to the crew during a SPE. Since the astronauts need to be warned of any incoming solar storm, the monitoring of the radiation level and heliospheric conditions are of the utmost importance.

On the other hand, Micro-Meteoroid and Orbit Debris (MMOD) protection also has an important structural mass impact, as it sizes the thickness of the shell. The LEO environment is considered to be the mission sizing case due to high MMOD density. However, due to limited data about interplanetary micrometeoroid environment significant uncertainties exist on the importance of this hazard for the crew safety and mission success.

Safety considerations

Crew safety aspects drive the design of the architecture at system and subsystem level. Safety for deep-space crewed missions is particularly challenging due to:
  • No real-time ground support (communications delay);
  • No re-supply vehicles or practical rescue missions;
  • No quick Earth return possibility.
Abort trajectories were considered for both Weekend Escape and Summer Holidays; for WE, no feasible return leg after 55 days was found, but up that point the propulsion stages provide enough delta-v capability to return to the Earth. In the case of SH, a mission abort and Earth return is feasible only for up to 7 days, past that point in the mission no return is possible.

Mass breakdown for Summer Holiday and Weekend Escape

For further information please contact:

Andrés Gálvez
General Studies Programme
Future Preparation and Stragic Studies Office (PPC-PF)
Tel: +33 1 5369 7623
Email: andres.galvez @ esa.int

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