LADEE Science Objectives

LADEEScience Objectives

LADEE Mission Concept Review (MCR)

December 10, 2008

Greg Delory – Deputy Project Scientist

Rick Elphic – Project Scientist Tom Morgan – Program Scientist

LADEE MCRDec 10 2008

Summary Overview

• The top eleven science goals identified in the National Research Council’s report, “Scientific Context for the Exploration of the Moon” include:

a. Determine the global density, composition, and time variability of the fragile lunar atmosphere before it is perturbed by further human activity

b. Determine the size, charge, and spatial distribution of electrostatically transported dust grains and assess their likely effects on lunar exploration and lunar-based astronomy.

• The LADEE mission was designed to begin to address these objectives.

2


Lunar Exosphere/Dust Environment

3


Lunar Exosphere (1)

4

ALSEP-LACE Detections on Apollo 17 Post Apollo Ground-based

• The Exosphere is a collisionless atmosphere. Atoms due not collide with each other to thermalize the gas ̶ terms like temperature must be treated with care.

• Some of the exosphere is externally derived ̶ Solar Wind and Interplanetary dust ̶ but most is originated from the regolith and reflects composition.

• There are various source and sink mechanisms, dependent on species, location on the Moon, position of the Moon in its orbit, internal activity and external events (meteor showers, solar conditions)

Remote observations of the Na exosphere (Potter & Morgan, 1998)

Global distribution of Ar-40 at 30 km and 50 km (D. Hodges).


Lunar Exosphere (2)

5

Lunar Atmospheric and Composition Experiment (LACE) 40Ar Measurements

• Identified Constituents are: 40Ar, 36Ar, 222Rb, He, H, Na, K; the upper limit on density of the exosphere is a few times 107 atoms /cm3

• Upper limits established for most other species – but solid detections of important components such as C, N, O, CO2, CH4 + metals remain elusive


Lunar Dust

6

Based on mechanical techniques the accepted wisdom is that:

1. Regolith is fine-grained2. Roughly:

- 10% smaller than 10 mm- 20% smaller than 20 mm

3. Predictable physical properties (porosity, thermal conductivity, shear and bearing strength, angle of repose, tribology)

4. Notice the shape/surface area

• All size fractions are present, but the very smallest are difficult to collect or to adequately count.

Plenty of dust, particularly in the size range below 10 - 20 um for which electrostatic forces dominate over gravity.

100 m


Evidence of Dynamic Dust

7

Gene Cernan sketches from Apollo Command Module

McCoy and Criswell, 1974

From lunar orbit… …and on the lunar surface

Berg et al., 1976

TerminatorsLunar Ejecta and Meteorites experiment (LEAM)


SDT Findings - Exosphere

8

• Species that make up exosphere:1) prevalent at 50 km altitude 2) maximize at sunrise terminator3) peak densities at equator

• Species can be categorized by their sources, including solar wind, regolith and radiogenic

• Ar, He, H/H2, OH, CH4, CO, CO2, Na, K, Si, Al, Fe all of interest

• No one instrument/technique can obtain all species of interest

• A NMS will likely detect Ar, He, H2 but will have great difficulty with trace species requiring a supporting instrument

• Mission lifetime of a year is ideal, but new, interesting science can be done in 3 mo.

Model results @50 km from R. Hodges for the LADEE SDT


SDT Findings - Dust

9

• Two dust components: 1) Dust of Lunar Origin (DoLO)2) Interplanetary Dust (IDP)

• DoLO: Electrostatically lofted or secondary ejecta

• DoLO may peak near terminator with densities at ~10-4/cc at 50 km

• IDPs/secondary ejecta detected at all longitudes

• Distinct Targets-of-Opportunity for improved dust observations:

˗ Known meteor shower/comet tails

˗ Magnetotail/plasma sheet crossings

˗ Solar storms

• DoLO single impacts very difficult to detect since grains are submicron and slow

• Remote sensing UV/VIS instrument will provide critical complementary data to in situ observations

Stubbs et al., 2006

LADEE MCRDec 10 2008 04/21/23

10

Baseline Science

In order to accomplish the science objectives, the LADEE mission shall meet the following baseline science requirements:

•Measure spatial and temporal variations of Ar, He, Na, and K over time scales from several (3) lunar orbits to one lunation. •Detect or obtain new lower limits for other species for which observations have been made. These include the following elements or compounds and the current limit* (part/cm3); CH4(1104), S(150), O(1103), Si(48), Kr(2104), Xe(3103), Fe(3.8x102), Al(55), Ti (1), Mg(6103), OH(1106), and H2O(100). •Search for other species (beyond those listed in the previous two bullets) or positive ambient ions of these species and other atoms or compounds in the 2-150 Da mass range. •Detect or set upper limits as small as 10-4 dust particles /cm3 from 1.5 to 50 km altitude for particles as small as 100 nm via occultation measurements.•Detect or set upper limits on the dust population at 50 km.

*Limits measured against Table 1.1, S. A. Stern, Reviews of Geophysics, 37, 453, 1999. (Stern states no limit for H20)

Science Flowdown - Payload

60 cm

100 mm OpticalModule

Modems

Control

Electronics

Dust Detector (DD) – TBDDetect sub-micron dust (size, charge)

Neutral Mass Spectrometer (NMS) UV Spectrometer (UVS)

Lunar Laser Com Demo (LLCD)

To be selected3 kg, 5W envelope

Several well-understood candidates available

LCROSS heritageMSL/SAM Heritage

Dust and exosphere measurements

In situ measurement of exospheric species

Technology demonstration

Science synergy: high data rates

51-622 Mbps

300 Dalton range/unit mass resolution


Science Flowdown – s/c & Trajectory

12

• Ideal orbit: Circular, retrograde, low inclination

• Most active science location: Periseline at < 50 km over sunrise terminator

• Retrograde orbit keeps instruments in ram but out of sunshine

• Equatorial orbit preferred over polar orbit: Densest portion of exosphere, don’t expect emitted polar water from cold traps. LADEE can contribute to search for water by “following the OH” and examining the terminator desorption processes

• Spacecraft is “dirty” and will have the potential to contaminate instruments via outgassing, thruster firings, and EMI


Science Flowdown - Operations

13

Guiding Considerations• Spatial and temporal variations crucial to

LADEE science objectives• Science operations need to cover

variations ranging from ~3 orbits to ~1 month or more.

• Spatial variations across noon-terminators-midnight important

UVS:• Sufficient time to achieve SNR ~5 for

most species• Occultation (dust) and limb (dust,

exosphere) modes

NMS:• Sufficient time to resolve major species• Ram, nadir, and rotisserie modes

DD:• Similar to NMS

Operations Profiles1 UVS

2 NMS/DD

3 UVS

4 Comm/PWR

5 NMS/DD

6 UVS

7 NMS/DD

1 UVS

2 Comm/PWR

3 UVS

4 NMS/DD

5 Comm/PWR

6 NMS/DD

Duty cycled profiles to optimize observations vs. bus resources

NMS/DD: 40% duty cycleUVS: 1500-3000 s integration times w/cycled occultations


Example Orbits

14

NMS/DD Orbit UVS Orbit

LLCD Orbit

Ram-pointing configuration

Limb-pointing

Sub-pointing


Follow-On Analysis

• Pointing w.r.t. lunar limb– Uncertainty in position of the Moon (lunar ephemeris)– Active trade underway examining slewing/coverage/SNR

• Altitude knowledge (ties into previous bullet)– 3 km is current goal; 1 km desired– Part of larger trajectory/navigation trade analysis

• Continue orbit/science observation trade– Trajectory/fuel trade may lead to more variable orbits– OK with science with modified observation profiles– Potential coverage at low (20 km) altitudes very compelling from

PSWG perspective

15


Summary

• Top level spacecraft and mission capabilities meet the baseline science objectives

• Current activities focused on optimization• Implementation of baseline science remains flexible and

can be responsive to changes in the mission design

16

Documents

LADEE Science Objectives