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31 Aug 2010 Vexag Venus W/S 1

Venus Express Mission

Håkan Svedhem ESA/ESTEC

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Scientific Objectives

•  The original objectives are defined as seven major Themes. Each theme is divided into several topics and corresponding measurements are defined –  Atmospheric Dynamics –  Atmospheric Structure –  Atmospheric Composition and Chemistry –  Cloud Layer and Hazes –  Radiative Balance and Greenhouse Effect –  Surface Properties and Geology –  Plasma Environments and Escape processes

•  Planetary evolution and comparative planetology has been added for the mission extension in 2008

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Science Payload

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Mission Timeline •  Launch 9 November 2005 •  Arrival at Venus 11 April 2006 •  Start of nominal operation June

2006 •  End of nominal operation/start of

extended operation September 2007 •  Mission presently approved until end

of 2012 peding a technical confirmation in 2010

•  In late 2010 the Science Programme Committee will consider an extension until end 2014.

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• Polar orbit • 24 h period • 66000km apocentre height • 250-400km initial pericentre height control band • Latitude of pc drifting at 3 deg per year, from 78 deg N at arrival, and passed 90 deg N mid 2009 • Pericentre height control band reduced to 175-275km late 2008

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Pericentre evolution due to solar perturbation

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VeRa occultation coverage

Now about 400 profiles available

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Thermal structure from VeRa (40-90 km)

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VeRa open loop double structure data

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Magnetometer measurements: From Solar Wind to Venus‘ Wake   Solar wind

  Interplanetary Coronal Mass Ejections   Thin current sheet   Mirror mode structures

  Ion pickup in the solar wind   Bow shock

  Location   Structure of the bow shock

  Magnetosheath:   mirror mode waves

  Magnetic barrier   Induced magnetosphere   Boundaries

  Terminator region   Giant Vortices   Turbulence

  Ionopause and flux ropes   Lightning

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Disappearing of the Magnetic Barrier

Zhang et al, GRL, 2009

The MAG team has been very productive and has published over 50 refereed articles

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Whistler-Mode Signals - Lightning

•  Lightning generate whistler waves - electromagnetic waves near 100 Hz propagating in the conditions of the ionosphere of Venus.

•  The VEX magnetometer samples these signals at 128 Hz. Since it requires two samples to define a wave, a signal can be properly reconstructed only below 64 Hz. The signas is thus sonewhat undersampled.

•  The magnetometer reduces the amplitude of signals above 64 Hz but does not eliminate them. Signals from 64-128 Hz will have reversed handedness.

13

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Venice, Italy VEX/SWT #24 Yoshifumi Futaana

VEX IMA ACE comparison on Aug 2007

Moment calculation by VEX/IMA and comparison with ACE data

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Atmospheric Evolution: -SpicaV/Soir measures HDO and H2O -Aspera measures escaping water

HDO/H2O ~ 0.07-0.1 average This equals 240±25 times the ratio in the Earth’ ocean. As Deuterium is heavier than Hydrogen it cannot escape as easily. This is a strong indication that Venus has lost a large portion of water in the past

ASPERA measures hydrogen and oxygen still escaping from the planet. The ratio is 2 to 1, thus corresponding to water! Escaping Helium is measured as well.

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15-27/2 16-23/2 2.3µm lower cloud imaging

120

110

100

90

80

70

60

6/14-21/1 oxygen 5577

28/2-6/3 1.18µm deep atm

Altitude (km)

4-10/5 T(z) O2 airglow

4-10/5 1.18µm

17-23/1 winds, CO(z), T(z)

HDO, SO, SO2

12-13/6 winds, CO(z), T(z)

15/2 winds cloud tops/CO2

28/2-6/3 T(z) O2 airglow

Mar16-22 winds, T(z), non-LTE

Apr2-7 winds, T(z), non-LTE

2009 Jan Feb Mar Apr May June

June2-7 winds, T(z), non-LTE

Coordinated Space-Ground Observ. Campaigns

(T. Widemann)

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Atmospheric Drag measurements

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Drag measurements

•  Atmospheric density is estimated from orbital decay and from direct Doppler tracking during the drag

•  A first assessment by ESOC Flight Dynamics group showed that no detectable drag could be seen from the orbital decay measurements

•  A detailed analysis has been performed by the drag team (Ingo Mueller Wodarg et al). They use the high rate open loop data recording of the Doppler signal from the drag pass itself (4 hours around pericentre) and as reference the same signal for another 10 hour per orbit.

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ESOC Flight Dynamics Results

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Preliminary results from drag campaign #2, October 2009

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VExAde Results 2009-2010

(P. Rosenblatt 2010)

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Torque measurement

s/c com SA cop

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8 Hz TM Analysis

-750 -500 -250 250 500 750sec

-0.0006

-0.0004

-0.0002

0.0002

0.0004

Nm

-750 -500 -250 250 500 750sec

-0.0002

0.0002

0.0004

Nm

Example: X-Axis

Total disturbance torque

Total disturbance torque minus SRP and gravity gradient torque

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8 Hz TM Analysis

190 200 210 220 230Height (km)

0

1

2

3

4

5D

ensi

ty (

10^-

13 k

g/m

^3)

Before pericentre

After pericentre

Model (Min)

Model (Max)

Model (Nom)

Venus Atmosphere Density

SA Tilt on 25/02/2010

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Combined Results 2009-2010

(P. Rosenblatt)

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Torque measurements

•  Some obvious advantages: –  It is more straightforward to analyse –  Seems more sensitive to atmospheric density and can therefore

work up to a higher altitude –  Can provide higher time resolution (=altitude resolution) –  Is not dependent on the radio propagation path Venus-Earth –  Is less dependent on modelling of the gravity field

•  But, it is yet a fairly unproven technique and a combination of both torque and drag measurement seems most promising

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Working towards an 1-D atmospheric engineering model

(I. Mueller-Wodarg)

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Heat flux during aerobraking

•  Future orbit modifications for Venus Express may include aerobraking. Studies on the effects on the spacecraft are on going. In particular the heat flux is a critical parameter.

•  Both the drag coefficient and the heat flux coefficient depend strongly on the mode of interaction between the impinging air molecules and the surfaces.

•  For an elastic collision (reflection) on a perpendicular surface the drag coefficient is multiplied by 2 compared to an non scattering case. Similarly the heat flux coefficient goes to 0 for an elastic collision.

•  The mode of collision is therefore of great importance for planning of aerobraking and a ‘windmill’ experiment can provide information on the character of the collisions between the air molecules and the spacecraft

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V s/c

Induced torque

Impinging air molecules

“Windmill experiment” (cf. Magellan)

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Measurements of Hydrogen in the solar system

•  Joint observations VenusExpress-MarsExpress and VenusExpress-SOHO have been performed in Lyman-alpha during 2008 in order to measure the hydrogen between the orbits of Venus and Earth/Mars (SipcaV, SpicaM and Swan)

•  Additional measurements are planned between Venus Express and Messenger (SpicaV and MASC)

Atoms between SOHO and Venus: I2-I4 or I3-I1 Cross-calibration: I4+I3=I1+I2

Interstellar Wind of H atoms

I1

I2 I3

I4

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Future work

•  The next years will provide extended coverage in latitude and local time for the atmospheric measurements and better surface coverage of the surface temperature and emissivity maps.

•  At the time of the lowest pericentre altitude atmospheric drag and torque measurements will be performed in order to beetr characterise the atmosphere at altitudes between 200 and 160 km.

•  A further pericentre altitude reduction will allow atmospheric drag measurements by on board accelerometers that should give very high precision data on the density and temperature in the 145-200 km altitude range.

•  Joint operations with the Japanese Venus Climate Orbiter in 2011 will enable simultaneous observations from different positions and provide improved temporal coverage.

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