reconnaissance observations of rosetta target 67p/c-g

Reconnaissance Observations of Rosetta Target 67P/C-G

Stephen Lowry Centre for Astrophysics and Planetary Science

University of Kent, UK

RAS – Dec 2010

ESA’s Cornerstone Rosetta mission -

The OSIRIS Project

Research Programmes SEPPCoN - Survey of the Ensemble Properties of Cometary Nuclei

The Asteroidal YORP Effect:

Detection and Theoretical

Investigations

Near Earth Asteroids - Physical Properties and Composition

RAS SDM: Dr. Stephen Lowry – Observations of 67P/C-G

•  Discovered in Oct 22nd, 1969 by Klim Ivanovic Churyumov (Univ. of Kiev, Ukraine), and Svetlana Ivanovna Gerasimenko (Inst. Of Astrophysics, Dushanbe, Tajilistan) at the Alma-Ata Astrophysical Institute, Russia.

•  Selected to be new target of ESA’s Rosetta mission in 2003. At this time very little known about nucleus.

•  Encouraged extensive observations since (initially by Lowry et al., Boehnhardt et al, and HST measurements by Lamy et al.)

67P/Churyumov-Gerasimenko Discovery and Selection as Rosetta Target


67P/Churyumov-Gerasimenko Orbital Characteristics

Lamy et al. (2006) Space Sci. Rev.


•  The orbital evolution of comet 67P/C-G is chaotic due to repeated close encounters with Jupiter (Carusi et al., 1985; Belyaev et al., 1986; Krolikowska, 2003).

•  The encounter which occurred in February 1959 at a distance of only 0.0518 AU considerably modified the orbit: the perihelion distance dropped from 2.74 to 1.28 AU, the eccentricity increased from 0.36 to 0.63, its orbital period was shortened from 8.97 to 6.55 yr

67P/Churyumov-Gerasimenko Orbital Evolution


•  Constraining the nucleus size, shape, spin state and surface properties is critical for establishing likely surface properties (active area, thermal properties):

Important for interpreting ground based observations of coma activity – specifically Thermo-physical models of surface properties that attempt to describe observations (see De Santis et al., Icarus 2010: Shape and obliquity effects on the thermal evolution of the Rosetta target 67P/Churyumov–Gerasimenko cometary nucleus)

Also critical for lander phase!

•  Important to set a context for in-situ measurements. - What is the baseline activity throughout orbit? - When does it start outgassing, relative to initial encounter by Rosetta? - Also, how is comet behaving as seen from ground based observations, during encounter? Can use wider suite of instruments from ground.

67P/Churyumov-Gerasimenko Importance of reconnaissance observations


•  Critical to have detailed models of dust environment that Rosetta will be subjected to, as well as lander.

•  Critical to establish models based on ground-based observations (+ HST etc) ahead of encounter so that analysis techniques can be verified ahead of time. Provides robust credibility of remote sensing methods.

67P/Churyumov-Gerasimenko Importance of reconnaissance observations


Nucleus HST Observations

•  Shortly after selection of a new suitable cometary target for Rosetta, Lamy et al. were granted Director Discretionary Time on the Hubble Space Telescope to pin down the size of its nucleus as this was a critical issue for the safe landing of the Philae surface module.

•  The observations were performed on 12 and 13 March 2003, when the comet was 2.50 AU from the Sun, 1.52 AU from the Earth, and at a phase angle of 4.8°.

•  Applied coma subtraction technique to ascertain nucleus properties.

HST WFPC2 (PC mode) image from March 2003 with the F675W filter (Lamy et al., A&A 2006)


Nucleus HST Observations

- R-band radius = 1.98 ± 0.02 km (assuming A, β = 0.04)

- The rotation period of the nucleus was determined from seven different period searching methods and is 12.41 ± 0.41 hours

-  The spheroidal solution implied by the double-peaked light curve has semiaxes lower limits of a = 2.41 km and b = c = 1.55 km.

HST WFPC2 (PC mode) image from March 2003 with the F675W filter (Lamy et al., A&A 2006)


Aphelion Monitoring from ESO 67P/Churyumov-Gerasimenko

Observing Team: Alan Fitzsimmons, Colin Snodgrass, Olivier Hainaut, Laurent Jorda, Mikko Kaasalainen, Philippe Lamy, Miriam Rengel, Imre Toth.

• 26 February, 2004 (VR) (Rh = 4.48 AU [outbound], Δ = 3.96 AU, α = 11.45°)

• 23 April, 2004 (VR), 27 April 2004 (VRI) (Rh = 4.72 AU [outbound], Δ = 3.72 AU, α = 1.12°)

• May 10th, 12th, and 14th, 2004 (VRI) (Rh = 5.60 AU [Q = 5.71 AU], Δ = 4.59 AU, α = 0.07-0.84°)

• July 16th, 19th, 22nd, 2007 (3 nights in VRI) (Rh = 4.63 AU [inbound], Δ = 3.72 AU, α = 6.04-7.28°)

• 13 September, 2007 (0.5 night in VRI) (Rh = 4.40 AU [inbound], Δ = 4.25 AU, α = 13.21°) - Encounter point!

3.5m NTT



3.5m NTT

•  Three nights of data taken with the 3.5m New Technology Telescope (European Southern Observatory, La Silla Chile), under good conditions on May 10th, 12th and 14th, 2004.

•  The photometry consists of visual broadband CCD imaging with VRI filters covering the 0.55-0.79 micron range, taken with the EMMI instrument.

•  Optical observing sequence for each object was 6R-8V-6R-8I-6R-8V-6R…, which allow colour indices to be determined without the confounding effects of rotation.

•  Data set is includes full lightcurve coverage in V, R and I filters.

•  This will allow robust determinations of the size, projected shape, synodic spin rate, colour indices (or possible presence of colour variations across surface).


Aphelion Monitoring from ESO

May 10 71x100s R-filter images



67P/Churyumov-Gerasimenko



1. Mean apparent R magnitude is 22.41 ± 0.03 Δm = 0.38 ± 0.04 (a/b ≥ 1.42 ± 0.05)

2. Mean effective radius is 2.26 ± 0.03 km (for AR of 0.04 and β = 0.035 mags./deg.) 3. Semi-axes: a = 2.07 ± 0.04, b = 2.94 ± 0.15 km

4. Colour index: (V-R) = 0.41 ± 0.04 (Spectral Gradient ~4.7 %/100nm)

5. No dust trail seen in R-filter ultra-deep images



Synodic rotation period = 12.72 ± 0.05 hrs



1.  Observations obtained of comet 67P/C-G in May 2005 when comet was at 5.6 AU from Earth and at opposition (4 additional data sets obtained)

2.  Comet appeared stellar on co-added ultra-deep images.

3.  Best-fit synodic rotation period is 12.72 ± 0.05 hours

4.  Asymmetric light-curve observed with full magnitude range of 0.38 ± 0.04, which corresponds to a/b ≥ 1.42 ± 0.05.

5.  Mean apparent magnitude is 22.41 ± 0.03, which corresponds to an absolute magnitude HR of 15.34 ± 0.03.

6.  Mean effective radius is 2.26 ± 0.03 km (assuming AR of 0.04 and β = 0.035 mags./degree).

5.  Semi-axes are: a = 2.07 ± 0.04, b = 2.94 ± 0.15 km (same AR and β).

6.  Colour indices: (V-R) = 0.41 ± 0.04 (S′ ~ 4.7 %/100 nm)

7.  No dust trail seen in R-filter ultra-deep images.


HST/ESO shape Model


HST May 10

May 12 May 14

Prograde

Retrograde



Pole Solutions


HST May 10

May 12 May 14



67P/Churyumov-Gerasimenko VLT/FORS2 observations

•  ESO VLT images: June 2004, May 2006, August 2006 (Tubiana et al. A&A 2008)

•  Rh = 4.89 – 5.62 AU, α = 0.5 – 10.6°

•  Rotation Period = 12.7047 ± 0.0011 hours

•  Effective radius = 2.38 ± 0.04 km (A = 0.04)

•  Phase coefficient = 0.076 ± 0.003 mag./°

•  No colour variation with rotation seen, based on optical spectra and broadband colours (May 2006)

•  Optical dust trail seen (a: I-filter, b: R-filter, c: V-filter, d: VRI composite), Extends more than 153.25”x8” or 4.8x105 km and 2.5 x 104 km at comet.


67P/Churyumov-Gerasimenko VLT/FORS2 observations: Optical Dust Trail

•  ESO VLT images: June 2004, May 2006, August 2006 (Tubiana et al. A&A 2008)

•  Rh = 4.89 – 5.62 AU, α = 0.5 – 10.6°

•  Rotation Period = 12.7047 ± 0.0011 hours

•  Effective radius = 2.38 ± 0.04 km (A = 0.04)

•  Phase coefficient = 0.076 ± 0.003 mag./°

•  No colour variation with rotation seen, based on optical spectra and broadband colours (May 2006)

•  Optical dust trail seen (a: I-filter, b: R-filter, c: V-filter, d: VRI composite), Extends more than 153.25”x8” or 4.8x105 km and 2.5 x 104 km at comet.


67P/Churyumov-Gerasimenko Spitzer observations: Nucleus & Dust Trail

•  Spitzer MIPS 24µm images: Feb 2004, spanning 12.5 hrs (Lamy et al. A&A 2008)

•  Rh = 4.48 AU, α = 12.1°

•  Rotation period not well constrained

•  Semi-axis: 4.40–5.20 km, 4.16–4.30 km, and 3.40–3.50 km (Effective radius = 1.93 – 2.03 km)

•  Albedo = 0.039 – 0.043


67P/Churyumov-Gerasimenko Spitzer observations: Nucleus & Dust Trail


•  Rh = 4.48 AU, α = 12.1°

•  Rotation period not well constrained

•  Semi-axis: 4.40–5.20 km, 4.16–4.30 km, and 3.40–3.50 km (Effective radius = 1.93 – 2.03 km)

•  Albedo = 0.039 – 0.043

•  The success of the landing of the Philae surface module now remains critically dependent upon the bulk density of the nucleus. A value of 0.1-0.37 g cm−3 (Davidsson & Gutiérrez, 2005) would ensure a safe landing. But this estimate could be as high as 0.5-0.6 g cm−3

Lamy et al. (A&A 2008),

Hilchenbach et al. (in ‘The New ROSETTA Targets’, 2004)


Kelley et al. (ApJ 2009)

67P/Churyumov-Gerasimenko Spitzer observations: IR Dust Trail & Nucleus



•  Rh = 5.5 – 4.3 AU, α = 11-13°

•  Radiometric effective radius 2.04 ± 0.11 km

•  Albedo = 0.054 ± 0.006

•  Maximum grain size 5µm. The grain number density near the nucleus is (1.33 ± 0.03) x 10−12 m−3, assuming that the dust is composed of millimeter-sized grains. The density corresponds to a Rosetta dust impact probability upper limit of 0.3% during the spacecraft’s approach to the nucleus in 2014.

•  Rotation period well constrained 12.7047 hrs

67P/Churyumov-Gerasimenko Spitzer observations: IR Dust Trail


Kelley et al. (ApJ 2009) •  Spitzer MIPS 24µm images: Feb 2004, spanning 12.5 hrs (Lamy et al. A&A 2008)

•  Rh = 5.5 – 4.3 AU, α = 11-13°

•  Radiometric effective radius 2.04 ± 0.11 km

•  Albedo = 0.054 ± 0.006

•  Maximum grain size 5µm. The grain number density near the nucleus is (1.33 ± 0.03) x 10−12 m−3, assuming that the dust is composed of millimeter-sized grains. The density corresponds to a Rosetta dust impact probability upper limit of 0.3% during the spacecraft’s approach to the nucleus in 2014.

•  Rotation period well constrained 12.7047 hrs

The Rosetta lander (delivery planned at 3 AU), may collect a monolayer of grains larger than two microns every five days.

While the Rosetta probe orbiting at a distance of 10 km from the nucleus will collect the first monolayer of grains larger than two microns after the first four months

(i)  grains larger than 1 cm are ejected from the coma;

(ii)  the mass loss rate is larger than 100 kg s−1;

(iii) the mass loss rate is dominated by the largest ejected grains;

(iv) the dust coma brightness (e.g. the Afρ quantity) depends significantly or even mainly upon the largest ejected dust grains;

(v) a low Afρ = 0.1 m is consistent with such a scenario;

(vi) the first dust monolayer will be collected by the lander in the first week;

(vii) the first dust monolayer will be collected by the orbiter in the first four months spent at distances less than 10 km from the nucleus;

(viii) since the dust brightness is dominated by, or at least significantly depends upon the largest ejected grains, spurious identifications of stars by the navigation cameras is a primary danger for the mission: robust software against false nucleus detection is mandatory.

67P/Churyumov-Gerasimenko Dust Environment (Fulle, A&A 2004)


67P/Churyumov-Gerasimenko Jet Structures

Lara et al. (A&A 2011)


67P/Churyumov-Gerasimenko EPOXI images of 103P/Hartley 2

Image Credit: NASA. 2010


67P/Churyumov-Gerasimenko Coma Production Rates

- The Rosetta lander (delivery planned at 3 AU), may collect a monolayer of grains larger than two microns every five days (Lara et al. A&A 2011).

- C2 and CN production rates are stable around perihelion (also no major outbursts detected – unlike 17P/Holmes – 0.5m fold increase in brightness – occurred on previous orbits – would represent major risk to spacecraft.

- Rates imply ‘typical’ classification: Could imply Oort Cloud source for 67P/C-G – which in turn implies an original formation location at the Uranus-Neptune region and slightly beyond.


Rosetta phase

Perihelion

Nucleus Monitoring periods

67P/Churyumov-Gerasimenko Future Observability

Aphelion


67P/Churyumov-Gerasimenko Future Observability Rosetta phase

Perihelion Aphelion

Nucleus Monitoring periods


reconnaissance observations of rosetta target 67p/c-g

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