Image of the Week

Gaia's sensors scan a lunar transit

   
 

Gaia's perspective on the lunar transit. A video is available here.

 

Update: plots showing the temperature variations during the lunar transit on 6 November 2015 are available here.

 

Located 1.5 million km from the Earth, ESA's Gaia spacecraft is scanning the sky to conduct the most detailed census of stars in our Galaxy. However, on 6 November, it will be perfectly placed to witness a rare event that will involve objects much closer to home – a lunar transit across the Sun.  

Transits occur when planets move in front of a nearby star. Such events occur occasionally in our Solar System when we see Venus and Mercury pass across the Sun's brilliant disc. As the small worlds drift across the illuminated stellar disc they appear as dark dots or circles. They also block some of the light and heat emitted by the star.  

The Moon sometimes covers the Sun – an event known as an eclipse - but we never see Earth's satellite as a small, dark circle crossing the face of the Sun.  However, Gaia is far enough away to observe such a transit.

Although its sensitive optics must be pointed well away from the Sun and Moon, Gaia's temperature sensors (thermistors) will be able to detect a lunar transit that will not be visible to any human eye or other spacecraft.

For more than 10 hours on 6 November, a near perfect alignment between Gaia, the Moon and the Sun will enable the star mapping spacecraft to chart how its temperature changes during the transit.

In order to monitor its health and physical condition, thermistors monitor the temperature at many places in the Gaia spacecraft. A series of measurements is taken every minute and subsequently sent back to Earth. During the passage of the Moon in front of the Sun, the Gaia team expects to detect a noticeable drop in the spacecraft's temperature.

During the first year of Gaia's science mission scientists have already seen small variations in the temperature data collected. The yearly change in solar distance due to the elliptical orbit of Gaia about the Sun results in a variation equivalent to ± 3.4% in the solar flux. This leads to a change of ± 1.2 °C in the temperature records.

Changes are also seen during the 6-hour spin period of Gaia. On the antenna, the temperature follows a very regular cyclical pattern, changing by 2 °C over each rotation-period.

The passage of 6 November 2015 starts at 08:18 UTC (GMT) with the first contact (outer contact) between the disc of the Moon and that of the Sun. The solar flux will gradually decrease for about 2 hours, until the Moon's entire disc is visible in front of the Sun. Over the next 6 hours the levels of incoming solar radiation will remain steady until the third contact. During the next two hours the Moon will move away from the solar disc and the event will end at 18:35 UTC.

The total duration of more than 10 hours equals 1.7 spin periods of the Gaia spacecraft. This should be enough to see a temperature drop of about 1.5°C and to estimate the thermal inertia of the spacecraft's components.

No temperature increase is expected to be measurable in the most insulated part of the spacecraft, but the Gaia team is looking forward to examining carefully the temperature records to see what really happened.

Gaia's orbit at L2, 1.5 million km from Earth in the anti-Sun direction, was aligned so that the number of lunar transits would be minimised and the spacecraft's thermal environment would be as stable as possible.  However, a similar celestial conjunction will occur in May 2016, and grazing transits, when only a fraction of the Moon's disc will cover the Sun, are predicted for April 2016 and February 2019.

Credits: image and movie courtesy of F. Mignard, Observatoire de la Côte d'Azur, Nice

[Published: 06/11/2015]

This text is also available on the Sci-Tech website.

 

Image of the Week Archive

2017
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.