Planet Earth now visible in STEREO Behind’s Inner Heliospheric Imager

STEREO spacecrafts are moving closer to the points in their orbits at which they will be directly opposite Earth on the other side of the Sun. Earth has been visible in the HI2 telescopes since launch, but this is the first time it’s been visible in either of the HI1 telescopes, which image areas closer to the Sun.

The STEREO Behind spacecraft has now moved far enough in its orbit for Earth to enter the HI1-B field-of-view. Earth will be soon visible in STEREO Ahead’s HI1 and in the coronographs.

It is expected that STEREO spacecrafts pass behind the Sun in 2015 (from the point of view of the two spacecrafts). There will be substantial periods during the year 2015 when one or the other STEREO spacecraft will be out of contact because its apparent angular distance from the Sun is too small, or it’s even actually behind the Sun. The current estimates are that the Ahead spacecraft will be out of contact between March 17 and July 14, 2015. The Behind spacecraft has two periods when it is expected to be out of contact that year, first between January 20 and March 28, and again between August 25 and October 19. Fortunately, there’s only a short period, March 17-28, 2015, when both STEREO spacecraft are expected to be out of contact.

After the two STEREO spacecraft pass behind the Sun, they’ll essentially be “upside-down” from their normal orientation. This is done to keep the high gain antenna pointed towards Earth. At the same time, the SECCHI Heliospheric Imager cameras will remain pointed at the region between the Earth and the Sun.

STEREO/SECCHI Imagers: Fields of View

Each STEREO spacecraft carry five imaging instruments and they are known as STEREO’s SECCHI imaging suite. SECCHI can image all the way from the Sun to the orbit of Earth.

There are three sun centered imagers: EUVI (extreme UV light out to 1.7 times the Sun’s radius), Cor1 (visible light coronagraph which observes from 1.5 to 4 solar radii) and Cor2  (visible light coronagraph which observes out to 15 solar radii) and there are two Heliocentric Imagers (HI), visible light imagers which are pointed away from the Sun: HI1 (observes visible light from the outer part of Cor2′s field of view to about a third of the distance to Earth’s orbit) and HI2 (completes the Sun-Earth view by extending the view to Earth’s orbit).

STEREO/SECCHI Home Page

The STEREO A and B orbits from Oct. 2006 to Oct. 2019 (animation)

Source: STEREO/GSFC

Featured image: Earth seen on STEREO B on December 17, 2012 (Credit: STEREO/GSFC)

Planet Earth now visible in STEREO Behind’s Inner Heliospheric Imager

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