Since mid-April, the STEREO-A spacecraft (more specifically its Heliopsheric imagers, HI) has been exposed to a spectacular view of the Sky! Indeed, not only is it observing a good portion of the Solar System’s planets, but the telescopes are pointed towards the heart of the Milky Way! In this blog post I dive into detail regarding the objects that STEREO-Ahead’s HI instruments are currently observing (planets and deep sky objects).
Remark: This blog post was originally intended to be published a week ago, at the time when STEREO/SECCHI images from May 6th were the most recent. Since then, many more images have appeared! To keep this post “up to date”, I’ve consequently added a few extra figures with STEREO/SECCHI images taken later than May 6th (the latest of them being from May 12th). These newer figures are marked with an asterisk “*”.
Below are individual STEREO/SECCHI HI1-A and HI2-A images taken on May 6th, 2018. As one can see, the Milky way’s galactic plane, along with several planets are clearly visible, even prominent!
Figure 1: Beautiful view of the Solar System and the Milky Way as seen by the HI1-A telescope. The vertical streaks associated with the planets are caused by saturation (pixel bleading). Image credit: NASA/NRL STEREO/SECCHI HI1-A.
Figure 2 Beautiful view of the Solar System and the Milky Way as seen by the HI2-A telescope. The vertical streaks associated with the planets (and one star) are caused by saturation (pixel bleading). Image credit: NASA/NRL STEREO/SECCHI HI1-A.
The relative motion of the planets as seen by STEREO HI
Over the course of April until mid-May, six planets transited the HI FOV. These include all the inner planets (Mercury, Venus, Earth and Mars), as well as Jupiter and Saturn. This does not include Neptune, which was only briefly visible in rolled STEREO data (from May 8th). Their brightness and/or motions differ drastically due to their individual orbital periods, as well as their relative distance from the STEREO-A spacecraft. Figure 3: Timelapse showing the relative motion of the planets as seen from the STEREO-A spacecraft, with the heart of the Milky Way laying in the background. Notice the solar corona on the left side of the images. Image credit: NASA/NRL STEREO/SECCHI HI1-A.Figure 4: STEREO/SECCHI COR2-A image showing Mercury. Pluto is also located in the FOV, but its brightness is way below the limiting magnitude of the COR2-A instrument to be detected. Notice the Solar corona radiating from behind the occulting disk. Image credit: NASA/NRL STEREO/SECCHI COR2-A.
As can be seen in figure 3, 5 and 6, Mercury rapidly arcs around the Sun (leaving the HI1-A FOV in late April) and heads into the COR2-A FOV by May (see figure 4). In comparison to the other five planets, Mercury has a rapid motion, while also moving in the opposite direction. This is caused by the planet’s significantly shorter orbital period in comparison to that of the STEREO-A spacecraft (88 days versus 346 days), as well as its location near the opposite side of the Sun relative to STEREO-A.
Despite Venus having a relatively similar velocity to STEREO-A (orbital period of 243 days), its apparent motion is very rapid due to the planet’s proximity with the spacecraft. It enters the HI1-A FOV in late May, and then the HI2-A FOV only a week later.
Earth only appears in the HI2-A FOV, with no apparent motion. This is due to the spacecraft having been purposefully placed in an orbit quasi-identical to it (orbital period: 365 days). Earth appears significantly fainter than Venus, due to its greater distance from STEREO-A.
Despite being larger than Mercury, Mars appears alot fainter due to its greater relative distance from the spacecraft. Its slow motion relative to the apparent motion of the other planets is due to its rather similar orbital period to STEREO-A (only about twice as long).Figure 5: The motion of the inner planets and the STEREO spacecraft relative to Earth. Knowing that STEREO-A and STEREO-B moves practically at the same velocity as Earth, one can interpret this as being the relative motion of the planets as seen from both STEREO-A and STEREO-B. Image credit: NASA/SSC STEREO Orbit tool
With Jupiter and Saturn being located much further away from the spacecraft relative to the inner planets, their apparent brightness does not [at all] reflect their intrinsic luminosity. Indeed, Jupiter appears similar to Earth, while Saturn‘s brightness is comparable to that of Mars! The rapid motion of both these planets reflects their great distance from the spacecraft, relative to the inner planets. In fact, Saturn has such a large distance from the spacecraft that its relative motion is comparable to that of the background star field! Figure 6: The motion of the outer planets and the STEREO spacecraft relative to Earth. Knowing that STEREO-A and STEREO-B moves practically at the same velocity as Earth, one can interpret this as being the relative motion of the planets as seen from both STEREO-A and STEREO-B. Image credit: NASA/SSC STEREO Orbit tool
By combining the HI1 and HI2 images, one can create a “family portait” of the different planets visible in those images. My attempt of creating such an image is shown in figure 7a. Note that the planets all show vertical lines, due to saturation. The star Antares (in the HI2 FOV) is the only non-planetary body bright enough to saturate the HI cameras (which is currently visible in STEREO/SECCHI). An updated version of this “portrait” is shown in figure 7b.Figure 7a: Image extract from the composite figure 8. It’s an HI “Family portait” of the Solar system, the the Milky Way‘s galactic plane in the background! Mercury is located in the COR2-A FOV (figure 4). Uranus and Neptune are located outside SECCHI FOV. Image credit: NASA/NRL STEREO/SECCHI HI-A and Trygve Prestgard.Figure 7b*: Update: This is an HI2-A image extract from May 12th (latest available high-quality HI2 image at the time of this blog post). At this date all the five planets shown in figure 7a had entered the HI2-A FOV. Note that this is an exctract from figure 2b. Image credit: NASA/NRL STEREO/SECCHI HI2-A.
Figure 8*: The faint presence of Neptune in rolled STEREO data from May 8th. Image credit: NASA/NRL STEREO/SECCHI HI2-A.
STEREO’s current view of the Milky Way
Perhaps more spectacular than the six planets described above is the is Milky Way‘s star-rich galactic plane and dusty star-forming regions! Indeed, the heart of our home galaxy covers a significant portion of the HI FOV, as one can see in figure 9! Figure 10 is a combination of image extracts I put together from the Mellinger survey, meant for comparison with figure 9.Figure 9: Combined and enhanced HI1-A and HI2-A images meant to bring out the Milky Way. Image credit: NASA/NRL STEREO/SECCHI HI-A and Trygve Prestgard
Figure 10: Composite images from the Mellinger survey of the same region currently observed by STEREO/SECCHI HI. Image credit: Mellinger Aladin Lite.
Some Deep sky objects currently visible in STEREO
When looking closer at the Milky Way, other then the million stars, one can distinguish some quite obvious and interesting deep sky objects, including star clusters and nebulae. Figure 11 shows some examples of such objects.Figure 11*: Combined image showing several Deep Sky Objects, including Antares (and its neighbouring nebulae), the dark nebula complex that includes LDN 52, Dobashi 47 and 48 (among many other!), the active star forming regions M8 and M20, and the visual asterism Ferrero 2. Image credit: NASA/NRL STEREO/SECCHI HI and Trygve Prestgard.
Figure 11, 1 is an HI2-A image extract from May 12th showing the Rho Ophiuchus complex. This is an immense region of star formation, hence it includes numerous HII regions and dark nebulae. The latter are clearly visible in this HI2 image extract, just north of Antares. Antares is one of the brightest objects in the night sky, with a visual magnitude +1 (but reaching negative magnitudes in the near-Infrared!). The star’s brightness is such that it saturates the STEREO HI detectors, causing vertical “spikes” to appear ( due to pixel bleeding). In figure 11, 1 one can also see the reflection nebula asociated to this star (faint “cloud” surrounding Antares).
Another complex system of dark nebulae, similar to the ones observed in Figure 11, 1, can be seen in Figure 11, 2. This vast cluster of dark nebulae include LDN 52, Dobashi 47, Dobashi 48, among many others!
M8 (Lagoon Nebula) and M20 (Trifid Nebula) are a couple of bright star forming regions in the Sagittarius constellation. Both of these nebulae have been known for centuries, and are faintly visible to the naked eye (best seen in the Southern Hemisphere). In STEREO HI their nebulous nature is obvious, as one can see in Figure 11, 3!
Ferrero 2 was discovered in the early 2000s by amateur astronomer Laurent Ferrero (France) using a 60mm refractor. The brightest stars of this asterism are just below naked eye visiblity, hence in STEREO-A HI they are very obvious! As one can see in figure 11, 4, it’s a very well defined group of stars, in the shape of an irregular polygon.
The Small Magellanic Clouds (SMC) and Large Magellanic Cloud (LMC) are the two dwarf satellite galaxies of the Milky Way. The former is an irregular galaxy while the latter is a disrupted barred spiral galaxy. Both these galaxies are visible to the naked eye, and hence, they popular visual targets (in the Southern Hemisphere). During the roll of the STEREO spacecraft on May 8th, the HI2 camera was briefly pointed towards these objects. STEREO (accidentally) caught two images of them (one of those being figure 12).
Figure 12*: HI2-A image taken during a STEREO “roll”. Note the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). These correspond to the fuzzy patches on the right side of the FOV (SMC to the left, and LMC to the right). The galactic plane of the Milky Way is partially visible at the very top of the FOV.
Wikipedia (2018) STEREO, Available at: https://en.wikipedia.org/wiki/STEREO
SIMBAD (2018) Antares, Available at: http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Antares&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id
Wikipedia (2018) Lagoon Nebula, Available at: https://en.wikipedia.org/wiki/Lagoon_Nebula
Wikipedia (2018) Trifid Nebula, Available at: https://en.wikipedia.org/wiki/Trifid_Nebula
Ferrero, L. (2018) Mon catalogue d’amas d’étoiles, Available at: http://splendeursducielprofond.eklablog.fr/mon-catalogue-d-amas-d-etoiles-p563496