As I wrote in BAN 789, the European Space Agency recently launched Hera, a space probe that will travel to the asteroid Didymos and its moon Dimorphos, the target of the NASA impactor DART. Hera is a follow-up mission designed to look at the aftermath of DART slamming into Dimorphos at high speed. It will examine the teeny moon’s surface, including the crater DART made, and look to see what changes have occurred in the system. 

As I also showed you in that issue, Hera took some images of the Earth and Moon on its way out. It has several cameras on board, including an infrared one called TIRI, for Thermal Infrared Imager. That camera took quite a few images as the spacecraft headed away from us, and what’s new now is the folks at ESA made a cool video showing the moon’s motion, which is a combination of the moon’s orbital motion around Earth plus the changing perspective of the spacecraft as it moved away.

The video spans the dates October 10 – 15, and the spacecraft moved from 1.4 million to 3.8 million km away (the Moon is 380,000 km from Earth for comparison). If you look closely you can see the Moon is half full; the Sun was shining from the right. As I explained in that previous issue, Earth still looks full because it’s warm! The night side of Earth only cools off a little, so it still glows in infrared. The Moon rotates much more slowly, so the night side cools off a lot, and is much fainter in IR.

On October 23, the spacecraft fired its three main thrusters for 100 minutes, putting it on a trajectory for Mars. Once at the Red Planet in March 2025 it will use a gravity assist to catch up with the binary asteroid at the end of 2026.

I cannot wait to see the images once it gets there. These asteroids are fascinating, and DART really put the hurt on Dimorphos. If we ever see an asteroid headed for Earth, we need to know exactly what to do to push it out of the way, so the results from Hera could literally save the world.

So have astronomers found a runaway supermassive black hole creating stars in its wake, or just a reeeeeeeeally long flat bulgeless edge-on disk galaxy?

OK, let’s back up. I first wrote about this a year ago; astronomers found a long, thin streak of light in an image that pointed right toward a distant galaxy. Given its weird structural details, they thought it might be due to a black hole that got tossed out of the galaxy. Every big galaxy has a supermassive black hole, and if two galaxies collide and merge, it’s possible to eject the black holes. The most likely way is if two black holes come together to form a binary system, orbiting each other, and a third big one comes along and interacts with the pair. Both the binary and single third black hole can then get tossed out of the galaxy by the powerful gravitational slingshots involved, heading away in opposite directions. 

As it plows through the thin gas between galaxies, stars could form in its wake, accounting for much of what they saw.

Another team of astronomers disputed this, though, saying it could be an extremely flat galaxy seen edge-on to us. Most disk galaxies, like our Milky Way, have a bulge in the center, but not all do. This kind of galaxy isn’t common, but also not so rare that you could exclude it. The first team rebutted this claim, but the status of the situation has remained a question mark.

However, new observations taken by the second team seem to support the idea it’s not a runaway black hole. They took very deep Hubble images, using filters where hot gas excited by the passage of the black hole would be brightest. They were looking for two features: a bow shock at the end of the feature where the black hole was still plowing through material, and a “counter wake”, looking for the stream of stars where the other black hole was ejected in the opposite direction.

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