I wonder if it would be cheaper and less hassle to buy a listing with Google?
EDIT: or maybe not, I just had a read of their "About" page. The best you could do would be one of the paid for adds that are search term related. One off payment and a cost per click.
How about advertising in the Bintel newsletter?
On a related note, I just noticed they used one of my Aurora photos. I'm not really fussed at not being contacted before they used it, especially as there is no way to contact me listed on the web site they took it from. Still it was a bit of a shock to see it there.
https://www.bintelshop.com.au/NightSky/NightSky.pdf