And a reply from a college professor specializing in Egyptian symbolism.
Thanks for writing. I'll see what I can do to help. My first instinct in looking at the papyrus is to say that it's a lotus flower, but the leaves of the lotus tend to be wider and more closely clumped in Egyptian art than the figure on this papyrus. There's an outside chance that it's a subspecies of lotus, but I doubt it, as depictions of the lotus remained fairly constant throughout Egyptian history.
I can concieve of three ways in which you may be able to discover what the object is, but they may require a good bit of research, so it depends on how important this identification is to the thesis of your research. I don't know if you're already gone down these paths, but these may be good avenues to pursue:
1. The caption underneath the image mentions that this is a vignette from the Book of the Dead. You might access a translation of the Book of the Dead (I would recommend Raymond O. Faulkner's, and that you steer clear of E.A. Wallis Budge at all costs) and try to match up this vignette with the images in Faulkner's book. Faulkner's translation includes tons of pictures, so you're likely to find it, and the presence of Nut will make this image really stand out when you're flipping through the pages. Once you find the image, there may well be a description of the object in the translation; as you may know, the Egyptians were incredibly descriptive when they wrote religious texts, much to the advantage of modern day researchers
2. The caption mentions the name of the Egyptian whom this vignette is written for, and that it is housed in the Egyptian Museum of Cairo. You might go to the Egyptian Museum's website and see if you can do an online search of their collections, by typing in the name of the Egyptian. Depending on the quality of their catalogue records, there may be a description of the sphinx and the object on its back.
3. If you can't find the object on the Egyptian Museum website, you might Google the name of the Egyptian mentioned in the caption. Be sure to do it in quotes --- "[Name of Egyptian]", and you may find that other researchers have used the papyrus in their studies. Maybe they have analyzed the image for its artistic symbolism. I doubt you'd have to comb through many search results, as this papyrus is probably the only mention of this Egyptian's name that has survived antiquity.
4. If all else fails (or, maybe you could start with this), you can always track down a professional Egyptologist who specializes in papyrology and ask them the same question. They may or may not respond, however --- academics can be fickle helpers.
In reference to your crackpot author, he's no doubt one of the many conspiracy theorists who also believe that aliens built the pyramids for the Egyptians, or some other such nonsense. However, if you're trying to use this specific papyrus to refute his argument, I would caution you --- the sphinx in this papyrus is not likely *the* Sphinx of Giza. Rather, the Egyptians were probably using the sphinx form in a metaphorical sense (as can be seen in the many sculptures of pharaohs as sphinxes), or it may be a representation of one of the goddesses who appeared in sphinx form in mythology, such as Sekhmet.
However, refuting a claim that UFOs landed on the back of the Sphinx shouldn't be too hard to do otherwise --- what a claim! The burden of proof is definitely on the guy who thinks aliens hung out with Khufu.
If you do discover what the object on his back is, let me know. I'm always eager to add another piece of information to my Egyptian symbolism repetoire.
I'm glad my research was of assistance, and I was happy to help with your query. Good luck with your detective work!