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SYFY WIRE Bad Astronomy

Hubble Shoots the Moon. Again.

By Phil Plait

I worked with Hubble Space Telescope (HST) for many years. First it was with early data (taken weeks after launch) for my PhD research, and then several years helping to build and calibrate a camera on board called STIS. Working with HST (as those of us in the know call it) and doing as much outreach as I do, I learned quickly that there are a lot of misconceptions about the orbiting observatory.

One of the most frequent is that it canât observe the Moon, because our natural satellite is too bright. Trying to snap a shot of it would damage Hubbleâs detectors.

Thatâs not true. Well, not totally true. Some cameras on HST are very sensitive, and could be damaged if pointed to a bright source. The ultraviolet camera I worked on was so sensitive it would fry if it looked some kinds of stars too faint to even see with the naked eye!

But other cameras are just fine with bright sources, and that includes the Advanced Camera for Surveys. On Jan. 11, 2012, it took this pretty amazing picture of the Moon:

Thatâs the crater Tycho, arguably the most famous on the Moon. First, itâs pretty easy to spot near Full Moon with just binoculars; plumes of material that splashed out when the crater formed over 100 million years ago fell back to the surface, creating long streamers called rays that radiate out from the crater. Theyâre bright and obvious, and delightful through a small telescope. You can see a hint of them in the Hubble picture.

Also, Tycho was where the Monolith was found, buried 4 million years ago by extremely advanced aliens. So thereâs that.

Tycho is actually quite round. It only looks elliptical in the Hubble image because the telescope saw the crater at an angle. Judging from the short axis to long axis ratio, it was pretty close to 45°. Notice that the craters around it are similarly distorted. For proof, hereâs a picture of Tycho taken by the Lunar Reconnaissance Orbiter, looking straight down on the massive impact site:

See? Round. And mind you, what youâre seeing is huge: Tycho is more than 85 kilometers (53 miles) across! If whatever hit the Moon to form Tycho had instead hit the Earth, we wouldnât be here to talk about it. That asteroid was probably bigger than any mountain on Earth.

Iâll note that this is a wide-angle image from LRO. It also has a camera that has more magnification, and it took this, one of my favorite pictures of the Moon of all time, showing the mountains in the very center of Tycho:

So there you go. The Moon is not too bright for Hubble. Funny though, it is hard to observe by HST, but thatâs actually because itâs moving too fast in the sky. Hubble isnât designed to track that quickly, so what they do to observe it is put it in âambush modeâ: Aim Hubble in the sky where the Moon will soon be, then wait. When the Moon moves in, Hubble grabs the snapshot. This has been done many times, actually (like in 1999 and 2005).

In this case, the shot of Tycho was taken as preparation for the transit of Venus last year. I know, it sounds weird, but the idea was that when Venus passed in front of the Sun, sunlight would be transmitted through the atmosphere of Venus. The different molecules in the planetâs air would then selectively absorb very discrete colors of sunlight. Astronomers hoped that fingerprint would be visible in their observations of the Moon, lit by that same Venusian-filtered sunlight. In this way, they might be able to make similar observations when exoplanets (alien worlds) transit their own stars as seen from Earth, possibly leading to a detection of those planetsâ atmospheric constituents. Itâs a clever idea.

And, Iâll note, it was done using STIS, the camera I worked on! So itâs neat to see this go full circle.

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