Hubble Snaps a Photo of the Crater Tycho on the Moon

The entire universe in blog form
March 10 2013 8:00 AM

Hubble Shoots the Moon. Again.

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.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

One of the most frequent is that it can’t observe the Moon, because our natural satellite is too bright. Trying to snap a shot of it would damage Hubble’s detectors.


That’s 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:

Hubble picture of Tycho
Hubble was used to take this dramatic picture of the lunar crater Tycho. Click to embiggen.

Image credit: NASA, ESA, and D. Ehrenreich (Institut de Planétologie et d’Astrophysique de Grenoble (IPAG)/CNRS/Université Joseph Fourier)

That’s the crater Tycho, arguably the most famous on the Moon. First, it’s 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. They’re 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 there’s 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, here’s a picture of Tycho taken by the Lunar Reconnaissance Orbiter, looking straight down on the massive impact site:

LRO image of Tycho
The Lunar Reconnaissance Orbiter took this picture of the crater Tycho looking straight down. Click to enlunenate.

Image credit: NASA/Goddard/Arizona State University

See? Round. And mind you, what you’re 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 wouldn’t be here to talk about it. That asteroid was probably bigger than any mountain on Earth.

I’ll 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:

Another LRO shot of Tycho
From an angle, LRO was able to see the central mountains in Tycho cast long shadows. Click to penumbrenate.

Image credit: NASA Goddard/Arizona State University

So there you go. The Moon is not too bright for Hubble. Funny though, it is hard to observe by HST, but that’s actually because it’s moving too fast in the sky. Hubble isn’t designed to track that quickly, so what they do to observe it is put it in “ambush mode”: 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 planet’s 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 planets’ atmospheric constituents. It’s a clever idea.

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



Slate Plus Early Read: The Self-Made Man

The story of America’s most pliable, pernicious, irrepressible myth.

Rehtaeh Parsons Was the Most Famous Victim in Canada. Now, Journalists Can’t Even Say Her Name.

Mitt Romney May Be Weighing a 2016 Run. That Would Be a Big Mistake.

Amazing Photos From Hong Kong’s Umbrella Revolution

Transparent Is the Fall’s Only Great New Show

The XX Factor

Rehtaeh Parsons Was the Most Famous Victim in Canada

Now, journalists can't even say her name.


Lena Dunham, the Book

More shtick than honesty in Not That Kind of Girl.

What a Juicy New Book About Diane Sawyer and Katie Couric Fails to Tell Us About the TV News Business

Does Your Child Have Sluggish Cognitive Tempo? Or Is That Just a Disorder Made Up to Scare You?

  News & Politics
Sept. 29 2014 11:45 PM The Self-Made Man The story of America’s most pliable, pernicious, irrepressible myth.
Sept. 29 2014 7:01 PM We May Never Know If Larry Ellison Flew a Fighter Jet Under the Golden Gate Bridge
Dear Prudence
Sept. 30 2014 6:00 AM Drive-By Bounty Prudie advises a woman whose boyfriend demands she flash truckers on the highway.
  Double X
Sept. 29 2014 11:43 PM Lena Dunham, the Book More shtick than honesty in Not That Kind of Girl.
  Slate Plus
Slate Fare
Sept. 29 2014 8:45 AM Slate Isn’t Too Liberal, but … What readers said about the magazine’s bias and balance.
Brow Beat
Sept. 29 2014 9:06 PM Paul Thomas Anderson’s Inherent Vice Looks Like a Comic Masterpiece
Future Tense
Sept. 30 2014 7:36 AM Almost Humane What sci-fi can teach us about our treatment of prisoners of war.
  Health & Science
Bad Astronomy
Sept. 30 2014 7:30 AM What Lurks Beneath The Methane Lakes of Titan?
Sports Nut
Sept. 28 2014 8:30 PM NFL Players Die Young. Or Maybe They Live Long Lives. Why it’s so hard to pin down the effects of football on players’ lives.