Why the Hubble Space Telescope can't see the flags on the Moon

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The manned missions to the Moon of the Apollo program one of the main icons are the U.S. flags planted and raised six times by the astronauts; observing them directly would be a very important evidence to clarify the doubts that may arise in some people about the veracity of the facts. Our The main observer of the universe is the Hubble space telescopebut nevertheless he is not able to see them, why?

About the flags

The main motive behind the manned missions to the Moon was to demonstrate the industrial and economic power of that potential that would succeed in getting a crew to the surface of the Moon first and bring them safely back to Earth. For this reason one of the tasks of the astronauts was to raise high the flag of their country, in this case the flag of the United States of America.

From the images and videos of them a lot of comments arise looking for denying the moon landings and using as an argument an alleged error, that of a flag apparently waving in the wind despite there being no atmosphere on the Moon..

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Jack Schmitt posing next to the U.S. flag and in front of the Apollo 17 Rover and Lunar Module.

Often a static photo of a flag is shown next to an astronaut, a first impression may give the impression of being waving due to the wind, but when the image is detailed it is easy to see a mast that holds the flag from the top, this allows it to remain raised.

A single image does not really allow us to be sure if it is flaming, for that we need to watch a video.

The video is accelerated in order to show the movement of the flag over a long time interval and to verify that it remains static once the astronauts disturb it when they touch it.

Observing the Moon with Hubble

The Hubble Space Telescope is one of NASA's most iconic achievements. Since its launch more than 30 years ago, it has given us eyes to explore the far reaches of the universe. It has captured many images of the planets of the solar system, nebulae, star clusters, galaxies and an incredibly long etc., anyone would think that being able to observe objects hundreds, thousands or tens of millions of light years away should have no problem with something we have right next door.

An analogy that allows us to easily understand this is to compare our ability to observe an ant at about 100 meters and a mountain at 10 kilometers, the first will be incredibly difficult to see (not to say it would be impossible), the second despite being much farther away we can see it without much difficulty. Something similar happens with Hubble, galaxies are billions of light years away, but they are hundreds of thousands of light years in size and are composed of stars that make them very bright.. On the other hand, the planets, the Moon and the flags despite being, in comparison, much closer are also much smaller. In this case it would be useful to use some unit of measurement to easily compare different objects.

In the case of sky observations it is very useful to work not with real sizes of the objects, but with how much space they occupy from our vision, this new measure is known as angular size.. Assuming the sky as a sphere we can divide the horizon into 360 equal parts, these are called degrees of arc. Then we will take one of them and divide it into 60 equal parts that we will call minutes of arc, repeat the process and get the seconds of arc.

The smallest angular size capable of being seen by any device is known as resolution, for Hubble this takes a value of 0.05 arcseconds (for comparison the human eye has an angular resolution of 40 arcseconds).that is, each pixel represents 0.05 seconds of arc in the sky. For the Moon this represents approximately 90 meters, therefore, to be able to resolve any human object it would have to be more than 180 meters long.

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The Moon's largest crater, Tycho, is photographed by Hubble. Credits: NASA/ESA

Angular sizes of objects observed by Hubble

Just as the misconception of the observatory's ability to observe the lunar surface in sufficient detail to observe the flags is believed we should be able to see with enormous resolution the other planets, however, it could not be further from the truth.

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The 8 planets of the solar system and Pluto with scale sizes. Credits: NASA

Due to the elliptical trajectory of the bodies around the sun, the distance to the Earth changes during the course of the year, and so do the angular sizes; the differences are more noticeable with nearby planets such as Venus or Mars.. Despite appearing to be very similar in size, each pixel represents very different lengths.

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Planets and Pluto under the same scale as seen from Earth

Other ways to view the flags

Although Hubble may not actually have the capability to observe the flags or debris left behind from the Apollo missions to the Moon there are other alternatives to see it. One of them is the NASA Lunar Reconnaissance OrbiterThe satellite, reaching up to 50 kilometers above the surface, allows observation with a resolution of up to 0.5 meters per pixel. 

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Apollo 11 lunar landing site as seen from lunar orbit by the LRO, the debris left behind and the descent stage of the lunar module can be seen. Credits: NASA

Other countries such as India with Chandrayaan-2 and China with Chang'e have provided direct images but with lower resolution.

Francisco Andrés Forero Daza