Astronomy

ASTROLETTERS

8 ASTRONOMY • DECEMBER 2018

Image vs. impression

In your Snapshot article, “The first

interstellar asteroid,” in the April issue,

you commit the cardinal sin of not label-

ing an artist’s drawing as such. Neither

the caption nor the story indicates that

the picture of ‘Oumuamua is an artist’s

impression, not a real image. I’m sure you,

too, have been annoyed at mainstream,

non-scientific news sources implying that

we’ve captured actual images of objects,

like Earth-like exoplanets. Please assure

me that this was a one-time oversight.

— Ray Gedaly, The Woodlands, TX

The concept that a speed greater than the

escape velocity is needed is only valid in a

one-shot deal, after which your rocket

then coasts on its own.”

So, you are exactly right: Escape veloc-

ity is the speed needed to achieve, in one

shot, escape from the surface of a body.

Rockets keep firing over time, so they can

move slower than the escape velocity and

still break free of gravity’s pull, given

enough time. Because rockets are so famil-

iar a concept and the option we currently

use to achieve spacef light, they seemed the

natural example in this case. Next time,

we’ll try to illustrate this concept in a way

that leaves less opportunity for confusion.

— Alison Klesman, Associate Editor

An undeniable talent

Praise should be given to illustrator Roen

Kelly for organizing so much information

in an understandable and eye appealing

way. The rocket summary on page 20 of

the August issue is a work of art.

— James McLeod, Charlotte, NC

Escape velocity and acceleration

I was reading an article on page 13 of

the July 2018 issue of Astronomy that’s

titled “How fast must a rocket travel to

leave each planet?” but that’s not the

same as escape velocity, which is what

the speedometer shows. A rocket doesn’t

have to travel 7 miles a second to leave

Earth because it has constant accelera-

tion to counteract the effects of gravity.

Escape velocity is the speed to leave the

atmosphere if there’s no added energy.

The title is a bit misleading since rockets

have perpetual thrust. — Alexander Zaleski,

Greenwich, CT

Astronomy responds

Your letter harks back to a statement

made by Bob Berman in his January 2018

column, “Earth’s gravity: A downer?” In

it, he writes about a debate he had with

the astrophysics chair at Columbia

University over this very point. He clari-

fies the issue succinctly by stating,

“Escape velocity simply doesn’t apply if

you’re supplying further energy to the job.

ESO/M. KORNMESSER

We welcome your comments at Astronomy Letters, P. O. Box 1612, Waukesha, WI 53187;

or email to [email protected]. Please include your name, city, state, and country.

Letters may be edited for space and clarity.

32 Elkay Drive, Chester, New York 10918 845.469.4551 http://www.TeleVue.com

A Family of Great Products,

From a Family Business,

For Your Family

to Enjoy!

With over forty years of design-

ing and building our vision for

eyepiece and refractor perfor-

mance, along with photo/visual

accessories to match, Tele Vue

continues its mission to make your

observing ”...even better than

you imagined.” Call for expert

and accurate technical advice.

The best $

eyepiece you’ll

ever buy.

No computer required. Battery-

powered 7” color monitor included.

High Point Scientic

Agena AstroProducts

Oceanside Photo & Telescope

Woodland Hills Telescope

Skies Unlimited

Orange County Telescope

REVOLUTION IMAGER

RevolutionImager.com

M16 Eagle Nebula

8” Celestron Evolution

Metropolitan Skies