It’s space-probe celebration time again, boys and girls, as
we mark the 32nd anniversary of Mariner 10’s first flyby of the planet Mercury.
On March 29, 1974, Mariner 10 became the first—and, to date, only—spacecraft
ever to make a survey pass of our solar system’s innermost planet, achieving
but one of its myriad aerospace distinctions in the process. Mariner 10 made
two additional passes at Mercury—on Sept. 21, 1974 and March 16, 1975—gathering
landmark photographic data along the way.

So what’s the big deal about 32-year-old pics of a nearby
planet? The Hubble Space Telescope has snapped images of extrasolar
planets, after all.

Well, until Mariner made its flybys, no one had been able to
take a direct look at Mercury because of the planet’s extreme proximity to the
sun. In simplest terms—way simpler than the intricate
astronomic-optical-physics explanation—the solar glare was simply too bright
for direct Mercurial observation.

That all changed with Mariner 10, which snagged roughly 2,800
images of Mercury during its trio of flybys and revealed an arid,
crater-blasted surface similar to Earth’s moon. The similarity may end there, however,
as Mercury has an appreciable helium atmosphere and a relatively significant
magnetic field, thanks to its iron-infused core. And Mariner 10’s observations
helped either discover or confirm all of these basic facts about Mercury.

Yet, while Mercury made Mariner 10 famous (among space
program aficionados, anyway), the probe also enjoys some technical distinctions
separate from the first rock from the sun. First, Mariner 10 was the first
space probe that successfully observed more than one planet, grabbing impressive
images of Venus in addition to its work with Mercury. But it’s what Venus did
for Mariner 10, rather than vice versa, that should truly raise some
astrophysical eyebrows.

Mariner 10 was the first spacecraft to use a so-called gravitational slingshot as part of its
interplanetary flight plan. Rather than expending fuel to undertake a course
correction, Mariner 10 intentionally used Venus’s gravity to pull itself in the
direction of Mercury. Numerous space probes have made similar use of the
gravitational slingshot since the Mariner 10 mission, but Mariner was the first
to employ the technique.

What’s more, the gravitational slingshot was not the only
first-of-its-kind unconventional navigational tactic employed by Mariner 10. It
was also the first spacecraft to use another propulsive method—one that’s still
elusively experimental even today.

WHAT STILL-EXPERIMENTAL PROPULSIVE TECHNIQUE WAS MARINER 10
THE FIRST SPACECRAFT TO USE?

What still-experimental propulsive technique was Mariner 10
the first spacecraft to employ, adding an often overlooked distinction to its
still-revered space exploration service record?

Mariner 10 was the first spacecraft to intentionally use
radiation pressure as a propulsive method—the same basic principle behind solar
sail propulsion. To date, no one has been able to successfully design, build,
and test a craft that uses radiation pressure as a primary means of propulsion,
but several space probes have used radiation pressure to alter course—and
Mariner 10 was the first spacecraft to pull off the feat.

For those unfamiliar with the radiation pressure/solar sail
concept, it breaks down like this: Anytime electromagnetic radiation impacts a
surface, it imparts an almost infinitesimal amount of kinetic energy. The more
reflective the surface (relative to the band of EM radiation involved), the
more kinetic energy imparted from the EM bounce.

In everyday life, this kinetic energy is utterly irrelevant,
but in the microgravity of space, this force is sufficient to move objects.
Indeed, given enough time, radiation pressure can actually cause some
incredibly massive bodies such as asteroids to experience rotational spin or
even alter their solar orbits—a phenomenon known as the Yarkovsky Effect. (Don’t let the Yarkovsky reference fool you—this is the extremely dumbed-down
version of how radiation pressure works.)

The trick to harnessing radiation pressure is getting a wide
surface area with a low mass, so that the maximum amount of electromagnetic
radiation—such as light from the sun or a high-energy laser
cannon—bounces off the object or craft, but the craft itself isn’t too
heavy to move. That’s why you need a wide, thin, light, highly reflective
surface—a solar sail—to make
radiation pressure propulsion work. So far, no one has been able to build a
large enough and light enough design that can use radiation pressure alone to
move a useful spacecraft.

For its part, Mariner 10 used its solar panels and high-gain
dish antenna as crude solar sails, tacking through the solar wind to make
course corrections on its way to the inner planets. Several other probes have
used similar tactics in the intervening years, but Mariner 10 was the first
craft ever to do so, earning it a place in the space propulsion hall of
fame—and the annals of Geek Trivia.

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The Quibble of the Week

If you uncover a questionable fact or debatable aspect of
this week’s Geek Trivia, just post it in the discussion area of the article.
Every week, yours truly will choose the best post from the assembled masses and
discuss it in the next edition of Geek Trivia.

This week’s quibble comes from the March 15 edition of Geek
Trivia, “Me,
myself, and Ides.” TechRepublic member Creyes53 busted me for my anachronistic understanding of Roman
numeral notation.

“You say, ‘Thus, just as IV would mean one before five (or four)’.
This implies the Roman use of the letter V.
Not so! The Romans would write five as IIIII
and nine as IIIIIIIII. The V was a 17th-century
invention.”

You’re at least partly right, Creyes53, as ancient Romans
wrote four as IIII in ancient Rome—not
as IV. V, or some other symbol for the distinct notation of five, was in use, however. There are various
theories as to why there was an exception for four—a prominent one holding that
the IV was shorthand for the god
Jupiter, making it inappropriate to use IV to notate four.

In any case, you get half-credit on the quibble, and you’ve
given me a great subject for a future column. Thanks for the ideas, and keep
those quibbles coming!

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The Trivia Geek, also
known as Jay Garmon, is a former advertising copywriter and Web developer who’s
duped TechRepublic into underwriting his affinity for movies, sci-fi, comic
books, technology, and all things geekish or subcultural.