TUNGESTEN VALLEY - We’ve been told there’s no such thing as a free lunch. We’ve been lied to. It exists, courtesy of some physics, and it’s a key element in moving around our solar system.

I’m talking about gravity-assisted (slingshot) maneuvers. They’re a way of increasing or decreasing spacecraft velocities merely by shooting the vehicles past planets or the moon at particular angles and very close distances.

Here’s how these shots work. A probe coasts through space, past a planet (or the moon). The probe is weightless; it feels no forces. Like a baseball after it’s been struck by a bat, its path is a free-fall conic-section, the shortest route possible in the space-time continuum. As seen from the planet, this is a hyperbola. It’s almost straight on its incoming and outgoing ends, but arcs sharply in the middle at its smallest distance (called the peri-point or periapsis) from the planet. 

Someone standing on the planet sees the probe gain speed as it’s incoming, hit maximal speed at the periapsis, and then lose speed as it’s outgoing. The incoming and outgoing speed profiles are identical.

It's the same as dropping a marble into a U-shaped trough. From a motionless start, the marble accelerates downhill, goes fastest at the bottom, symmetrically slows as it goes uphill on the far side, and comes to a halt at the top of the far side. Neglecting friction, the marble neither gains or loses energy. Similarly, the probe gets no net change in speed between the incoming and outgoing legs of its planetary encounter.

But we don’t care what the planet’s beings see. We fly space probes relative to the sun’s frame of reference, not the planet. Let’s look at the probe as seen by someone standing on the sun.

As the diagram shows, putting the planet’s own motion around the sun (its solar vector) into our picture changes everything. Summing the sun’s vector with the in-and-out planetary vectors gives different sizes for incoming and outgoing speeds! The sun’s frame “breaks the symmetry,” as physicists say. That’s our free lunch. The closer the periapsis distance, the bigger the net speed shift.

Full disclosure: The planet does lose or gain as much energy as the probe gains or loses. But planets are so massive that the effect on their own speeds is immeasurably small.

If the probe flies past the planet’s trailing edge, it gains energy, as in the diagram. If the probe flies in front of the planet in a leading-edge intercept, it loses energy. Flight planners use both methods to buzz around the solar system. Pioneer 10 and both Voyagers used trailing edge Jupiter intercepts to accelerate through the outer solar system and reach the outer planets much faster than if they had just used their own engines. Conversely, probes like Parker Solar and Bepi Colombo, to the sun and Mercury, are brought into lower-energy orbits in the inner solar system by flying energy-robbing leading-edge encounters with, say, Venus.

Lunar missions use leading edge gravity-assist encounters at the moon to reduce energy and get into lunar orbit, and then gain energy via trailing edge maneuvers for return to the earth. Here’s to the no-cost salad bar.

In November Skies:

The sun begins the month in Libra, entering Scorpius on November 25. At mid-month, days and nights are 9.5 and 14.5 hours long, respectively.

The moon’s dates are: Full (Beaver Moon) November 5; Last Quarter November 12; New November 19; First Quarter November 26.

Standard Time begins on November 2; we gain time as we move our clocks backward one hour at 2 a.m.

November Meteors: The North Taurid shower (parent comet Encke) peaks November 11-12. The Leonid shower (parent comet Tempel-Tuttle), one of the year’s best, peaks November 18.

Best Sky Viewing Nights (Minimal Moon): November 12-26.

Sunset (Mid-Month): The Milky Way glistens, southwest to northeast. Fomalhaut rises with the Pleiades and Capella. The Andromeda galaxy is high in the northeast, presenting a nice binocular view. Deneb is directly overhead, with Vega a bit westward and Altair southward.

Midnight (Mid-Month): Gemini twins Castor (above) and Pollux (below) are high in the east. Below them and to the right is Procyon. Orion and Sirius are in the southeast. The Great Nebula that births stars in Orion’s sword is naked-eye visible; it’s a terrific view in binoculars. Andromeda is high in the northwest. Brilliant Capella is nearly overhead; Cassiopeia is high in the northwest.

Sunrise (Mid-Month): Spica is low in the southeast; Arcturus is high in the east; the Big Dipper is overhead. Capella is high in the northwest as Orion sets.

Mercury, in Scorpius, is lost in the sun at sunset.

Venus, in Libra, is the Morning Star, low in the east at sunrise.

Mars, in Scorpius, is lost in the sun at sunset. 

Jupiter, in Gemini, rises at 9 p.m., is high in the east at midnight and is in the southwest at sunrise.

Saturn, in Aquarius, is high in the southeast at sunset, setting at 2 a.m.

Notable Space Missions: The U.S.-European Sentinel-6B satellite mission to track and record rapidly increasing global sea levels will launch on November 21 from Vandenberg AFB. Its data will also improve short-term hurricane intensity forecasts.

Frank Sanders, a spectrum scientist at the U.S. Department of Commerce in Boulder, takes astronomy-related inquiries at backyardastronomy1@gmail.com.