In “Mission: Risk Averse” [Graphic Science], John Matson expresses concern about the apparent risk-averse attitude at nasa leading to a preponderance of Mars missions at the expense of exploration of the rest of the more challenging parts of the solar system. I would suggest that there is another psychology at play: catering to the persistent hope of manned exploration. If such exploration is at all feasible, then Mars is the place (never mind that it is also an obvious dead end), and it is then natural to put so many eggs in the Mars basket.

But this reveals the deeper choice that confronts nasa: How long will the capability to do real science be hobbled by the fabulously expensive Buck Rogers fantasies of manned missions? Given human psychology, this will not be readily resolved.

John Gaffin
Myers Flat, Calif.


In “The End of Orange Juice,” Anna Kuchment suggests orange growers can best defend against the citrus disease huanglongbing (HLB) by growing genetically modified trees or by applying chemicals to existing trees. But she fails to consider a third, more sustainable option: interplanting orange trees with other crops, such as guava.

Guava trees emit compounds that repel the insects responsible for HLB's transmission, and growers in Asia have for years deterred these insects by planting guava trees among their orange trees. Based on controlled studies, researchers at the U.S. Department of Agriculture have indicated that this approach deserves more attention in the U.S.

Rob Schaaf
via e-mail


Who could object to developing cute little “RoboBees”—bee-size flying robots—as described in “Flight of the RoboBees,” by Robert Wood, Radhika Nagpal and Gu-Yeon Wei? The article highlights beneficial uses such as searching for survivors of a natural disaster and acting as substitute pollinators for honeybees wiped out by colony collapse disorder. But the “gee whiz” factor masks numerous possible darker contingencies.

What if the RoboBees were slightly altered electronically and released as a colony of thousands to spread deadly bacteria, viruses or fungi? And what about more unregulated surveillance? Further, these machines are meant to be “disposable” (the researchers assert that there's no problem if individual units fail “periodically”). Yet are they not constructed with many of the same toxic materials found in all electronics?

Andrew Degen
via e-mail


In “The Inner Life of Star Clusters,” Steven W. Stahler proposes that binary stars—pairs of stars that orbit each other—in stable stellar groups called open clusters fuel the expansion of those clusters. A related diagram shows a heavy interloper forming a stable pair with the heavier of two stars in a pair, throwing out the lightest. Yet consider the last state depicted with all velocities reversed: the least massive star comes in and ends up throwing out the heaviest!

Ken Knowlton
Sarasota, Fla.

When a binary pair in an open cluster ejects a third star, Stahler explains, the latter encounters other members of the cluster and shares its energy with them, effectively “heating” the cluster and causing it to expand. But doesn't the ejecting pair lose energy in the process, perhaps orbiting more closely, and keep the cluster more compact?

K. Cyrus Robinson
via e-mail

Stahler states that stars “coalesce within vast clouds composed chiefly of hydrogen molecules, along with other elements and a small admixture of dust.” The process by which elements were created has been established, but what is the nature of this “dust,” and how was it created?

Harold W. Simons
Weiser, Idaho

STAHLER REPLIES: Regarding Knowlton's question, interacting systems of stars evolve along well-defined evolutionary paths. That they do so demonstrates that the system's entropy increases along these preferred directions. It is always possible, at least conceptually, to reverse all stellar velocities and cause the system to regress to a lower-entropy state. Yet the probability of such an occurrence becomes vanishingly small as the number of stars involved increases.

Robinson is correct that the binary pair left behind after the ejection of a lighter star has lower energy. Thus, its component stars orbit each other more tightly. But this separation is tiny compared with the overall size of the cluster, which increases.

As to the dust: interstellar dust grains are solid, submicron-size particles composed of silicates and other minerals, along with an outer mantle of water ice. They condense out of the slow winds emanating from red giant stars. During the process of star formation, these lowly grains form rocky planets—and therefore us!


“Ready. Aim. Investigate,” by the Editors [Science Agenda], cites automobile safety research and policy implementation in arguing that research into the causes and prevention of gun violence, previously hampered by the National Rifle Association, is needed.

Research into making guns safer—without compromising speed and effectiveness—is a worthwhile goal. The analogy between automobile and gun deaths, however, is flawed. Automobile deaths are incidental, collateral damage to a car's purpose, whereas death or incapacitation is a gun's primary purpose.

Robert H. Miller
Prescott, Ariz.

I, as a legal gun owner, have taken due diligence that has ensured I have never had an incident. I cannot and will not suppose that owners of illegal firearms do the same. If research is to be conducted and solutions found, make sure it includes ways to keep the guns from entering the hands of the criminally minded. If this can't be done, and I suppose it can't, don't limit or remove our capability to defend ourselves from that element.

There will always be examples of poor firearm safety—even the best gun owner can slip. But most gun violence is perpetrated overwhelmingly by criminals and the psychologically challenged.

Ken Ridgley
via e-mail


In criticizing potential abuses of expanded DNA collection in “The Government Wants Your DNA,” Erin Murphy cites the case of Shannon Kohler, who had initially refused a DNA test in a search for a serial killer but was then forced to submit DNA under a court order despite having exonerating details that included an alibi.

There is a long history of police departments publicly accusing and convicting innocent people without valid evidence, but this does not require DNA evidence. In fact, Kohler was exonerated because of the DNA testing; without it, he may have easily been convicted.

Ethan Solomita
via e-mail