Making the Most of Mars

Making the Most of Mars

From Earth’s point of view, Mars is unique. It’s the most Earthlike world we know; backyard telescopes can sometimes show polar caps, surface markings, seasonal white clouds, and windblown dust. Mars also behaves uniquely in our sky. It spends most of its time far away as a tiny blob in a telescope, then every 2.1 years it swings much closer for just a few months around opposition.

Moreover, Mars comes almost twice as close at some oppositions than at others (because Mars has a significantly elliptical orbit that is also near Earth’s orbit). The near and far oppositions (“perihelic” and “aphelic”) come and go in a 16-year cycle.

We’re now near the bottom of that cycle. Mars reaches opposition and makes its closest approach to Earth in late January 2010. But it appears only 14.1 arcseconds in diameter at the time. Mars will appear larger than 12″ through the end of February, and larger than 10″ through late March (see the graph on the bottom). That’s pretty small.

And next time around, in March 2012, Mars will be only 13.9″ wide at closest approach. Not until July 2018 will it peak out again, at 24.3″.

But astronomy is all about making the most of very distant, difficult views. And at least Mars will cross the night sky high for Northern Hemisphere observers. It spends the next five months in and near Cancer, at declinations +16° to +23°.

In a good 4-inch or larger telescope on a night of steady air, you may first make out the bright north polar cap. In February or March you may notice the cap shrinking in the Martian northern spring. Dark surface markings may be harder to discern, depending on which side of Mars is facing Earth when you look. Watch also for bright limb hazes, occasional white clouds, and possibly the obscuring bright patch of a dust storm moving from day to day. But don’t expect impressive views. Every bit of what you see will be a hard-won prize.

A Strange "Comet" Among the Asteroids

Comet P/2010 A2 from WIYN Observatory
The cometary object P/2010 A2, as captured on January 11, 2010, by the 3.5-m WIYN telescope in Arizona. Note the small distinct object (arrowed). Astronomers are trying to determine if the diffuse cloud is normal cometary outgassing or the result of a collision between two objects. For more detail about this image, click here.

January 7th's announcement that the LINEAR telescope had spotted a new periodic comet wasn't all that interesting: a 20th-magnitude blip out in the asteroid belt in a benign orbit that wouldn't come anywhere near Earth. Designated P/2010 A2 (LINEAR) by the IAU's Minor Planet Center, it was just another notch on the finderscope for this discovery machine near Socorro, New Mexico, which has chalked up 77 periodic comets (and a couple hundred one-timers) since coming online in 1998.

But as other observers chipped in positions over the next week, it became clear that this was an object worth watching. For one thing, the now-precise orbit was looking less like a comet's and more like an asteroid's. And images of the interloper showed a tail growing in length yet without a clearly defined head. The online chatter got more animated — just what was this, anyway?

On January 14th, Javier Licandro and others used the Nordic Optical Telescope in the Canary Islands to get a better view, and they discovered something completely unexpected: a small asteroid lay 2 arcseconds to P/2010 A2's east and was moving along with it. Moreover, the "comet" showed no central condensation and looked more like a narrow dust swarm about 110,000 miles (177,000 km) long.


Orbit of The newfound "comet" designated P/2010 A2 has an orbit that's squarely in the asteroid belt, circling the Sun ever 3.4 years. Click here for a larger view.

Licandro quickly enlisted the biggest aperture in the island's observatory complex: the Gran Telescopio Canarias. Dozens of images taken three days ago using its immense 34-foot (10.4-m) aperture confirm that the "comet" is being shadowed. It's hard not to conclude that we are watching the aftermath of a collision in the asteroid belt. But it's still too early to know for sure. Licandro and his colleagues are analyzing the GTC images carefully — and they hope to make them public soon.

Meanwhile, comet specialists are hoping to observe the strange goings-on with both the Hubble and Spitzer space telescopes. Neither has been given the green light yet, but if/when that happens the observations would be made within the next few days. According to Caltech astronomer William Reach, Spitzer no longer has the ability to look deep in the infrared, but it can still record at 3.6 and 4.5 microns, where the cometary gases carbon monoxide and carbon dioxide have strong emissions.

New Report Spotlights Impact Hazards

Apophis and Earth in 2029
On Friday the 13th in April 2029, a 1,000-foot-wide asteroid named Apophis will pass close enough to Earth (within 20,000 miles) to briefly appear as a 3rd-magnitude star in the night sky.


New Report Spotlights Impact Hazards

Given the odds of some giant space rock crashing into Earth and what it might do when it hits, scientists now estimate that on average 100 people will die each year from a cosmic impact. How much this number scares you depends on how far out you want to look into the crystal ball. Within the next couple of centuries a Tunguska-like blast might match the devastation of the earthquake that just devastated Haiti. Or fast forward 10 million years, and you can expect a titanic crash powerful enough to wipe out a billion people worldwide.

In 1998 Congress felt worried enough about the sky falling that it tasked NASA with finding 90% of all the near-Earth asteroids at least 1 km across within 10 years. (Anything this size would likely trigger global devastation during and after its collision.) Then Congress raised the bar in 2005, mandating that NASA find 90% of all the threatening asteroids with diameters down to 140 meters (460 feet) by 2020.

It looks good on paper, but neither Congress nor NASA have ever anted up the dedicated funds to do those searches. Back in the mid-1990s, NASA scientist David Morrison famously observed that fewer people are watching for asteroids able to collide with Earth than work in a typical McDonald's franchise. And while more observers are warily sweeping the sky these days, including many capable amateur astronomers, they're doing so by coattailing on other space surveys not optimized for the task.

In short, there aren't enough bodies or telescopes in the impact game to meet the 2020 deadline set by Congress. Don't take my word for it — read the 149-page report released today by an A-list panel of experts assembled by the National Research Council. Titled Defending Planet Earth,, it explores both the best ways to track down all the hazardous near-Earth objects (NEOs) and what to do once it becomes clear that one of them is destined to have a run-in with Earth.

The NRC report both summarizes the state of the searches to date and lays out the steps that NASA or the National Science Foundation (which funds most professional astronomy in the U.S.) would need to take to get serious about cosmic threats. "This is a huge milestone," observes small-body specialist Richard Binzel (MIT), who had no role in the committee or its findings. "The asteroid problem is now a grownup, joining the table of other natural disasters for which mitigation strategies are developed."

Pointedly, the NRC panel states that the Congressional target simply can't be met by 2020. If NASA and NSF managers decide they want to complete the survey as soon afterward as possible, then they'll need to fortify observers not only with dedicated ground-based efforts like the proposed Large Synoptic Survey Telescope but also with a infrared-sensing space observatory. Or ground-based telescopes could go it alone, which would keep costs down but delay the completion date until about 2030.

Concerning mitigation strategies, the panel assessed four approaches and found them all viable and complementary. For the smallest and thus localized impact threats, the most cost-effective approach is simply to move people out of harm's way, either by sheltering or evacuating them.

Bigger NEOs, ranging up to 100 m across, would affect too large an area to make civil defense practicable. So a passive space-based defense — using a spacecraft to pull or push the body enough to alter its path — would work better. For still-larger impactors, up to 1 km in size, a salvo of spacecraft might need to strike it head-on to change its course. But both of these methods would make only slight trajectory redirections and thus require many orbits, over decades of lead time, to avert a disastrous collision.

For the biggest threats, or if one of the other methods fails or if the lead time is short, the panel concludes that the "only current, practical means" are nuclear explosions. These wouldn't be used disrupt the incoming body but rather to give it a Really Big Push all at once. (No need to cue Bruce Willis and his Armageddon team — these would be delivered robitically.)

There's more. As noted in its preliminary findings, released last year, the NRC panel emphasizes the crucial role being played by the unique radar capabilities of Arecibo Observatory Puerto Rico — a facility that an NSF review team felt ought to find its funding elsewhere or be shut down.

Saturn's Prometheus: Just Plain Weird!

NASA's Cassini spacecraft has been orbiting Saturn for only 5½ years, but in that time this remarkable craft has had occasion to pass near many of the planet's 61 moons (six of which were discovered in its images). It's an amazing bunch of bodies — variously big and small, smooth and cratered, gas-gushing and quiescent.

But the close-up of Prometheus seen here represents new territory, in a sense. This elongated body, just 74 miles (119 km) long, has an alien quality to it. Note how all the edges are rounded and the craters are filled in. We're just not used to seeing moons (or asteroids) with such muted features. Heck, I've seen potatoes with more detailed surfaces — but no mere spud has ever fascinated me nearly as much as this picture does!

Cassini captured this view on December 26th, from just 36,000 miles (59,000 km) away. We're not likely to see Prometheus with this kind of detail (about 1,150 feet, or 350 meters, per pixel) for a very long time, if ever again.

Discovered in 1980 by Voyager 1, Prometheus and like-sized Pandora are the F ring's gatekeepers, "shepherd satellites" that circle just inside and outside the faint ribbon of dust. Their gravitational influence confines the F ring, keeps its particles from dispersing, and gives a tortured appearance.

But all this gravitational jostling forces each moon into a very slightly eccentric orbit, one part in 450 (0.0022) for Prometheus and one in 240 (0.0042) for Pandora. The slight orbital eccentricity causes Prometheus to make headlong dives in the F ring once per orbit (every 15 hours) into Saturn's F ring. You'd lose the rough edges too if you endured all that peppering. Pandora, which got its Cassini close-up in 2005, looks somewhat the same — though not nearly as blanketed in ring dust.

I'll bet that Carolyn Porco, the lead scientist for Cassini's imaging team, finds this picture particularly satisfying. As a young scientist on the Voyager mission, she specialized in the gravitational interplay of rings and moons. While there's been no shortage of amazing sights to grab her attention throughout Cassini's long-running romp, I suspect that right now she's thinking long and hard about how this funny little moon, along with its buddies Pandora and Atlas (the A ring's shepherd), came to be.