Space Friday: Transit of Mercury, Boiling Water on Mars, and Earth-like Worlds

One If by Land, Two If by Sea (SpaceX Does It Again)

Having accomplished two successful landings of its Falcon 9 first stage, SpaceX went for the “hat trick” and nailed it. On Friday morning, it landed successfully for a second time on the Atlantic-serving Autonomous Spaceport Drone Ship named “Of Course I Still Love You” (OCISLY), giving the company three successful first stage landings, including the first on December 21, 2015, which occurred on land. Each attempt has been secondary to the main mission of lofting payloads into orbit, and this third try was deemed particularly difficult and not given more than 50–50 odds of success. The main payload was a Japanese communication satellite headed for geostationary orbit 36,000 kilometers (22,000 miles) out, requiring a higher velocity than a trajectory to low-Earth orbit. Like the landing, that too was successful.

For viewers of the live video feed from OCISLY, the landing looked like a spectacular magic trick as cameras adjusted for the darkness of a nighttime landing were overwhelmed by the brightness of the approaching rocket’s exhaust plume, and the rocket itself was not seen descending into view. After a dazzling “white out,” the image went dark, and an audible “awwwww” sounded from the crowd at SpaceX’s Hawthorne, California, headquarters, expecting that predictions of another “rapid unplanned disassembly” had come true. Then, cheers erupted as the camera readjusted and showed the first stage, standing tall and steady on its landing legs, practically dead-center on the circular SpaceX logo painted on the landing platform.

In adding to increasing its total number of successful landings and boosting confidence that it can bring its rockets home under a wide variety of challenging conditions, SpaceX is moving toward its eventual goal of reusing those same boosters to dramatically reduce the cost of spaceflight. –Bing Quock

How to Safely Observe the Coming Transit of Mercury

On the morning of Monday, May 9, part of the Sun’s disk will be blocked from view by the planet Mercury—a really, really TINY part. Similar in principle to a solar eclipse, this event is called a transit. But while Mercury is slightly larger than the Moon, it’s also a lot farther away—about 84 million kilometers (52 million miles) distant, or about 218 times farther than the Moon, which is 384,000 kilometers (240,000 miles) from Earth. At that distance, Mercury’s apparent diameter is only about three-thousandths of one degree, or about 1/150 that of the Sun—admittedly, not an especially impressive image. Even if weather permits, you won’t be able to see it without magnification, meaning that a properly-accessorized telescope or a viewing device specifically designed to safely magnify the Sun’s image is a must. (More on safe viewing below.)

Mercury is one of only two planets, along with Venus, whose orbits lie inside Earth’s, allowing them to move between Earth and the Sun and be seen transiting across the Sun’s disk. In the case of Mercury, this can happen 13 or 14 times per century, and only in either May or November. This pattern results in the first transit of Mercury that we’ve seen since 2006. The next won’t be until 2019. The last transit of Venus occurred in 2012, and we produced a video about how to observe it, which you can view here. Although Mercury is a much smaller object than Venus (and might even be mistaken for a sunspot), many of the same principles and cautions apply.

The transit begins at 4:12:19 a.m. PDT (7:12:19 a.m. EDT). This moment is called first contact and is when the leading edge of Mercury’s tiny silhouette crosses the edge of the Sun’s disk. A little more than three minutes later, second contact occurs at 4:15:31 a.m. PDT (7:15:31 a.m. EDT), when Mercury’s entire disk is silhouetted against the Sun. Observers on the West Coast may have noticed that this is before sunrise (which for San Francisco specifically, occurs at 6:05 a.m. PDT). By that time, Mercury will already have chugged about a quarter of the way across the solar disk. Crossing the Sun completely will take another five hours and 37 minutes. Third contact—when Mercury’s leading edge touches the far edge of the Sun’s disk—is at 11:39:14 a.m. PDT (2:39:14 p.m. EDT), and the transit ends when Mercury’s silhouette is completely clear of the Sun’s disk (fourth contact) at 11:42:26 a.m. PDT (2:42:26 p.m. EDT).

As with any observations involving the Sun, eye safety is a top priority! There are plenty of safe ways to observe the transit, from observing directly through a filter made for solar viewing to using the optics of a telescope or pair of binoculars to project a large, focusable image onto a white screen. You can also log onto one of several websites that will be streaming the transit so you can watch on your computer. Weather permitting, Academy docents will be on our Living Roof with special telescopes for visitors to use.

Transits were once used to help astronomers determine the sizes and precise orbital speeds of Mercury and Venus, to detect their atmospheres (or lack thereof), and even to calculate the distance to the Sun. It also turns out that transits across other stars enable modern astronomers to detect planets orbiting other stars. By measuring the tiny reduction in a star’s light as planet moves in front of it—a technique called transit photometry—the orbital period of the planet, its average distance from its star, and its rough diameter can be determined. As the star’s light passes through the planet’s atmosphere (if any), the makeup of the atmospheric gases can be determined by identifying the wavelengths of light that are characteristically absorbed by certain elements. Combining transit data with information gleaned from other methods allows astronomers to further deduce additional physical characteristics, such as the planet’s mass and density, offering clues to its overall composition and structure.

All that mostly from observing a planet move in front of its star! So if you can watch Monday’s transit of Mercury across our Sun, you’re also seeing how astronomers have found many of the thousands of other worlds now known to circle the distant suns in our galaxy. –Bing Quock

Capturing Mars’s Boiling Water

When I was a kid, my grandfather had a cabin on the shores of a tiny Finger Lake in upstate New York. During the summers, my family would visit, and we’d all go trekking into the woods to drink from the spring that fed the lake. The water was cold and clear and delicious, and it flowed downhill, at first just running across the surface but eventually gently eroding the ground to create shallow banks along the borders of a small stream. It’s beautiful in its simplicity. It’s also fairly typical of water bursting from the ground on Earth.

On Mars, springs are a bit more… exciting.

The Celsius scale for measuring temperature is designed so that water boils at 100°C (212°F), but that figure assumes atmospheric pressure of Earth at sea level. With less air pressure, that boiling point drops, which is why recipes are different in California versus Colorado. On Mars, where air pressure can be as low as one percent that of Earth, the boiling point can be as low as 0°C (32°F). Martian water bursting from below ground starts to boil instantly. Because this process is energetic, even explosive, it can deform the landscape around it. A new study has detailed the process.

The team placed blocks of water ice and brine ice (saltwater ice) on a slope of dirt in a pressurization chamber set to resemble either Earth’s atmosphere or Mars’s atmosphere. Under Earth-like conditions, when the pure ice melted, it flowed across the surface, dampening the soil, and then was absorbed back into the dirt. When the briny ice melted, it got farther, because adding salt to water lowers the boiling point. This brine survived long enough to erode a small rivulet in the soil before succumbing to evaporation.

Under Martian conditions, however, the results were far more energetic. The water ice boiled rapidly in the low-pressure environment, and the reaction was strong enough to dislodge grains of dust with enough force to cause small dry avalanches. The result was a series of ridges forming a terraced slope downhill from the original ice block. When the ice contained salt as well, it was slower to boil, but each boiling event was still very strong, so the result was a significantly deeper channel left behind in the dirt and a much rougher surrounding area.

These results give us strong new insights into the processes that mold the Martian surface. As we continue to map and study the Martian landscape, we have new tools to describe how the features formed and what that could mean for the history of the Red Planet. The results also provide crucial context for studying the recurring slope linae that show present-day water flowing down hillsides in the southern hemisphere’s summer. And of course we have to address the question of life on Mars, which seems much less likely at present if the liquid water necessary for that life evaporates so quickly and violently. However, in the distant past, when Mars had a much denser atmosphere, the water would have been much more stable on the surface, covering at least a fifth of the surface in a vast ocean and creating a fairly habitable environment.

The study also has ramifications for the future of Martian exploration. If humans ever want to spend long periods on the ground, we’re going to need a lot of water. Storing and transporting that water is going to be a critical measure, and if breaches in our pressurization can have explosive results, we’re going to need contingencies to prevent such a scenario.

This isn’t some far-off concern, either. NASA plans to put humans on Mars by 2035. A private company, SpaceX, is shooting for an earlier landing of 2026. This might seem overly ambitious, but SpaceX has a plan. In 2018, they will land their first large payload on Mars. While this has been done before, it’s never been done by private enterprise. It’s also never been done at this scale. The Curiosity rover, which landed in 2012, came in at about 900 kilograms—the size of a typical cow. SpaceX’s payload is planned to be about 6,000 kilograms—the size of a large elephant!

How is SpaceX going to accomplish such a huge endeavor so quickly? They’ve been preparing. As part of their development of Dragon v2, a capsule for delivering astronauts to Low Earth Orbit that plans to have its first crewed flight in 2017, SpaceX tested a variety of abort-scenarios that could keep passengers safe in emergency situations. One of these tests involved a retro-rocket hovering system that could allow heavy payloads to land on a planet from high altitudes with a great degree of precision. Essentially, it turns the capsule into its own maneuverable craft. While this is essential for astronaut safety on Earth, it’s also exactly what we’re looking for when going to Mars. So while SpaceX has ostensibly been working on sending astronauts to the International Space Station on American rockets for the first time since 2011, this system is also easily modified into the SuperDraco system that could allow a Dragon v2 capsule (called Red Dragon when it’s headed for Mars) to achieve a vertical landing similar to the ones displayed by SpaceX’s Falcon 9 or Blue Origins’ New Shepard rockets.

Someday soon, humans could be living on another world, drinking water that bursts in springs from the Martian surface. But we should probably stand back. –Colin Elliott

Three Potential Earth-like Worlds

The ESO’s TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST) at the La Silla observatory in Chile has identified three potentially Earth-like planets orbiting the star2MASS J23062928-0502285 (now thankfully also known as TRAPPIST-1), only 40 light years away in the constellation of Aquarius. The report was published this week in Nature and the system is being hailed as the current best place to look for life outside our solar system. But not because it is a perfect match of our own solar system.

While all three planets are roughly Earth-size and within their star’s habitable zone, the star itself is anything but Sun-like. TRAPPIST-1 is an “ultracool” brown dwarf not too much larger than Jupiter, which means it isn’t massive enough to sustain hydrogen fusion at its core. Because of this, it is very cold and doesn’t produce much visible light. As a result, the three habitable-zone planets are huddled extremely close—so close that it takes only 1.5 and 2.3 Earth days, respectively, for the first two to make full orbits of the star and between 4.5–72.8 for the third (about 20–100 times closer than Earth is to the Sun). For most stars, this would be a blow-torch orbit. However, since the star is so dim, the first two planets probably have below boiling temperatures at their surface and only receive two to four times the radiation that Earth does.

Additionally, it appears that each world maybe tidally locked, meaning only one side of the planet would always face the star leaving the other side in permanent night.

So why look around small, cool stars, such as this one, for signs of life?

“The reason is simple,” Gillon explains. “Systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look.”

As we mentioned above, one current method for analyzing an exoplanet’s atmosphere involves detecting starlight that passes through the planet’s atmosphere as it transits the star. Brighter stars swamp the subtle effect with starlight, but a dim star allows the effect to be detected from here on Earth. Additionally, these worlds are close enough for us to get much more precise mass and density measurements.

If nothing else, these planets are the first found around an ultracool brown dwarf, and since brown dwarfs are fairly common, the discovery opens a new chapter in exoplanet hunting.–Elise Ricard

​Image: Mercury transit, 2006, edhiker/Flickr