Japanese Craft Sail Off to Venus

Akatsuki spacecraft at Venus
Japan's Akatsuki spacecraft is intended to operate for at least two years after it begins orbiting Venus in December 2010.

Japanese Craft Sail Off to Venus

The Americans have done it. So have the Soviets and the Europeans. Now the Japanese are giving it a go.

"It" is putting a spacecraft in orbit around Venus.

The journey of Akatsuki, Japan's latest interplanetary probe, began on May 21st at 6:58 a.m. local time (21:58 Universal Time the previous evening) when a powerful H-IIA rocket soared into an overcast sky from a launch pad on Tanegashima Island. Officials from the Japan Aerospace Exploration Agency (JAXA) report, albeit briefly, that so far everything is going smoothly. The spacecraft has already relayed a few test images of a crescent Earth taken from 150,000 miles (250,000 km) away.

Akatsuki means "dawn" or "daybreak," which is where you'll find Venus in early December when the spacecraft reaches its destination. Depending on the source, this spacecraft is also known as "Planet C" (its designation prior to launch) and "Venus Climate Orbiter" — take your pick.


Crescent Earth from Akatsuki
Hours after its launch on May 21, 2010, the Venus-bound Akatsuki spacecraft recorded the receding crescent Earth at near-infrared (left) and ultraviolet wavelengths.

This craft is all about understanding the Venusian atmosphere. Scientists want to know why the dense airmass is "superrotating," that is, circling completely around in just 4 or 5 days when the planet itself takes 100 times longer. How does its opaque blanket of clouds form? Are volcanoes actively spewing sulfur dioxide (SO2) into the air? Does lightning — long suspected but as yet unproven — really occur, and if so where and when?

A few weeks after settling into a polar orbit that'll range from 200 to 50,000 miles (300 to 80,000 km) above the planet, the boxy, half-ton craft will get to work. Five instruments will take images: one recording ultraviolet wavelengths (to study SO2), two for near-infrared work (ground and lower atmosphere), one looking deep in the infrared (night-side studies of winds and cloud motion), and one keyed to selected visible-light bands for detecting lightning flashes and airglow. A sixth "instrument," an ultrastable radio transmitter, will permit mission scientists to map temperatures as its signal passes through the upper atmosphere.


IKAROS, Japan's solar sail
Measuring 46 feet (14 m) square, IKAROS is a "solar sail" designed to navigate through interplanetary space using the pressure of sunlight for propulsion.

Tagging along with Akatsuki is a secondary payload called IKAROS, a cutesy acronym derived from Interplanetary Kite-craft Accelerated by Radiation Of the Sun. This "solar sail," a giant sheet of thin polymer membrane about 46 feet (14 m) square, will be unfurled in a few weeks, and engineers hope to use it to demonstrate how future spacecraft might glide through the inner solar system powered by nothing more than solar radiation pressure.

Louis Friedman, executive director of the Planetary Society, has championed the potential of solar sailing for decades. In fact, the organization hopes to once again launch its LightSail craft next year. Click here to get Friedman's take on IKAROS and LightSail.

But wait — there's more! The H-IIA also lofted four other small satellites built by Japanese universities. Three of these will remain in orbit around Earth; the fourth, initially called UNITEC 1 but renamed Shin-en ("abyss"), is a test of computer durability that's coasting toward Venus as well.

Faint Comet in the June Dawn

Comet C/2009 R1 (McNaught) on May 19, 2010
On May 19th, when Michael Jaeger shot this image from Austria, Comet C/2009 R1 was 8th magnitude and showing the characteristic green comet color in large telescopes. Note the thin blue gas tail.

We rarely see a good comet when it's at its best. Most comets are brightest when nearest the Sun — just when they’re most likely to be hidden in the Sun’s glare or below the sunrise or sunset horizon.

That's the situation this spring with Comet C/2009 R1 (McNaught). Even so, observers in the Northern Hemisphere should be able to pick it up with telescopes, and possibly binoculars, just before dawn for at least part of June, during its runup in brightness.

And in fact, the comet is turning out to be 1 or 2 magnitudes brighter that we predicted in the June Sky & Telescope (page 60). Let's hope this behavior keeps up!

Comet Timetable

As of mid-May the comet was about magnitude 8.5 (compared to the 10 we originally predicted), as it rose about an hour before the start of astronomical twilight for mid-northern observers. Throughout this apparition it will be low in the east or northeast when dawn begins to brighten.

May 31st will find McNaught, now hopefully 6th or 7th magnitude, passing 2½° southeast of the 2nd-magnitude star Beta Andromedae. At the beginning of astronomical twilight it’s a respectable 20° up as seen by observers at 40° north latitude. But the waning gibbous Moon will brighten the sky.


The June comet crosses Perseus, low just before dawn, when at its best in June.

On the morning of June 5th the comet skims just north of the large, loose open cluster NGC 752. On June 6th and 7th it’s within about 2° of the 2nd-magnitude double star Gamma Andromedae. The Moon is much thinner then, but also closer to the comet.

Mid-June is when Comet McNaught should be most interesting, offering the best compromise between its increasing brightness and its decreasing altitude at the start of dawn. Moreover, the sky will be free of moonlight.

The helpful conjunctions continue as the comet passes about 1° north of the open cluster M34 in Perseus on the morning of June 10th, and 3° south of 1.8-magnitude Mirfak (Alpha Persei) on the 13th. It’s still about 15° high in the northeast as the sky starts to grow light on June 15th, but it appears roughly 1° lower every day after that. The comet passes zero-magnitude Capella on the 21st, and it’s very low by the 24th, when it passes 2nd-magnitude Beta Aurigae. By now Comet McNaught may be as bright as 4th or 5th magnitude, but moonlight is returning.

The comet will be lost to view by June’s end — just before it reaches perihelion on July 2nd, 0.405 astronomical unit from the Sun. It remains far from Earth throughout this apparition, never venturing closer than 1.135 a.u. (in mid-June). After perihelion it will fade rapidly as it heads to the far-southern sky.

The comet is approaching on a hyperbolic orbit, which means that it’s making its first trip in from the Oort Cloud. So its brightness is even less predictable than usual. Will it flare unexpectedly or perhaps fizzle right out?

Many McNaughts

This particular Comet McNaught is one of 54 (and counting) named for Robert H. McNaught of Australia’s Siding Spring Observatory. He works in the Siding Spring Survey, funded by NASA to record large swaths of sky to find potentially hazardous near-Earth objects. The survey also turns up many other moving objects. McNaught found this comet (which will never come near Earth) at 17th magnitude on an image taken last September 9th. Pre-discovery images quickly established its orbit.

The most famous of the Comet McNaughts is C/2006 P1, also known as the Great Comet of 2007. It was an easy naked-eye sight when passing near the Sun in mid-January of that year, shining at magnitude –5 or –6, and in the following days it flung a gigantic, multi-banded tail across the Southern Hemisphere’s evening sky.

Sun's Size is "Rock Steady"

Large sunspot in Octeber 2003
In October 2003, the Sun’s disk included active region 10486, which became the largest sunspot seen by SOHO.

Sun's Size is "Rock Steady"

Admit it: At some point you've probably thrust the tip of your index finger (2° across, more or less) skyward to cover up the Sun's disk (½°, more or less). Isn't it amazing how a simple body part can momentarily obliterate a star nearly a million miles across?

Using gadgetry far more sophisticated than a fingertip, astronomers have been measuring the Sun's diameter ever since the 17th century — and they've gotten very good at it.

So has our star changed size in all that time? Hard to tell. You'll get a different result depending on the technique used, the wavelength of light used, and what exactly you define as the Sun's "edge."

A new result, announced this week by a team of solar physicists, tries to sidestep all of these vagaries by using only measurements taken by a single instrument over 12 years simply to see if the Sun's diameter has changed — and the answer, apparently, is "no."

Jeff Kuhn (University of Hawaii) and three colleagues find that the Sun's mean diameter hasn't changed more than one part in a million since 1998. And what makes them so sure? They've sifted through some 500,000 frames taken by the Michelson Doppler Imager aboard the Solar and Heliospheric Observatory. Launched in December 1995, SOHO has followed the Sun through a full 11-year solar cycle and then some. Moreover, it doesn't have to contend with turbulence in Earth's atmosphere.

"Getting above the atmosphere is an enormous advantage for astrometry, more than an order of magnitude improvement," Kuhn explains, "and we just don't see any solar-cycle variability." His team's analysis of SOHO observations will appear soon in Astrophysical Journal.

The same can't be said about measurements taken closer to home. Measurements using astrolabes, Mercury transits, and eclipse timings, among others, reveal variations 10 to 100 times larger. Yet they all show bigger swings in the Sun's size, which suggests that the ground-based measurements are indeed correct but systematically influenced by some kind of subtle atmospheric effect. (That said, measurements taken from high-altitude balloons in 1992-96 inexplicably show the same trend.)

Kuhn has a hunch that this apparent discrepancy between ground and space is "telling us something interesting about how the solar cycle affects the stratosphere."

"All mechanisms are speculative," he notes, but pointedly adds, "That the solar diameter is rock-solid is not speculation."

Fortunately, even better measurements will be in hand soon. The researchers plan to repeat their work using images from NASA's just-launched Solar Dynamics Observatory. By 2017, they'll have access to the exquisitely detailed views from Advanced Technology Solar Telescope being built at the summit of Haleakala in Hawaii.

Herschel's Cold, Wonderful Universe

Herschel space selescope
Launched on May 14, 2009, the Herschel Space Observatory boasts a primary mirror 3.5 meters across. To view faint far-infrared sources, the spacecraft uses a large Sun shield and liquid helium to keep its optics and detectors cold

A couple of weeks ago we celebrated the Hubble Space Telescope's 20 years of discovery, and today astronomers gathered in Noordwijk, The Netherlands, to recognize another milestone by an up-and-coming space observatory.

It's been almost a year since the European Space Agency launched its Herschel Space Observatory, which shared its ride to orbit with the Planck spacecraft. Both payloads are doing well: Planck has been busy compiling the most accurate map ever made of the cosmic microwave background (CMB), the relic radiation from the Big Bang.

But the focus at today's gathering was Herschel and its trove of amazing observations. The spacecraft boasts a mirror 11.5 feet (3.5 m) across, more than twice HST's light-gathering power. But Herschel looks almost exclusively at far-infrared and submillimeter-wave "light," from 55 to 670 microns wavelength. (Visible light is from 0.4 to 0.7 microns.) Such wavelengths show very cold objects "glowing" at only a few tens of degrees above absolute zero. Since a telescope's resolution decreases at long wavelengths, Herschel needs all that aperture to see fine-scale detail in its frigid targets.

It also needs help to keep its own "body heat" from swamping the faint signals from the depths of space. An onboard supply of liquid helium should keep the detectors hovering near absolute zero for another three years. Cooling the primary mirror is another story — it's too big to refrigerate, so it stays hidden behind a Sun shield to remain near -315°F (80 K).

Herschel carries three instruments: a pair of cameras (PACS and SPIRE) and a ultra-high-precision spectrometer (HIFI). The mission's one serious glitch came last August, when HIFI fell silent, but in January controllers switched to a set of backup electronics and it's been working fine since then.


Heavyweight star forming in RCW 120
Herschel’s observation of the interstellar bubble RCW 120, about 4,300 light-years away, has revealed an embryonic star in its rim (arrowed) that looks set to turn into one of the brightest, most massive stars in our galaxy.

Investigator Annie Zavagno's presentation, "The Dark Side of Star Formation," focused on the little-understood process that spawns beefy stars with at least 8 times the Sun's mass. These heavyweights are rarely found, both because just-forming stars exercise lots of self-control when gobbling up interstellar gas and because they exhaust their hydrogen fuel and die in just a few million years. Zavagno (Laboratoire d’Astrophysique, Marseilles) and her team have used Herschel to spot high-mass stars as they form, and it now seems that the trigger points occur along the margins of rapidly expanding ionized-hydrogen bubbles (called HII regions).

Meanwhile, the recently rejuvenated HIFI spectrometer has been tracking down concentrations of ionized water, which has a distinct and strong spectral signature. "Water is an excellent diagnostic tool to probe the chemical and physical structure of the interstellar medium," explains Alexander Tielens (Leiden University). In particular, he notes, water helps cool the gas and dust surrounding newborn stars by radiating infrared energy to space.


Views of Hosehead nebula in visible and infrared
The familiar visible-light view of the Horsehead nebula (left) compared to a far-infrared internal view from the Herschel Space Observatory, which reveals regions of intense star formation.

But what really caught my eye today was a side-by-side comparison of the famed Horsehead Nebula in Orion. it turns out that the dark "clouds" that give this showpiece its distinctive appearance in visible light are internally ablaze with star formation when spied in the far infrared. Steve Eales (University of Cardiff) showed the paired views to demonstrate the potential of the Herschel ATLAS (short for Astrophysical Terahertz Large Area Survey), which will map 550 square degrees of sky at five wavelengths. Eales expects the 600 hours of exposures to reveal some 250,000 galaxies.

Herschel is showing us the "cold universe" as never before, and these results demonstrate that we've only scratched the surface of what this spacecraft will be revealing in the months and years ahead.

Happy Birthday, Hubble!

Astronomy's crown jewel — the Hubble Space Telescope — has turned 20, and retrospectives are definitely in order.

Since soaring skyward aboard the Space Shuttle Discovery on April 24, 1990, the orbiting observatory has amassed more than 930,000 observations and snapped some 570,000 images of 30,000 celestial objects. That's enough data — 45 terabytes — to fill nearly 5,800 DVD movies. Astronomers have published more than 8,700 scientific papers that utilize HST's data, about 650 last year alone.

NASA and the European Space Agency (the project's seldom-recognized partner) have pulled out all the stops in celebrating HST's 20th anniversary. For its part, NASA has launched multiple websites: one of them requires Adobe Flash player; a second recalls the mission's greatest hits; and a third is geared toward students and educators. For its part, ESA is sponsoring a Hubble pop-culture contest to gauge the extent to which HST and its science have become aspects of everyday life.

There's also a new effort to engage the public in "citizen science." The Space Telescope Science Institute (STScI), which has operated the observatory from the outset, has teamed with the Galaxy Zoo team so that you can try your hand at classifying galaxies by diving headlong into one of HST's deep-field images.


Carina Nebula from Hubble Telescope
Hubble Space Telescope views reveal a portion of the Carina Nebula in visible (left) and infrared light. Located some 7,500 light-years away, these towers of dust-laced hydrogen gas are among the most active star-forming regions known. Click here for a larger view, and click here for more details about how HST's Advanced Camera for Surveys acquired these images in early 2010.

To cap off the celebrations, astronomers have released stunning new views of a small bit of the Carina Nebula, seen here. They've named this scene the Mystic Mountain. Many news outlets have published only the visible-light view at left, but there's also an infrared composite, at right, that reveals a host of stars in and behind the dust-laced clouds.

Also on the site is a wide-field view of much more of the Carina Nebula, on which you can hunt out the Mystic Mountain. Three hints: it's small, it's upside down, and in's in a bright/dark transition zone.

Located about 6,500 light-years away, the Carina Nebula is a rich star-forming region dominated by the brilliant supermassive star Eta Carinae. Hubble's portraits show a 3-light-year-long bit that contains two jet-pairs of dense knots (Herbig-Haro objects) designated HH 901 and HH 902. The streams of knots are being squirted from the vicinity of new-forming stars embedded in two tower tips.

The cool, dust-laced towers themselves, mostly hydrogen and other gases, have been eroded out of the wall of the main nebula in a scene reminiscent of HST's iconic "Pillars of Creation" view of the Eagle Nebula (Messier 16), released in 1995. Intense ultraviolet emission from newborn stars is boiling off the gas in many places, compressing back sharp, illuminated rims and sculpting the whole thing beautifully.

Despite nearing retirement age, HST remains incredibly productive and in high demand. Since its fifth and final servicing mission last year, astronomers have clamored for roughly six times as much observing time as can be accommodated. With careful use and a little luck, the Hubble Space Telescope should still be going strong when its 25th anniversary rolls around in 2015.