Gravity Probe B: Einstein Confirmed?

Want to take a guess at what NASA’s longest running continuous research program in physics is? The answer: Gravity Probe-B. Although the satellite wasn’t launched until 2004, its origins go back to 1959, with NASA funding beginning in 1964. GP-B is a laboratory in space, one that uses four precision gyroscopes to measure two effects that grow out of Einstein’s general theory of relativity. The geodetic effect is caused by the mass of the Earth warping local space-time. The frame-dragging effect results from the rotating Earth dragging local space-time along with it.

Gravity Probe B Studies General Relativity

Image: With its telescope aimed at IM Pegasi, a far-off guide star serving as a fixed reference point, the experiment measured tiny changes in the direction of spin of four gyroscopes. Credit: Stanford University.

And if these things seem far too minute to examine, we’re beginning to learn that GP-B is up to the challenge, at least as far as the geodetic effect is concerned. The first look at data from the experiment was released at the American Physical Society meeting in Jacksonville (FL) on April 14. GP-B’s gyroscopes should shift by a tiny amount due to the geodetic effect during the course of a year, departing from their initial alignment by 6.606 arc-seconds (0.0018 degrees).

So far that shift is confirmed to a precision of better than one percent. “It’s fascinating,” says Francis Everitt (Stanford), principal investigator of Gravity Probe B, “to be able to watch the Einstein warping of space-time directly in the tilting of these GP-B gyroscopes — more than a million times better than the best inertial navigation gyroscopes.”

Fascinating indeed. Centauri Dreams might have chosen the term ‘awe-inspiring’ — imagine noting an effect this small and using it to confirm insights that grew out of raw theory and the genius of a single man. The mind reels in admiration at both theorist and experiment.

The frame-dragging effect is still in play. Theory says it should cause the spin axis to shift by an angle of 0.039 arc-seconds (0.000011 degrees). That’s 170 times smaller than the geodetic effect and the Stanford team is still trying to extract its signature from the available data. Torque and sensor effects have to be modeled and removed from the result.

But here’s a much more interesting way to put this ongoing work, spoken by GP-B program manager William Bencze:

“We anticipate that it will take about eight more months of detailed data analysis to realize the full accuracy of the instrument and to reduce the measurement uncertainty from the 0.1 to 0.05 arc-seconds per year that we’ve achieved to date down to the expected final accuracy of better than 0.005 arc-seconds per year. Understanding the details of this science data is a bit like an archeological dig. A scientist starts with a bulldozer, follows with a shovel, and then finally uses dental picks and toothbrushes to clear the dust away from the treasure. We are passing out the toothbrushes now.”

Those toothbrushes are going to be busy. For more on the problematic torque and sensor effects, this Stanford news release is helpful. As to the GP-B spacecraft itself, data collection ended in September of 2005 and the team has been working on the analysis of more than a terabyte of information ever since. To perform these extraordinary measurements, the spacecraft and gyroscope spin axes were aligned with IM Pegasi, used as a guide star throughout the mission. Expect the data analysis to be completed in December. Regarding the outcome, we may well have a hunch, but Francis Everitt offers a useful reminder: “Always be suspicious of the news you want to hear.”

Dawn Mission Readies for Asteroid Belt

If you want to follow the Dawn mission to Ceres and Vesta in detail, you’ll want to know about Dawn’s Early Light, the newsletter being published online to keep scientists up to date about its progress. With a launch window opening in late June, Dawn will be worth following on many fronts, not the least of which are its targets: Ceres and Vesta. These tiny protoplanets seem to be at opposite ends of the planetary formation spectrum. Ceres shows signs of water-bearing minerals and an extremely tenuous atmosphere, while Vesta is dry and significantly cratered.

The asteroid and protoplanet Ceres

In fact, the large impact crater that covers much of Vesta’s southern hemisphere is thought to be the source of material we can study here on Earth. Howardite, eucrite, and diogenite (HED) meteorites are now thought to have been ejected less than a billion years ago by the crater-forming impact, which flung debris that fell millions of years later onto our planet. Can we really identify meteorites conclusively as coming from Vesta? The delivery mechanism seems sound and experimental work suggests Vesta as the ultimate source (click here for more on this work from Michael Drake at the University of Arizona’s Lunar and Planetary Laboratory).

Image: Ceres as viewed by the Keck Observatory (Mauna Kea, HI). What was once thought to be a flat surface now seems rich in features. Does Ceres contain water from the days of the Solar System’s formation? Credit: NASA, ESA, J. Parker (Southwest Research Institute).

As to Ceres, its surface shows no signs of water ice but studies with the Hubble Space Telescope have looked at its shape and spin rate, both of which imply a rocky core covered by a layer of ice that could be anywhere from 60 to 120 kilometers thick. Dawn should be able to tell us something about the internal structure of these protoplanets, both of which have remained intact since their formation. And that helps us understand the conditions under which these objects formed, as well as deepening our understanding about the basic building blocks of larger planet formation.

In mid-June, the spacecraft goes from the processing facility at Titusville FL to the launch pad preparatory to beginning its eight-year journey to the asteroid belt. From a propulsion standpoint, Dawn’s ion engines offer another chance to shake down a useful outer-planet technology. We’re talking tiny levels of acceleration here — about the weight of a piece of paper in your hand — but over long acceleration times, ion engines make it possible to fly missions that would be far heavier (and pricier) if performed with chemical rockets. Factors like that helped to keep the Dawn mission alive despite NASA’s current budgetary woes.

A Terrestrial Planet Finder That Works

Yesterday we looked at ESA’s Darwin mission, and the plan to use a fleet of space telescopes to see planets around other stars. How else could you accomplish this goal? One option is a starshade like New Worlds, working with a distant space telescope to null out glare from the star. Another is an internal coronagraph, a device within the telescope itself that masks the glare. Centauri Dreams has backed the starshade idea, looking at its practicality and advantages over existing coronagraph designs (click here to see a breakdown of the pros and cons of each).

JPL coronagraph test results

But what if the coronagraph were dramatically improved? Scientists at the Jet Propulsion Laboratory believe they have done just that. In fact, John Trauger, lead author on a paper on this work that has just appeared in Nature, has this to say: “Our experiment demonstrates the suppression of glare extremely close to a star, clearing a field dark enough to allow us to see an Earth twin. This is at least a thousand times better than anything demonstrated previously.” That’s a bold claim, but one rigorously developed in JPL’s High Contrast Imaging Testbed, which allows coronagraph technologies to be examined in a space-like environment.

Image: Three simulated planets — one as bright as Jupiter, one half as bright as Jupiter and one as faint as Earth — stand out plainly in this image created from a sequence of 480 images captured by the High Contrast Imaging Testbed at JPL. A roll-subtraction technique, borrowed from space astronomy, was used to distinguish planets from background light. The asterisk marks the location of the system’s simulated star. Credit: NASA/JPL-Caltech

So how do you get past the major coronagraph obstacles? Diffracted light can create a spike or ring pattern around stars in a telescopic image, blocking the faint light of planets. Trauger’s team uses a set of masks that clear the diffraction artifacts while blocking much of the starlight. A second problem is scattered light, the speckling caused by ripples in the telescope mirror. It is addressed by using a deformable mirror whose surface can be tweaked by computer-controlled actuators. Up next for the team is an improvement in speckle suppression as the technology battles for a place on a future terrestrial planet finding mission.

What lies ahead for this latest coronagraph concept? Ian Jordan (CSC/STScI) described Trauger’s work as ‘significant progress’ toward the goal of finding extrasolar terrestrial planets. And he agreed with Trauger and collaborator Wesley Traub that demonstrating the instrument’s sensitivity in broadband optical light — and not just in the coherent laser light thus far used — will further test and extend the idea. Says Jordan:

If the passband for adequate suppression is too small, then the throughput of the system will remain ‘slow’. Slow is problematic for several reasons: 1) exoterrestrial analogues are inherently faint and their photons are few and far between anyway, 2) spectroscopy of the planetary signatures is important for determining what the faint objects are and may demand relatively high system throughput.

Throughput is critical for finding and studying the faint objects the coronagraph is designed to hunt. “Telescope time is precious,” adds Jordan, “and preserving as many of the planetary photons making their way to the aperture from as much of the field of view as possible is a goal for TPF.” Another coronagraph advantage: ‘slew’ times — the time needed to move the telescope to a new target — are much shorter for internal coronagraph designs than for external occulters.

Centauri Dreams‘ take: This impressive work keeps the coronagraph in serious competition with starshade concepts as we work out the technologies for future planet finder missions. A major constraint will be cost — let’s see how the numbers match up as both technologies continue to be refined within budgets we can realistically hope to meet.

The paper is Trauger, John and Wesley Traub, “A laboratory demonstration of the capability to image an Earth-like extrasolar planet,” Nature 446 (12 April 2007), pp. 771-773 (abstract available).

ESA: Tuning Up the Darwin Mission

What on Earth (or off it) is an Optical Delay Line (ODL)? It turns out to be, according to the European Space Agency, “…a sophisticated opto-mechanical device that can introduce well-defined variations, or delays, in the optical path of a light beam…” And it’s a key player in the technique known as nulling interferometry, which ESA’s Darwin mission will use to dampen the glare of distant stars while exposing the light of their planets. Darwin will be a multi-satellite mission using multiple orbiting telescopes working together to produce a much larger effective aperture than any one of them can muster.

Artist's conception of Darwin telescope

As to that ODL, the optical delay it introduces has to be able to adjust the path of a beam of light with an accuracy measured in just a few nanometers (billionths of a meter). To achieve this, the agency is testing a design using magnetic levitation to control its mirror, a contactless and frictionless method ESA likens to the touch of a feather (a video clip is available). What’s more, the device has been tested at -233 Celsius, a cryogenic temperature demanded by the need to reduce interference from the satellites’ own thermal radiation. This is crucial because Darwin will operate at infrared wavelengths.

Image: One of Darwin’s telescopes, due to be part of a flotilla of four or five free-flying spacecraft. Credit: ESA 2002

Why infrared? Because the difference in brightness between star and planet is minimized at mid-infrared wavelengths. Assuming the ODL design (or its successor) functions as planned, Darwin will recombine the light from its separate telescopes aboard a hub spacecraft, and we may eventually be examining exoplanets and their atmospheres for signs of oxygen, carbon dioxide and other potential life-markers.

The target: Some 1000 of the closest stars and the small, rocky planets assumed to circle at least some of them. Orbiting from the L2 point 1.5 million kilometers from Earth, Darwin’s spectrometers could give us some of the most exciting data ever received in the exoplanet hunt, though just when it will launch is still a matter for conjecture.

Yuri’s Night To Be Observed Worldwide

April 12 is a memorable date, the anniversary of Yuri Gagarin’s 108-minute orbital flight in 1961. It’s also the date, some twenty years later, when NASA launched Columbia, the first Space Shuttle (and boy do I remember the trepidation of watching that one go up). Celebrating these milestones is Yuri’s Night, marked by 119 parties scheduled in 32 countries on six continents. Check the celebration site for parties near you. And for stay-at-homes, be aware that Second Life is hosting one of the venues online. Thanks to Frank Taylor for the tip on this.

And note (via Larry Klaes) David S. F. Portree’s fine tribute to Gagarin’s accomplishment. Nice cover of an early 1930’s Science Wonder Stories on the same page. Somewhere Hugo Gernsback is smiling…