David Wright, co-director of the Union of Concerned Scientists Global Security Program, is an established expert on the technical aspects of arms control, particularly those related to missile defense systems, missile proliferation, and space weapons.

(CNN) A missile capable of reaching the United States topped with a nuclear warhead is considered to be Pyongyang's ultimate goal.

On July 28, Pyongyang made major progress toward that goal, announcing it had successfully tested an intercontinental ballistic missile that can reach most of the mainland US, including some of the country's largest cities.

It's one of a number of ballistic missile tests this year, and showed North Korea has the ability to potentially strike a US state.

With each test, Pyongyang builds its expertise.

We asked David Wright, co-director of the Global Security Program at the Union of Concerned Scientists and an expert on arms control and missile proliferation, to break down what we've learned from the year's tests.

What did we learn from the July 28 test?

The July 28 test demonstrated by far the longest range missile North Korea has launched. As in its recent tests, Pyongyang fired the missile almost straight up to high altitude so it would land in the Sea of Japan/East Sea without flying over Japan.

Based on the height it reached in the test (2,300 miles), I calculate that if launched on a standard trajectory it would have the range to reach a number of major cities in the US, including those on the West Coast as well as Denver, Chicago, and possibly Boston and New York. Based on the information we have now, it appears it might fall slightly short of Washington, DC.

We don't know the mass of the payload used in the test; if a real warhead is significantly heavier, that would reduce the range the missile could reach.

We also don't have enough information yet to understand how this missile might differ from the one launched on July 4.

IStill, this latest test is a clear indication that North Korea has made very significant progress in its ability to launch long-range missiles. These missiles would be inaccurate and would not be able to destroy military targets, but would be able to attack large targets like cities.

What did we learn from the July 4 test?

The missile appears capable of traveling at least 6,500 kilometers (4,000 miles), which is a big increase over the May 14 test. This puts it in the ICBM category, which is defined as a missile with range longer than 5,500 kilometers (3,400 miles). Its range would allow it reach all of Alaska and parts of the Hawaiian island chain.

The advances demonstrated over the past few months strongly suggest that unless Pyongyang agrees to halt its missile testing, it will continue to increase its missile range and eventually have the ability to hit the US Mainland with a nuclear warhead.

Most important test before July 4

Calculations show the missile used in the May 14 test would have a range of about 4,800 kilometers (2,980 miles). We don't know the payload used in the test, and if it was lightweight, the actual range with a nuclear warhead would be significantly shorter. This range is long enough to reach US bases in Guam (3,400 kilometers from North Korea), but no other new targets.

This missile appeared to use an engine we had not seen flight tested previously, and which may have been designed by North Korea.

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It seems likely that North Korea would need a more powerful engine to make a bigger rocket that would have intercontinental range, but this successful test seemed to show that it was on a technically credible path to doing that. It also indicated its missile engineers were becoming more sophisticated.

In early June, a US official told a reporter that three failed tests in April had in fact been previous attempts to launch this missile, suggesting North Korea was running into technical problems with the design.

Is there one theme throughout all the tests?

On all but the shortest test on May 29, the missiles were flown on lofted trajectories. This is presumably so they'd have a shorter range and land in the sea off the Korean Peninsula, rather than overflying Japan, which has caused big international problems for North Korea in the past.

What have we learned about the Pukguksong-2?

On February 12 and May 21, we saw tests of the Pukguksong-2 (Polaris-2), which is a land-based version of the missile (the Pukguksong-1) North Korea launched last year from a submarine.

From the test, we know this missile could travel about 1,250 kilometers (776 miles), although we don't know how heavy a payload this test used. If it was lightweight, the range with a nuclear weapon could be significantly shorter. This range is comparable to its Nodong missile, and can target most or all of Japan.

A missile believed to be the Pukguksong-2, a land-based variant of the Pukguksong-1.

It is an important step for North Korea because the Pukguksong missiles use solid fuel rather than liquid fuel like the Nodong, which means they can be moved to a launch site and fired quickly.

The Nodong must be filled with fuel once it reaches the launch site, which can take an hour or so.

The history of other countries shows that it could take a decade or more to develop a long-range solid fuel missile. This missile has now had three successful tests — one from a sub and two from land -- after a string of failures.

What did the lone launch of multiple ballistic missiles tell us?

On March 6, North Korea launched four "Extended-Range Scuds" (Scud-ER) simultaneously from mobile launchers on a road.

This is a liquid-fueled missile with a range of about 1,000 kilometers (621 miles) with a 500-kilogram payload, and we think it is something North Korea has had for a while.

So while this test did not show new missile capability, it seemed to show that they were practicing operational use of the missiles, since you might fire multiple missiles in combat.

How did the May 29 test differ from previous short-range missile tests?

We don't know a lot about the May 29 test. It was apparently a Scud missile with a range of several hundred kilometers. What was new about it was a new front end, which had small fins that North Korea claimed help guide it during re-entry to give it high accuracy.

North Korea paraded several missiles with fins on the re-entry vehicle during its April 15 parade, which it presumably would like to use as an anti-ship missile. Guiding a missile with that accuracy is difficult and will require considerable effort to develop.

What did North Korea launch on June 8?

The June 8 launches were cruise missiles, which fly in the atmosphere like small airplanes, rather than ballistic missiles, which are powered only early in flight and then arc toward the target like a baseball. They flew about 125 miles and are likely another system being developed to attack ships.

What about the tests that failed?

In addition to the three failures in April there was also a failed test in March. Little is known publicly about those launches—why they failed or even what missile was fired in March. When tests are successful, North Korea releases detailed photographs and other information that allows us to understand more about the missiles.