Nelson Mandela is hovering on the political horizon like a human equivalent of Halley's Comet. The latter is returning, portentously, after 76 years of bottling around the cosmos.

The former seems set to return in similar fashion after nearly 25 years in a small cell, property of SA Prison Services. But while scientists can tell us how the comet will behave, what effect it will have and when it will disappear again no one can say with certainty what will happen when the African National Congress leader, serving a life sentence for sabotage, is back in his modest Soweto home.

Official sources and those close to his family indicate he is expected out in a matter of weeks. That this is being taken seriously is shown by the fact that his lawyer, Ismail Ayob, has made inquiries about office space in Johannesburg.

His release would be the single most important gesture the government could make to indicate that it is serious about allowing change to occur. It is one of the few moves being contemplated by the government that will have an instant and powerful effect on the majority of people.

It would increase PW Botha's local and international credibility and ease South Africa's negotiations with its international creditors considerably. Obviously that is why it is being considered -- not because of a sudden rush of compassion from the powers that be.

What is not clear, however, is the political effect Mandela's presence will have in an already tense situation. One can recall both the Iranian experience -- where the return from exile of the Ayatollah Khomeini presaged the collapse of the Shah's regime -- and the Namibian experience -- where the long-awaited and much-heralded release from prison of Andinba Toiva ja Toiva was quickly forgotten.

There is little doubt that Mandela -- having refused and apparently defeated attempts to impose conditions on his freedom -- will begin organising and rallying as soon as he is freed. His family is precise about what he will do: "He will take up where he left as leader of his people."

One can expect a series of mass rallies and maybe even protest marches attended by millions of people. Curiosity alone will fill any of South Africa's large sports stadiums. One senior PFP parliamentarian put it this way: "If he comes out and calls a meeting in Soweto, it will make the State President's speech at Zion City last year look like an NRP meeting".

One can expect a massive outpouring of public sentiment. Thousands of youths -- most born after his imprisonment -- have adopted him as their symbolic leader despite never having seen or heard him. Rumours that he had died last led to a brief outbreak of violence in Soweto.

The government has added to the tension and sense of expectation over release by drawing out the process. The first offer of a conditional release was made secretly by the then-Minister of Police, Jimmy Kruger, in 1977. Since then there have been countless offers and refusals, each offer from the government raising and international expectations, principled refusal on Mandela's enhancing his popular status.

The state will struggle to contain the popular reaction to his release, perhaps even banning or restricting him. If they do, one can expect Mandela to resist or even ignore them. When the government proposed previously to release him to the Transkei, he made it clear through his family that he would ignore such conditions and leave the area. His position will be a powerful one -- knowing the authorities would find it difficult to put him back in prison under any but the most desperate circumstances.

One can expect him, cognisant of the relative immunity he enjoys, to have little hesitation in breaking the law furthering the aims of the organisation he leads, the ANC. The government will have little option but to accept this -- probably within specific limitations that preclude the direct propagation of violence. This could lead to a separation between an external military wing of the ANC and an internal political wing, similar to the structure of Swapo in Namibia.

Mandela is almost certain to use his relative immunity to call for mass action, perhaps a national strike or a protest march. All this sounds like a scenario for mass upheaval. It could prove to be explosive. It comes at a time when there is a lull in unrest, but there is talk of a national schools boycott, national rent strikes and stay-at-homes.

It comes at a time when there has never been such widespread popular discontent, seemingly begging for direction. But there is a crucial difference between this and what happened in Iran. The cataclysmic return of the Ayatollah was done according to his own timing -- as the government was collapsing of its own accord.

The South African state may be under a great deal of pressure and looking less certain than it has for decades, but it is not collapsing. It may find it difficult to contain Mandela, but it will always have the final -- albeit desperate -- option of returning him to prison. On the other hand, a call for action from Mandela could peter out like many of the previous calls for a national strike, such as the one proposed last year by Bishop Desmond Tutu.

The mythology surrounding Mandela -- cultivated by a government that allowed no one but his immediate family and his fellow prisoners to hear or see him for a quarter of a century -- is far greater than any individual could possibly live up to. The reality is that, despite the euphoria building up over his release, the presence of one man is not enough to bring overnight change.

This means an almost certain disappointment -- the Toivo ja Toivo factor. The comparison is not a simple one. Toivo ja Toivo was not known as a powerful and charismatic leader. He was hailed as the "father of Namibia" because he played an important role in forming the group that came to be the germ of that country's liberation movement.

Mandela's history was very different. In his book "Black politics in South Africa", Tom Lodge described him thus: "... a tall, aristocratic-looking man with a remote but commanding personality ... prove(d) himself to be a pragmatic and astute strategist". American academic Thomas Karis described him as "tall, athletic, with a commanding bearing and a dominating personality ... a born mass leader".

But, on the other hard, at 68 he is no longer a young man, even if he is fit and healthy for his age, and has been in relative isolation for a quarter of a century. The question will be whether he can sustain the interest and momentum generated by his return.

He is likely to come out at a time when many of the organisations that have campaigned for his release and could provide him with a support network have been severely weakened by the repression of the State of Emergency.

One only has to visit the UDF head office in Johannesburg -- once a hive of activity, always packed with activists and volunteer workers, now occupied by a lone person to answer the phone -- to see that he will have difficulty finding the backup and structure so crucial to political mobilisation. An important factor will be whether his fellow prisoners will be released with him.

The government will have difficulty keeping the other Rivonia trialists (such as Walter Sisulu, Govan Mbeki, Ahmed Kathrada) in prison after his release. It is one thing to release an individual leader, another to release a range of powerful, experienced and respected ANC leaders.

No doubt there are some within the government who are supporting their release in the hope that it will bring division within the ANC. On the contrary, the release of Mandela is likely to boost the ANC, raising hopes for its unbanning and placing it firmly in the centre stage.